Online newspaper of professor Yasser Metwally

The author: Professor Yasser Metwally

http://yassermetwally.com

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INTRODUCTION

Primary brain tumors represent approximately 15% of pediatric cancers. 13 They are much less common in children when compared with adults. Nevertheless, primary brain tumors are the leading cause of cancer-related deaths in children younger than 15 years old. 53 Supratentorial tumors predominate from birth to 3 years old, and infratentorial tumors predominate from 4 to 11 years old. 53 Thereafter, there is an equal incidence in both locations. Astrocytoma, medulloblastoma, ependymoma, craniopharyngioma, and glioblastoma multiforme account for about 80% of pediatric brain tumors. 53

ASTROCYTIC TUMORS

Astrocytic tumors are the most common primary brain tumor in children, constituting 30% to 40% of all central nervous system tumors. 30 There are two major groups of astrocytic tumors. The first and most common group is the diffuse astrocytomas. They are numerically graded 2 to 4, with the higher grades showing increasing tendency toward anaplastic transformation. The second group is more heterogeneous and includes pilocytic astrocytomas, pleomorphic xanthoastrocytomas (PXA), and subependymal giant cell astrocytomas (SEGA) (Table 1).

Diffuse astrocytomas are more common in adults, whereas the astrocytic tumors of the second group are more common in children. Most childhood astrocytic tumors are pilocytic astrocytomas. 30 Diffuse astrocytomas are more common than pilocytic astrocytomas only within the cerebral hemispheres and brain stem. 30 In contrast to adults, astrocytic tumors in children are relatively slow growing and have a relatively favorable prognosis. For example, in adults, low-grade diffuse astrocytomas have a marked tendency toward anaplastic transformation, whereas in children, low-grade diffuse astrocytomas rarely undergo anaplastic transformation. 30 The extent of initial resection, patient age of less than 5 years, and tumor location are important factors affecting prognosis. 9,28 Tumors located within the cerebral hemispheres and cerebellum have a more favorable prognosis, compared with tumors in the optic nerve, hypothalamus, thalamus, brain stem, and spinal cord. 9,28 The overall survival for childhood low-grade astrocytic tumors is 94% at 4 years. 28 For low-grade cerebellar astrocytic tumors, the 20-year survival after gross total resection is 100%. 9,28

Table [1] ASTROCYTIC TUMORS

• Supratentorial Astrocytic Tumors

Supratentorial astrocytic tumors account for approximately one third of childhood supratentorial tumors. 53 The peak age incidence is 2 to 4 years and 7 to 8 years. 24,53 There is a slight male predominance. 24,45 Most tumors occur within the cerebral hemispheres, but they can occur in the optic nerve or chiasm (optic glioma), hypothalamus (hypothalamic glioma), thalamus, basal ganglia, and ventricles. Within the cerebral hemispheres, most astrocytic tumors are low-grade diffuse astrocytomas, and the most frequent location is the temporal lobe, followed in decreasing order by the frontal lobe, parietal lobe, and occipital lobe. 53 Approximately 45% are solid, and 55% appear as a “cyst” with a mural nodule. 53 The mural nodule is usually located medially. 53 The “cyst” wall is not epithelial lined as in a true cyst but consists of nonneoplastic glial tissue. The CT appearance of solid astrocytic tumors is variable and may resemble adult gliomas and glioblastomas. The solid portions are usually hypodense to isodense before contrast administration. Enhancement is variable. The tumors may enhance completely, partially, or not at all. The tumors may show areas of necrosis and calcification. On MR imaging, compared with white matter, they may be hypointense to isointense on Tl-weighted images and hyperintense on T2-weighted images. Enhancement is variable. In general, low-grade tumors tend to be well circumscribed and homogeneous and show minimal, if any, adjacent edema. High-grade tumors are more heterogeneous, show areas of hemorrhage and necrosis, and cause more adjacent edema.

• Cerebellar Astrocytic Tumors

Cerebellar astrocytic tumors account for 10% to 20% of all childhood intracranial tumors. 9 They usually occur between the ages of 1 and 40 years, with 70% occurring in childhood. 9 The mean age of occurrence in all patients including adults is 15 years. 9 The mean age of occurrence in children is 7 years. 9,28 There is no sex predilection. Cerebellar astrocytic tumors occur equally within the vermis and cerebellar hemispheres, and approximately 80% are pilocytic astrocytomas. 9 The median duration of symptoms before diagnosis is 3 months. 9 Symptoms usually reflect increased intracranial pressure and include headache, vomiting, and papilledema. Ataxia, dysmetria, and brain stem dysfunction also can occur. Signs of brain stem dysfunction may reflect brain stem invasion and usually portend a worse prognosis.

Figure 1. Supratentorial low-grade astrocytoma. (left) Sagittal Tl -weighted image shows well-circumscribed hypointense right parietal lobe mass. Axial T2-weighted (right) image show the mass to be hyperintense. There is no peritumoral edema. (Click to magnify figure)

Cerebellar astrocytic tumors are usually large and can be cystic, solid, or both. On CT, about 40% of tumors appear solid, 10% to 20% calcify, and 60% to 70% contain Cysts. 9,53 Of cystic tumors, approximately 47% appear as a “cyst” with a mural nodule; 45% are tumor-lined cysts, which result from necrosis within a solid tumor; and 8% are multicystic. 53 The “cyst” wall is not epithelial lined as in a true cyst but consists of nonenhancing nonneoplastic glial tissue. The cyst wall may appear mildly hyperdense on CT because of the compressed nonneoplastic glial tissue. 53 Resection of the mural nodule is adequate to prevent recurrence if the cyst wall does not enhance. If the cyst wall enhances (i.e., indicating a tumor- lined cyst), complete resection of the cyst lining is required to prevent tumor recurrence. 9 The cyst is unilocular, and the nodule is single but may contain cystic areas. The mural nodule can be round, oval, or plaquelike. On CT, the mural nodule is usually isodense but may be partially cystic and calcified. The mural nodule usually shows intense homogeneous or heterogeneous enhancement. The solid astrocytic tumors are isodense to hyperdense on CT. Enhancement is variable. These tumors may enhance completely, partially, or not at all. On MR imaging, the tumors are hypointense to isointense on Tl-weighted images and hyperintense on T2-weighted images. If a tumor appears homogeneous, it may be difficult to distinguish cystic from solid masses before contrast administration. Enhancement patterns on MR imaging are similar to those on CT.

• Optic Gliomas

Optic pathway gliomas can arise anywhere along the optic pathway from just behind the globe to the occipital cortex. 1 Optic pathway gliomas constitute 1.5% of all intracranial tumors. 50 Optic nerve gliomas constitute about 3% of all orbital tumors and are four times as common as perioptic meningiomas. Approximately 50% to 85% of all optic pathway gliomas involve the chiasm or hypothalamus. Optic chiasm or hypothalamic gliomas (OCHG) comprise a subset of optic pathway gliomas and are often considered a single disease entity because it is often difficult to distinguish clearly the site of origin. Approximately 60% of OCHG are pilocytic astrocytomas, and 40% are diffuse fibrillary astrocytomas. 1 Rarely, OCHG can be gangliogliomas. 1 Most optic pathway gliomas occur in the first decade of life, with 65% occurring in the first 5 years. They have been reported in patients up to 79 years old, however. 50 There is no sex predilection when considering all optic pathway gliomas; however, optic nerve gliomas have a slight female predominance. 50 Optic pathway gliomas are the most common intracranial tumor in patients with neurofibromatosis type I (NF-1). Approximately 15% to 40% of patients with NF-1 develop OCHG, 1,50 whereas 10% to 70% of patients with optic pathway gliomas have NF-1. 50 About 20% of patients with optic gliomas experience visual impairment. 50

Figure 2. Optic chiasm glioma. Sagittal precontrast (left) and postcontrast (middle) Tl -weighted images show solid and cystic mass involving the optic chiasm. The solid portion enhances homogeneously. Axial intermediate-weighted image (right) shows hyperintense well-circumscribed optic chiasm mass. (Click to magnify figure)

Although most optic pathway gliomas are benign, congenital lesions with limited growth potential, 20% to 30% of patients experience progressive visual decline, neurologic deficits, and even death. 1,50 Tumor-related mortality depends, on location, occurring in 5% of isolated optic nerve gliomas and 50% of OCHG. 1,50 OCHG are more aggressive with a higher recurrence and mortality rate in children younger than 5 years old. The overall 5- year survival is 40% to 88%. 1 In patients with optic pathway gliomas, the prognosis is much better if they also have NF-I compared with patients without NF-1. 1 Signs and symptoms of optic pathway gliomas include optic atrophy, visual loss, endocrinopathies, hemiparesis, seizures, and increased intracranial pressure from hydrocephalus. Optic nerve gliomas can be unilateral or bilateral, especially in NF-1. Typically the optic nerve is enlarged (>4 mm in diameter), often with buckling of the nerve. Cystic changes can occur. Calcification does not occur in nonradiated optic nerve gliomas, a differentiating factor from perioptic meningioma, which commonly shows stippled calcification. Enlargement of the optic canal may occur with extension through the optic foramen. Enhancement occurs after contrast administration. If the tumor grows into the perineural space of the optic nerve, circumferential enhancement may occur, making it difficult to distinguish from perioptic meningioma.

Figure 3. Juvenile pilocytic astrocytoma. Sagittal Tl -weighted image (left) shows cerebellar hypointense mural nodule within a cyst. Sagittal post-contrast Tl -weighted image (middle) shows intense enhancement of the mural nodule. Axial T2-weighted image (right) shows hyperintense mural nodule within a cyst. The mural nodule contains multiple cystic foci. There is peritumoral edema. (Click to magnify figure)

On imaging, OCHG are usually large at presentation, appear round or lobulated with sharp margins, are predominately solid but can be partially cystic, and show mild edema in the adjacent brain. They may encase and displace the vessels of the circle of Willis and invade in all directions into adjacent structures. Hydrocephalus is common. Necrosis and hemorrhage are uncommon . 39 On CT, they appear homogeneous and hypodense. On MR imaging, they are hypointense to white matter on Tl-weighted images and hyperintense to white matter on T2-weighted images. The solid portions enhance. Rarely, they can demonstrate cerebrospinal fluid spread. 1 A normal imaging study in a patient with NF-1 does not exclude future development of an optic pathway glioma, which questions the congenital nature of the tumor. 50

• Subependymal Giant Cell Astrocytoma (SEGA)

SEGA occurs at the foramen of Monro in patients with tuberous sclerosis. Whether or not SEGA occurs outside the setting of tuberous sclerosis is controversial. 30 Tuberous sclerosis (Bourneville’s disease) is an autosomal dominant disorder that has a prevalence of about 1 in 10,000. Approximately 10% of these patients develop a SEGA, and the average age at diagnosis is 10 years. 61 SEGA arise from subependymal nodules located on the wall of the lateral ventricles. By convention, subependymal nodules larger than I cm or that are symptomatic are considered SEGA. 30 These tumors are hypodense to isodense on CT. They are frequently calcified, well-circumscribed rounded intraventricular masses at or near the foramen of Monro and usually cause hydrocephalus. Multiple subependymal nodules are usually present. SEGA show heterogeneous signal on MR imaging. They are usually hypointense to isointense on Tl-weighted images and hyperintense on T2-weighted images. These tumors strongly enhance. Although histologically benign, spontaneous intratumoral hemorrhage can result in death. The overall 10- year survival is approximately 80%. 30

Figure 4. Subependymal giant cell astrocytoma. Axial CT scan without contrast (left) shows calcified solid and cystic intraventricular mass at left foramen of Monro (arrow). Calcified subependymal nodules are also seen at the heads of the caudate nuclei. Axial T1 -weighted image (middle) shows heterogeneous mainly isointense intraventricular mass at left foramen of Monro with central foci of dark signal. Axial T2- weighted image (right) shows heterogeneous mainly isointense intraventricular mass at left foramen of Monro with central foci of bright signal. Note hyperintense cortical and subcortical tubers. (Click to magnify figure)

Figure 5. A, Subependymal giant cell astrocytoma (frontal section). Multiple nodules elevate the ependymal wall of both frontal horns in this case of tuberous sclerosis (Bourneville’s disease). B, Subependymal giant cell astrocytoma (MT x100). Large cells with abundant cytoplasm, thick or filiform processes, and a large, vesicular eccentric nucleus that often contains a prominent ganglion-like nucleolus. Several calcospherites are visible. The vessels are generally thin-walled. (Click to magnify figure)

• Brain Stem Gliomas

Brain stem gliomas (BSG) are a heterogeneous group of tumors that comprise 10% to 20% of all pediatric brain tumors and approximately 25% to 30% of all pediatric infratentorial tumors. 25,26,34,53 The mean age at diagnosis is 7 years. 2,25,34 There is no sex predilection. 2,34 The common CT appearance of BSG is that of a hypodense, partially enhancing brain stem mass. Calcification and hemorrhage are uncommon. On MR imaging, they appear hypointense on Tl-weighted images and hyperintense on T2-weighted images. Enhancement is variable and often minimal or absent. In one large series of 119 pediatric BSG, approximately 25% showed enhancement, 23% showed hydrocephalus, 17% showed cystic change, 4% showed hemorrhage, and none showed calcification. 34 Features that correlate with poor outcome include short duration (<6 months) of clinical history, cranial nerve deficits at diagnosis, pontine location, greater volume of tumor and degree of brain stem enlargement, ill-defined tumor margins, peritumoral hypodensity on CT or Tl hypointensity, and ring enhancement. 25,26 Favorable prognostic indicators include symptom duration longer than 12 months, exophytic location, NF-1, calcification on CT, and pathology suggesting low-grade tumor. 21 The extent of enhancement does not correlate well with tumor extent; however, the area of T2 hyperintensity does correlate closely with microscopic extent of tumor. 2 Tumor cysts had a favorable effect on outcome. 2 Tumor necrosis and hydrocephalus had no effect on outcome. 2 The overall survival is poor, with 5-year survival rates of 20% to 30%. 26 One large series showed an overall 3-year survival of 13%. 34 However, the location of the tumor within the brain stem affects prognosis. The 2-year survival for BSG in the midbrain is 100%; in the medulla, 50%; and in the pons, 20% . 2 On imaging, the most important factor affecting prognosis is whether or not the tumor is focal or diffuse. 25 The cumulative 5-year survival for focal tumors is 72% versus 20% for diffuse tumors. 25 Four types of BSG are described: diffuse, focal, dorsal exophytic, and cervicomedullary tumors. 26

Figure 6. Brain stem malignant glioma with cystic and haemorrhage components. (Click to magnify figure)

Diffuse intrinsic tumors represent about 80% of BSG. 26 They are poorly defined, involve greater than half of the involved brain stem segment (e.g., midbrain, pons, medulla), or extend both superiorly and inferiorly into the adjacent brain stem segment. 25 They most often occur in the pons, causing diffuse enlargement. Tumors in the pons are exophytic in 82%, with most growing ventrally into the prepontine cistern, surrounding the basilar artery. 2 Cerebrospinal fluid spread is not uncommon. The duration of symptoms is short, about 1 month. Cranial nerve deficits especially cranial nerves 6 and 7, long tract signs, and ataxia are common. Only 10% of patients have hydrocephalus at initial diagnosis. 26 Radiation therapy is the usual treatment. The median survival is less than 1 year.

Focal tumors represent about 5% to 10% of BSG. 26 They are well circumscribed, occupy less than half of the involved brain stem segment, and show no evidence of infiltration or peritumoral edema. 25 They can have an exophytic component of any size, with exophytic growth defined as growth extending into an adjacent cistern or ventricles. 25 They can appear cystic, solid, or both. They are most frequently located in the midbrain and medulla but can occur anywhere in the brain stem. They present insidiously, with a long history of focal neurologic deficits. Increased intracranial pressure is uncommon except in patients with tectal region tumors, which may obstruct the cerebral aqueduct and cause hydrocephalus. Patients with focal tectal gliomas who present only with hydrocephalus may do extremely well with just cerebrospinal fluid diversion (e.g., ventriculoperitoneal shunt). Surgery is the treatment of choice for surgically accessible focal tumors, meaning that they extend toward the brain stem surface; can be approached above or below the facial colliculus; and have imaging features suggestive of low- grade histology, which include homogeneous enhancement and absence of peritumoral hypodensity on CT. 26 Radiation therapy can be used for surgically inaccessible focal tumors.

Figure 7. Diffuse brain stem glioma. Sagittal T1 -weighted image (left) shows hypointense mass causing diffuse expansion of the pons. Axial T2-weighted image (middle) shows well-circumscribed hyperintense Pontine mass encasing the basilar artery and compressing the fourth ventricle. There is no peritumoral edema. Axial postcontrast Tl -weighted image shows no enhancement (left). (Click to magnify figure)

Dorsal exophytic tumors represent about 10% to 20% of BSG. 26 They arise from subependymal focal medullary tumors in the floor of the fourth ventricle, which then grow dorsally into the fourth ventricle. 25,26 They are usually large, well-circumscribed tumors that often fill the fourth ventricle but do not invade the adjacent brain. Most are low-grade pilocytic astrocytomas. Patients present insidiously with failure to thrive in infants and increased intracranial pressure in older children. Cranial nerve deficits are common, but long tract signs are distinctly unusual. 26 Surgery is the treatment of choice.

Cervicomedullary tumors represent about 10% to 15% of BSG. 26 They arise in the upper cervical cord and grow superiorly beyond the foramen magnum. Their axial growth is limited by the pyramidal decussation located ventrally at the junction of the cervical cord and medulla, at which point the tumor grows posteriorly, causing a dorsal bulge in the medulla. 26 High-grade tumors have a greater invasive potential and tend to grow through rather than around the pyramidal decussation and therefore do not exhibit the typical dorsal medullary bulge of low-grade tumors. Patients present with lower cranial nerve deficits, long tract signs, and sometimes torticollis. Hydrocephalus is uncommon. Surgery is the treatment of choice.

Figure 8. Focal tectal brainstem glioma. Sagittal Tl- weighted image (left) shows isointense mass expanding the tectum. Axial T2-weighted image (right) shows well-circumscribed hyperintense tectal mass, without adjacent edema. (Click to magnify figure)

• Pleomorphic Xanthoastrocytoma

PXA is the most recent addition to the 1993 World Health Organization (WHO) classification of astrocytic tumors. 36 Since its first description in 1979, approximately 90 cases have been reported, and although there are isolated reports in the elderly, two thirds of cases occur in patients younger than 18 years old. 8,30 PXA accounts for fewer than 1 % of all brain gliomas. 8 There is no sex predilection. It arises almost exclusively in the cerebral hemispheres, most commonly the temporal lobes, followed in order by the parietal, occipital, and frontal lobes. 30,78 Rare cases have been reported in the cerebellum and spinal cord. 8 PXA is believed to arise from subpial astrocytes because of their superficial location. 86 Patients typically present with seizures, and the mean duration of symptoms before diagnosis is 5 years. 8 In typical PXA, mitoses are rare, and necrosis and endothelial proliferation are absent. 78 The 5- year survival for typical PXA is 80%. 8 In atypical PXA, there is necrosis and increased cellularity and mitotic activity. 8 No patients with atypical PXA have survived for more than 3 years. Of patients younger than 18 years old at initial diagnosis, approximately 30% have died of tumor progression. 78 Therefore there is likely a subset of PXA prone to anaplastic transformation. Surgery is the treatment of choice; however, there has been no proven difference in survival when comparing gross total resection versus subtotal resection of tumor. 8 On imaging, PXA typically presents as a superficial cortical temporal lobe mass appearing as a cyst with a mural nodule. The mural nodule always enhances. The cyst wall may or may not enhance. If the cyst wall does enhance, resection of the cyst wall is necessary. Less commonly, the tumor can be completely solid. 79 The tumor is well circumscribed, is often large, and commonly is attached to the dura. 8 The dura is usually not invaded, however. 8 There may or may not be enhancement of the overlying dura. Nonresected leptomeningeal invasion could account for tumor recurrences Rarely, there can be surrounding gyriform enhancement. 78 Occasionally the inner table of the skull can be eroded. 8 On CT, the solid portions of the tumor appear heterogeneous and either hypodense or hyperdense. Calcification is variable. On MR imaging, the solid portions of the tumor appear hypointense or isointense on Tl- weighted images and hyperintense on T2- weighted images. The differential diagnosis of PXA includes ganglioglioma, pilocytic astrocytoma, oligodendroglioma, anaplastic astrocytoma, metastasis, granulomatous disease (e.g., sarcoid or fungus), and meningioma.

Figure 9. A, Pleomorphic xanthoastrocytoma (MT x100). Large cells with abundant, often foamy cytoplasm. Irregular, frequently large and multiple nuclei. No mitoses. This tumour primarily affects young subjects and generally occurs superficially over the cerebral hemispheres (meningocerebral lesion). The dense network of reticulin fibres previously suggested a diagnosis of fibrous xanthoma of the meninges. B, Pleomorphic xanthoastrocytoma (GFAP x100). Large cells with sometimes large or multiple nuclei, and abundant, occasionally foamy cytoplasm with variable positivity for GFAP. (Click to magnify figure)

Figure 10. Pleomorphic xanthoastrocytoma. Axial Tl -weighted image (left) shows large hypointense left temporal lobe mass with mural nodule. The mass involves cortex. Axial postcontrast Tl -weighted image (middle) shows enhancing mural nodule and peripheral enhancement. Axial intermediate-weighted image shows isointense mural nodule and hyperintense cystic tumor (right). (Click to magnify figure)

• Glioblastoma Multiforme

Glioblastoma multiforme accounts for approximately 7% of all childhood brain tumors. 22,86 Of all glioblastomas, approximately 3% occur in children. 22 The overall average age at diagnosis is 10 years 22,86 ; however, a biphasic age distribution has been noted at 4 to 5 years old and 11 to 12 years old. 22 There is no sex predilection. About 90% of childhood glioblastomas occur in the cerebral hemispheres, 86 but they also occur within the brain stem, cerebellum, and spinal cord. 22 In children, the most common location is within the frontal lobe, followed by the parietal lobe. 22 In adults, the most common location is within the frontal lobe followed by the temporal lobe. 22 Signs and symptoms are nonspecific and include headache, vomiting, paresis, seizures, diplopia, increased intracranial pressure, and cranial nerve palsies. The average duration of symptoms before diagnosis is 10 weeks. 22 Glioblastomas are highly aggressive malignant tumors that behave similarly in children and adults. The extent of surgical resection is the most important prognostic indicator. In children with malignant astrocytomas, the 5- year survival after radical resection is 35% versus 17% after subtotal resection. 86 Anaplastic astrocytoma has a better prognosis than glioblastoma, with 5-year survivals of 44% versus 26% after radical resection. 86 On imaging, glioblastomas often appear ill defined and heterogeneous and show increased peripheral edema and mass effect. Hemorrhage is common and is more often found in glioblastomas than in anaplastic astrocytomas. Calcification is rare. Enhancement is heterogeneous, often with thick irregular rim enhancement and central necrosis. Glioblastomas can be diffusely infiltrating and often involve the corpus callosum. Cerebrospinal fluid spread is noted in 20% of cases. 86

PRIMITIVE NEUROECTODERMAL TUMORS

Primitive neuroectodermal tumors (PNET) are small cell malignant childhood tumors with a predominant location in the cerebellum and a noted capacity for divergent differentiation, including neuronal, glial, mesenchymal, and melanotic. 62 There is no consensus regarding classification of PNETS.

• Supratentorial Primitive Neuroectodermal Tumors

Supratentorial PNET probably represent less than 1 % of all pediatric brain tumors. Approximately 85% of cases occur before 10 years of age, and about 65% occur before the age of 5 years. 12,31 There is no sex predilection. Patients often present with seizures or signs of increased intracranial pressure. In one large series, the most common location was in the frontal lobes, followed in order by the parietal, temporal, and occipital lobes. 31 The tumor can also be intraventricular. 20 Approximately 50% to 70% of supratentorial PNET calcify, 62 and approximately 40% of cases demonstrate cerebrospinal fluid spread. 20,31 On CT, the tumor appears as a heterogeneous well-circumscribed mass. The solid tumor components appear hyperdense on CT, probably secondary to the tumors dense cellularity and high cellular nuclear-to-cytoplasm ratio. The solid tumor components enhance. Variable coarse calcification, cystic or necrotic change, and hemorrhage contribute to the tumor’s heterogeneity both before and after contrast enhancement. On MR imaging, these tumors show heterogeneous signal on both Tl-weighted images and T2-weighted images, which is likely due to variable foci of blood products, calcification, and cystic change. The tumors are often large but may demonstrate little peritumoral edema relative to the size of the lesion. 20,23 Hydrocephalus may or may not be present. The solid tumor components enhance after gadolinium administration. The differential diagnosis of a large well-circumscribed heterogeneous supratentorial mass includes PNET, ependymoma, oligodendroglioma, and astrocytoma.

• lnfratentorial Primitive Neuroectodermal Tumors

Most infratentorial PNET occur in the cerebellum; however, PNET can occur in the brain stem, particularly the pons. 3 The tumors can be ill defined or well circumscribed and may or may not have adjacent edema. The tumors may enhance homogeneously, heterogeneously, or not at all. Focal tumors, well-circumscribed margins, and lack of enhancement do not necessarily mean that the brain stem tumor is nonaggressive or benign. 3 Compared with PNET of the cerebellum, pontine PNET have a poorer prognosis. 3

• Medulloblastoma

Approximately 85% of PNET arise in the cerebellum and are commonly referred to as medulloblastoma. 62 Medulloblastoma is the most common posterior fossa tumor in children, accounting for 30% to 40% of childhood posterior fossa tumors and 15% to 25% of all pediatric brain tumors. 37,51,80 In comparison, medulloblastoma accounts for only about 1% of all adult brain tumors. 37 The average age at diagnosis is approximately 7 years. 51,80 There is a slight male predominance. 37,47,51,53,80 Approximately 40% of patients demonstrate cerebrospinal fluid spread, solid central nervous system metastasis, or both at initial diagnosis. 40 In almost all children, however, at least micrometastases along the entire cerebrospinal fluid pathway have to be assumed because the omission of craniospinal radiation leads to a dramatic increase in recurrence rates. 40 The overall 10-year survival is 60%; however, children younger than 3 years old have a poorer prognosis regarding both survival and quality of life. 5,66 Approximately 40% of patients relapse. 5 The median time from initial diagnosis to recurrence is 10.5 months, and approximately 75% of recurrences occur in the first 2 years. 5 The median survival time from relapse is 5 months, and only 25% to 30% of patients survive more than I year. 5 In patients with recurrent disease, approximately 50% have localized disease, and 50% have metastasis at initial diagnosis. 5 There is no significant difference in outcome for patients who have undergone gross total resection versus subtotal resection of tumor. 62 Both postoperative chemotherapy and radiation therapy are important in the treatment of medulloblastoma. Approximately 40% of patients have asymptomatic relapses that are detected by surveillance imaging or lumbar puncture, and these patients show longer survival than patients with symptomatic relapse. 5 Nevertheless, the overall outcome for patients with recurrent disease is uniformly poor, and the value of surveillance imaging or cerebrospinal fluid sampling remains controversial.

Figure 11. Medulloblastoma. Axial T2-weighted image (left) shows isointense fourth ventricular mass. Axial Tl -weighted image (middle) shows the mass to be hypointense. Sagittal post-contrast Tl -weighted image (right) shows hydrocephalus and intense enhancement within the mass. (Click to magnify figure)

Approximately 80% of childhood medulloblastomas are midline, whereas adult medulloblastoma is more often lateral and hemispheric. 37,62 The most common location for medulloblastoma is the middle and inferior portions of the anterior vermis. 51,62 More than 90% of cases have hydrocephalus. Most medulloblastomas are solid, well-circumscribed masses with convex margins. The tumor may occasionally spread through the fourth ventricular outlet foramina. 80 On CT, the tumor is isodense to hyperdense. Clumplike calcification is seen in 20%. 51,80 In comparison, 50% to 70% of supratentorial PNET calcify. 62 On MR imaging, the tumor is hypointense to isointense on Tl -weighted images and isointense to hyperintense on T2-weighted images. Signal on T2-weighted images is heterogeneous secondary to calcification, cysts, necrosis, hemorrhage, and vascular flow voids. Approximately 90% of medulloblastomas enhance. 62,80 Metastatic or recurrent medulloblastoma may or may not enhance, which suggests that metastatic or recurrent disease could be underestimated on postcontrast examinations. 51 Normal postoperative enhancement is usually linear, whereas tumor enhancement is usually nodular. Approximately 30% to 40% of patients demonstrate cerebrospinal fluid spread, solid central nervous system metastasis, or both at initial diagnosis. 37,40,62,80 The differential diagnosis of medulloblastoma includes ependymoma, astrocytoma, choroid plexus papilloma, and metastasis. Some suggest the combination of an isodense to hyperdense mass on CT that is hypointense on Tl- weighted images favors medulloblastoma. 80

A desmoplastic variant of medulloblastoma exists and usually occurs in adolescents and young adults. 43 The mean age at diagnosis is approximately 17 years. 43 The desmoplastic variant is usually located in the cerebellar hemispheres, in contrast to the midline location of classic medulloblastoma. In children, the desmoplastic variant of medulloblastoma may have a more favorable prognosis compared with classic medulloblastoma. 43

Medullomyoblastoma is a rare cerebellar tumor with histologic characteristics of medulloblastoma within which smooth and striated muscle is present. 43 There are approximately 30 cases reported in the literature. 43 The patients are usually younger than 10 years of age. 43 Patients usually present within 3 months from the onset of symptoms. Signs and symptoms include increased intracranial pressure, cerebellar signs, and lower cranial nerve palsies. Imaging findings are similar to those for classic medulloblastoma. The prognosis is worse than classic medulloblastoma. The largest series reports no patients surviving beyond 3 years. 43

Malignant rhabdoid tumor of the cerebellum is frequently misdiagnosed as medulloblastoma. Malignant rhabdoid tumor was initially described in 1978 as an embryonal tumor of the kidney. The term rhabdoid was used because the cells resembled rhabdomyosarcoma by light microscopy, but myogenic differentiation has never been proven. 49 Although the rhabdoid cell is the hallmark of malignant rhabdoid tumors, these tumors also contain zones of PNET, which explains why they have been previously misdiagnosed as medulloblastomas. Most patients present before 3 years of age. 49 Approximately 50% of malignant rhabdoid tumors occur in the posterior fossa; 20% occur in the cerebral hemispheres; and a few percent occur in the pineal gland, suprasellar region, and spinal cord. 49 About one third of cases have cerebrospinal fluid spread or metastasis at initial diagnosis. 49 Imaging findings are similar to those for classic medulloblastoma. Compared with medulloblastoma, the patients are usually younger (mean age 17 months) and have a worse prognosis, with death occurring within 1 year.

EPENDYMOMAS

Ependymomas are glial tumors derived from ependymal cells lining the ventricles and spinal cord central canal. Ependymal cell rests within the white matter or foramina of Luschka account for the extraventricular origin of these tumors. Ependymomas account for approximately 10% of childhood intracranial tumors. 27 They are the fourth most common posterior fossa tumor of childhood, after medulloblastoma, cerebellar astrocytoma, and BSG. 81 Approximately 70% of ependymomas are infratentorial. 17,27,53,56 There is a slight male predominance. 17,56 The average age for supratentorial ependymomas is 16 to 24 years. 27 Infratentorial ependymomas have two age peaks, around 5 years and 35 years. 17, 56,70,73,81 Symptoms include nausea and vomiting, headaches, personality changes, altered level of consciousness, ataxia, cranial nerve palsies, nuchal rigidity, torticollis, seizures, and delayed growth and development. 17,27,81 Most cases have a history of less than 6 months. Overall, approximately 70% of ependymomas are malignant; however, supratentorial ependymomas are more often malignant (85%) than infratentorial ependymomas (50%). 56 Approximately 50% to 60% recur, and about 10% to 20% metastasize. 56,61 Metastasis occurs along the cerebrospinal fluid pathways. Supratentorial ependymoma metastases are uncommon. When they occur, they are usually supratentorial. Infratentorial ependymoma metastases are common (one third) and are almost always intraspinal. The extent of tumor excision and the presence of residual tumor on postoperative imaging are the only significant factors affecting survival. 61,81 The 5-year survival rate has improved to approximately 64%. 81

Figure 12. Infratentorial ependymoma. Sagittal postcontrast Tl-weighted image (left) shows heterogeneous enhancing fourth ventricular mass causing hydrocephalus. Axial T2-weighted image (right) shows heterogeneous signal mass extending out the foramina of Luschka and compressing the medulla. (Click to magnify figure)

In general, the floor of the fourth ventricle is the most common location, followed by the lateral ventricles, cerebral aqueduct, third ventricle, spinal cord, filum terminate, and cerebral hemispheres. 81 Infratentorial ependymomas tend to be intraventricular, and supratentorial ependymomas tend to be extraventricular. 17 Supratentorial ependymomas are more often cystic (80%) and usually have large cysts, whereas infratentorial ependymomas are uncommonly cystic and contain small cysts. 17,70 On CT, ependymomas usually appear as an isodense, partially calcified fourth ventricular mass with variable, often heterogeneous enhancement. Approximately 50% of both supratentorial and infratentorial ependymomas calcify. The calcification is often small and round. 73 Cyst formation and hemorrhage can be present. Intratumoral hemorrhage is uncommon, occurring in 0 to 22% of cases. 70,81 On MR imaging, the solid tumor components are hypointense to isointense to white matter on Tl-weighted images and isointense to hyperintense to white matter on intermediate-weighted and T2-weighted images. Signal intensity is often variable because of the presence of calcification, cysts, and hemorrhage. Variable heterogeneous enhancement is most common; however, rarely, ependymomas do not enhance. 73,81 The following imaging features have been described in infratentorial ependymomas, 8l : a cleavage plane between the tumor and vermis, capping of the fourth ventricle, desmoplastic development (which is the tendency for ependymomas to spread through the fourth ventricular outlet foramina in a fashion likened to toothpaste squeezed from its tube), hydrocephalus, calcification, heterogeneous architecture, hemorrhage, and vessel encasement. Supratentorial ependymomas usually appear as a partially calcified mixed solid and cystic mass with heterogeneous enhancement. The cysts are often large.

The differential diagnosis for infratentorial ependymomas includes medulloblastomas, cerebellar astrocytomas, and BSG. Medulloblastomas are usually hyperdense on CT, have more homogeneous enhancement, less often calcify (15%), and usually calcify in larger clumps. Cerebellar astrocytomas often have a more homogeneous appearance and tend to occur in older children. BSG rarely calcify and tend to infiltrate and expand the pons. Any intracranial glioma of childhood could be considered in the differential diagnosis for supratentorial ependymomas; however, the presence of cysts and calcification favors ependymoma and ganglioglioma. Patients with ganglioglioma tend to be older and have a longer prediagnosis history.

GANGLION CELL TUMORS

Ganglion cell tumors are low-grade neoplasms that compose approximately 1% of all primary brain tumors, 11 and 4% of pediatric brain tumors. 32 They are composed of a mixture of ganglion cells and glial elements. Gangliocytomas (WHO grade 1) are composed only of neuronal elements and are also referred to as ganglioneuroma. Gangliogliomas (WHO grade 1-11) have both neuronal and glial components. Ganglion cell tumors are classified according to their stages of differentiation and relative proportion of neuronal to glial elements as follows: gangliocytoma, ganglioglioma, ganglioneuroblastoma, anaplastic ganglioglioma, and neuroblastoma. 11 Gangliogliomas can occur at any age, but 80% occur in patients younger than 30 years of age. 35 The average age at diagnosis is 10 years. 32 There is no sex predilection. Seizures are the most common presentation, and the average duration of symptoms is 2 to 11 years. 32,89 Gross total resection of tumor provides the best clinical outcome. 32,89 After surgery, 80% of patients are seizure-free, and 20% of patients have a reduction in seizure frequency. 32,39 In addition, if a gross total resection has been achieved, anticonvulsant medication can be tapered or discontinued in most patients. 32 Some gangliogliomas may have a malignant component . 32 The 3-year survival for gangliogliomas is 92%. 32

Gangliogliomas have nonspecific imaging findings. They can occur anywhere in the neural axis but are most common in the temporal lobe. 32,89 On CT, the tumor is usually hypodense and may be entirely solid, entirely cystic, or mixed. Forty percent calcify, 11,89 and occasionally a bizarre calcification pattern may be seen. 10 Approximately 50% have either homogeneous or heterogeneous enhancement. 10,11,32,89 The tumor is often well circumscribed, is peripheral in location, and shows little or no mass effect. A peripheral ganglioglioma may erode the adjacent calvaria (5% of cases). 53 Hemorrhage is rare but may be the presenting sign. 53 On MR imaging, there is variable signal intensity Tl-weighted images may be hypointense, isointense, or slightly hyperintense. 11 T2-weighted images are usually hyperintense but may also have variable signal intensity. 89 Rarely, gangliocytomas can occur in the sella, and approximately 60% of sellar gangliocytomas are associated with pituitary adenoma. 59 On imaging, sellar gangliocytoma and pituitary adenoma are indistinguishable. 59

Within the cerebellum, dysplastic gangliocytoma may occur, which is also known as Lhermitte-Duclos disease (WHO grade 1). Pathologically, it is thought to represent a hamartoma. 11 A familial occurrence has been reported. 41 Associated developmental anomalies include megalencephaly, heterotopia, microgyria, hydromyelia, and polydactyly 4l Coexisting conditions include neurofibromatosis and Cowdens disease (multiple hamartoma syndrome). 4l It typically presents in young adults (mean age of 34 years, age range, birth to 74 years) as a cerebellar mass with symptoms from increased intracranial pressure. 4l The cerebellar folia are enlarged (7 mm thick). 4l It usually appears as a large, nonenhancing cerebellar mass that typically has a striated appearance on MR imaging. 4l Calcification may occur. Long-term follow-up is recommended because of the possibility of symptomatic recurrence many years after gross total resection. 4l

• Desmoplastic Infantile Ganglioglioma and Superficial Cerebral Astrocytoma

Desmoplastic infantile ganglioglioma (DIG) is a WHO grade I tumor, with both neuronal and glial differentiation. DIG usually presents in the first year of life. 84 Boys are affected more than girls. 76,84 Seizures and macrocephaly are the most common presentation. 76 Typically the tumor presents as a large hemispheric cystic mass with an enhancing solid portion adjacent to the meninges. The solid portion of the mass contains the desmoplastic tissue. 63,76,84 The frontal and parietal lobes are most commonly affected, 1,48,76,84 with relative sparing of the occipital lobes. 84 DIG is a benign tumor with a favorable prognosis after excision.

The differential diagnosis in the infantile age group includes classic ganglioglioma, supratentorial ependymoma and astrocytoma, PNET, malignant teratoma, and sarcoma. DIG is different from classic ganglioglioma in that the presentation is in infancy, the location is usually frontal or parietal, there is dense desmoplasia, and there is inclusion of immature neuroepithelial cells. 76 Ependymomas usually enhance heterogeneously and are often calcified. PNET are more heterogeneous, with cysts, calcification, hemorrhage, and necrosis.

Superficial cerebral astrocytoma is another tumor, which presents in infants younger than 1 year of age, has a large size, has large cysts, is superficial and predominantly frontal lobe in location, is attached to the dura, shows intense enhancing leptomeningeal reaction, and has a favorable prognosis. 21,74 Superficial cerebral astrocytoma represents 1% of infant brain tumors. 74 The main difference between DIG and superficial cerebral astrocytoma is that superficial cerebral astrocytoma lacks a neuronal component, whereas DIG contains both neuronal and glial differentiation. 84

OLIGODENDROGLIOMAS

Oligodendrogliomas are neuroepithelial tumors composed of glial cells resembling oligodendrocytes. Approximately half of the tumors are actually mixed gliomas containing other glial elements, most commonly astrocytic. 42 Oligodendrogliomas account for approximately 5% to 7% of all brain tumors, and about 5% to 10% of these occur in patients younger than age 20. 44,60,68 Oligodendrogliomas constitute approximately 1 % to 3% of pediatric brain tumors. 60 Pure oligodendrogliomas account for about 4% of primary brain tumors. 68 There is a biphasic age distribution of 6 to 12 years and 26 to 46 years. 60,77 There is a slight male predominance. Patients usually present with a long history of seizures and headache. The average duration of symptoms before diagnosis is 3 years. 68 After surgery, there is an approximate 30% reduction in seizure frequency The overall 5-year survival for pediatric oligodendrogliomas is 65% to 70%. 60 The 15-year survival for oligodendrogliomas overall is 24%. 68 Children with histologic low-grade tumors, pure oligodendrogliomas, and age younger than 12 years have a better prognosis. 60 Approximately 95% of oligodendrogliomas occur in the cerebral hemispheres, most commonly the frontal lobes. 42,44,68 The second most common location is either the temporal or the parietal lobes. 42,44,68,77 Oligodendrogliomas have been reported in the sella, ventricles, posterior fossa, and spinal cord as well. 44,60,77 Cerebrospinal fluid spread occurs in approximately 9% of cases. 44

On CT, oligodendrogliomas usually appear as a well-circumscribed hypodense, partially calcified mass in a cortical or subcortical location. Approximately 40% of oligodendrogliomas calcify, 42,68,77 and 20% show areas of hemorrhages. 42,44 Twenty percent can show areas of cyst formation. 42 Enhancement is usually heterogeneous and is seen in approximately 25% on CT and 80% on MR imaging. 77 Skull erosion can be seen in about 17% of cases. 42 On MR imaging, these tumors are usually hypointense on Tl-weighted images and hyperintense on T2-weighted images. Peritumoral edema occurs in less than 25% of cases. 60 Compared with adults, pediatric oligodendrogliomas less frequently show calcification, enhancement, and peritumoral edema. 77 Approximately 10% of cases have hydrocephalus. 77

Figure 13. Oligodendroglioma. Coronal precontrast (left) and postcontrast (middle) Tl -weighted images show hypointense right temporal lobe mass, which demonstrates homogeneous enhancement. Axial T2-weighted image shows well-circumscribed hyperintense mass without peritumoral edema (right). (Click to magnify figure)

The differential diagnosis mainly includes supratentorial ganglioglioma, astrocytoma, and ependymoma. Gangliogliomas are usually located in the temporal lobe and have a deep location. Both astrocytomas and ependymomas also usually have a deep location. Supratentorial astrocytomas usually lack calcification. Supratentorial ependymomas usually enhance and have large cystic components.

CHOROID PLEXUS TUMORS

Choroid plexus tumors constitute 1% of all brain tumors, 16 and approximately 3% of pediatric brain tumors. 16 About 70% of patients with choroid plexus papilloma (CPP) are younger than 2 years of age. 16 The first year of life is the most common time for CPP to occur, 16 when they account for 10% to 20% of brain tumors during that time. 47 Cpp is classified as a WHO grade I tumor. 36 In children, CPP usually occur in the lateral ventricle, whereas in adults, they occur more frequently in the fourth ventricle. Third ventricular CPP are rare, and most of these occur in children younger than 1 year of age. 16 Patients present with symptoms from increased intracranial pressure secondary to hydrocephalus. The average duration of symptoms before diagnosis is 1 month. 55 The cause of the hydrocephalus may be due to multiple factors, including cerebrospinal fluid overproduction, ventricular obstruction, 16 and impaired cerebrospinal fluid resorption. In addition, for reasons that are unknown, the hydrocephalus may persist after tumor resections. 55 Patients younger than 2 years old also present with macrocephaly. CPP are benign tumors, and the 5-year survival after resection is 100%. 55

Figure 14. Choroid plexus carcinoma. Axial Tl -weighted image (left) shows large lobulated intraventricular mass in right atrium, which contains foci of bright signal representing hemorrhage. Foci of dark signal represent calcification and vascular flow voids. There is marked mass effect and hydrocephalus. The mass invades the adjacent parenchyma. Axial post-contrast Tl -weighted image (middle) shows intense homogeneous enhancement within the mass. Axial T2-weighted image (right) shows heterogeneous isointense mass with peritumoral edema. (Click to magnify figure)

Approximately 30% of choroid plexus tumors are choroid plexus carcinomas (CPC). 4,16 CPC are classified as WHO grade 111-IV tumors. 36 The average age at diagnosis is approximately 2 years, 4 which is slightly older than the age for CPP. There is no sex predilection. 4 Symptoms include signs of increased intracranial pressure, diplopia, cranial nerve palsies, motor deficits, weakness, and irritability. The average duration of symptoms before diagnosis is 1 month. 4 The extent of surgical resection is the only significant prognostic factor. 4 After gross total resection, the 5-year survival is about 86%. 4 The overall 5-year survival for CPC is 26% to 40%. 4,55 CPC has a high propensity for recurrence, and in cases in which recurrence is suspected, a second operation is recommended. 4 Because choroid plexus tumors are large, fragile, highly vascular tumors, there is a substantial risk of uncontrollable intraoperative hemorrhage, especially in infants. For this reason, there has been a move toward preoperative embolization. 55 Superselective catheterization of feeding arteries is often difficult, however, and is the main reason for preoperative embolization failure. 55

On CT, the tumors appear as an isodense to hyperdense intraventricular mass with punctate calcification and marked homogeneous enhancement. They can occur anywhere along the choroid plexus but are most commonly centered in the region of the trigone. On MR imaging, CPPs are hypointense to isointense on Tl-weighted images with a lobulated contour and may contain areas of signal loss resulting from calcification or vascular flow voids. Cerebrospinal fluid trapped between the tumor papillae may cause a mottled appearance. Heterogeneous signal on T2- weighted images is likely due to calcification, blood products, and vascular flow voids. Approximately 8% of choroid plexus tumors are extraventricular and are located in the cerebellopontine angle cistern. 55 The average tumor size is approximately 4.5 cm. 55 Calcification occurs in approximately 25% of cases. Hydrocephalus occurs in 90% of cases. 55 Hemorrhage may occasionally occur. Unifocal lesions occur in 86% of cases, and bifocal lesions (i.e., simultaneous supratentorial and infratentorial locations) occur in 14% of cases. 4 Cerebrospinal fluid spread is seen in about 15% of cases. 4 In general, CPC cannot be distinguished from CPP by imaging. Some features, however, such as parenchymal invasion or metastatic nodules in the ventricles, may suggest CPC. 4 The differential diagnosis includes ependymoma, PNET, astrocytoma, and medulloblastoma.

DYSEMBRYOPLASTIC NEUROEPITHELIAL TUMOR

Dysembryoplastic neuroepithelial tumor is a rare benign cortical tumor (WHO grade I) associated with medically intractable seizures without neurologic deficit or with stable congenital deficit. There is usually a long history of partial complex seizures with an age of onset of less than 20 years. 75 The average age of seizure onset is 9 years. 38 Males are affected more than females. The term dysembryoplastic is used because there are several factors suggesting an origin in dysembryogenesis, which include foci of cortical dysplasia, young age of symptom onset, and overlying calvarial deformity. 75 All reported cases in the literature have a supratentorial location. 19,30,75 The most common location is the temporal lobe, followed by the frontal lobe. 38 On MR imaging, there is usually a focal well-circumscribed cortical mass, which is hypointense or rarely isointense on Tl-weighted images and hyperintense on T2-weighted images. There is no adjacent edema. Enhancement is usually minimal to none. On CT, the tumor is usually hypodense with a cystic appearance. Overlying calvarial deformity and thinning of the inner table may be seen. Calcification can occasionally occur. Patients with complete or incomplete resection do not show evidence of clinical or radiologic recurrence.

The differential diagnosis includes low-grade gliomas and gangliogliomas. Gangliogliomas are more often calcified and show enhancement. Dysembryoplastic neuroepithelial tumor may resemble a simple cyst or show increased signal intensity on intermediate- weighted images suggesting a more complicated lesion.

CRANIOPHARYNGIOMAS

Craniopharyngiomas are benign epithelial neoplasms and account for approximately 3% of all intracranial tumors. 52,64 Although half of all cases occur in adults, craniopharyngiomas are the most common nonneuroepithelial tumor in children. 52 They constitute more than 50% of all pediatric suprasellar tumors 39,52 and approximately 10% of all pediatric brain tumors. 15 Approximately 94% of craniopharyngiomas are suprasellar, but they can also be entirely intrasellar or intraventricular within the third ventricle. 15,18,52 They can extend into the anterior, middle, or posterior cranial fossas and often involve adjacent structures, such as the hypothalamus, thalamus, optic chiasm, third ventricle, medial temporal lobes, midbrain, and brain stem. 15,52 Thus, impairment of endocrine control, vision, memory, and cranial nerves may occur. 15,18,52, 85 Two distinct clinical, histologic, and radiologic subtypes have been described. 18,52,64 In the classic craniopharyngioma, the epithelium resembles tumors of tooth-forming epithelium similar to adamantinomas and is thus called adamantinomatous craniopharyngioma. 52 The other subtype of craniopharyngioma lacks the distinctive features of the adamantinomatous type, consists of more regularly stratified squamous epithelium with papillary projections, and is thus called squamous papillary craniopharyngioma. 52 Mixed forms of craniopharyngiomas also exist. 52 Adamantinomatous craniopharyngiomas cala occur at any age but have a peak age incidence between 10 and 20 years. 18 Squamous papillary craniopharyngiomas occur almost exclusively in adults, with a peak age incidence between 40 and 50 years. 18 There is no sex predilection in either type.

Table [2] Pathological types of craniopharyngioma

In adamantinomatous craniopharyngiomas, approximately 90% are cystic, 90% calcify, and 90% enhance. 47,64 The calcification is often curvilinear and along the periphery of the tumor. 82 The enhancement involves the solid tumor portions and the cyst walls and helps to distinguish craniopharyngiomas from epidermoids, which do not enhance but frequently occur in the suprasellar region. On CT, the tumor typically appears as an enhancing, heterogeneous, partially calcified suprasellar mixed solid and cystic mass. On MR imaging, the tumor typically appears as an enhancing heterogeneous lobulated mixed solid and cystic mass. The cysts are often hyperintense on Tl-weighted images and either hypointense or hyperintense on T2-weighted images. The cyst signal intensity is mainly influenced by a protein concentration greater than or equal to 90 g/L and the presence of free methemoglobin. 64 Rupture of the cyst, either spontaneously or at surgery, can cause a chemical meningitis. 52,82 Hydrocephalus is seen in approximately 50% of cases. 85 After gross total resection, the recurrence rate is approximately 18%, and after subtotal resection with radiation therapy, the recurrence rate is about 43%. 85 The overall 10- year survival is 91%. 67 Features that favor adamantinomatous craniopharyngiomas over squamous papillary craniopharyngiomas are occurrence in children, calcification, predominantly cystic with hyperintense cysts on Tl- weighted images, lobulated shape, vessel encasement, brain invasion, and propensity for tumor recurrence. 52,64

Figure 15. Craniopharyngioma. Sagittal (left) and axial (middle) postcontrast Tl – weighted images show an enhancing cystic and solid suprasellar mass. Axial T2-weighted image (right) shows hyperintense cysts and hypointense solid portions, some of which are calcifications. (Click to magnify figure)

In squamous papillary craniopharyngiomas, approximately 50% are cystic and 50% predominantly solid. 64 The cysts are usually hypointense on Tl-weighted images. 64 Most intraventricular craniopharyngiomas are of the squamous papillary type. 18 The absence of calcification, spherical shape, propensity to involve the third ventricle, and potential for rapid growth distinguish squamous papillary craniopharyngiomas from adamantinomatous craniopharyngiomas. 18,64 There does not appear to be a significant difference in resectability, efficacy of radiation therapy, and overall survival between the two subtypes. 18

The differential diagnosis includes Rathke’s cleft cyst, epidermoid, cystic optic chiasmatic or hypothalamic glioma, necrotic pituitary adenoma, thrombosed aneurysm, and pituitary abscess. Rathke’s cleft cyst is usually found in adults and is small, intrasellar and homogeneous and does not enhance or calcify.

PINEAL REGION TUMORS

Pineal region tumors constitute approximately 1% of all adult brain tumors and 10% of all pediatric brain tumors, with a slightly higher incidence in Northeast Asia. 13,33 There are many types of pineal region masses, including germ cell tumors (GCT) (50% to 70%), pineal cell tumors (PCT) (15% to 30%), gliomas (12%), meningiomas, CPP, metastases, vascular lesions, and nonneoplastic masses (such as pineal cysts, arachnoid cysts, inflammatory cysts, lipomas, epidermoids, and dermoids). 13,88 This discussion focuses on GCT and PCT.

Of intracranial GCT, 55% to 60% occur in the pineal region, 30% occur in the suprasellar region, 15% occur in the basal ganglia or thalamus, and 15% are multiple (usually pineal and suprasellar). 14,72 Of pineal GCT, 65% are germinomas, 25% are nongerminomatous GCT (i.e., immature teratomas, embryonal carcinoma, endodermal sinus tumor [yolk sac tumor], choriocarcinoma, and mixed nongerminomatous GCT), and 10% are mature teratomas. 14 Mixed germinomatous and nongerminomatous pineal GCT occur in 15% to 20% of cases. 7,14,57 One third of pineal tumors are benign, for which surgery alone is usually curative. 7 The overall operative mortality for pineal region tumors is 4%. 7 Germinomas are often curable with radiation therapy or chemotherapy, whereas nongerminomas are relatively radioresistant. 14 The 5-year survival for germinomas and mature teratomas is 80% to 90% and for nongerminomas is 49%. 14,33,57 Cerebrospinal fluid spread occurs in about 8% to 18% of GCT and PCT. 7,14 Paralysis of upward gaze (Parinaud’s syndrome) occurs with about 50% to 75% of pineal tumors. 7,14 Increased intracranial pressure, diplopia, headache, and vomiting also occur. Hydrocephalus occurs in about 70% of cases. 14 The peak age for intracranial germinomas is 10 to 15 years. 14,33,72 About 75% to 90% of pineal germinomas occur in males 14,33,88 ; there is no sex predilection for suprasellar germinomas. 47

Figure 16. Mixed germinomatous germ cell tumor. Axial Tl -weighted image (left) shows isointense pineal mass with small focus of bright signal representing hemorrhage. Axial T2-weighted image (middle) shows heterogeneous pineal mass. The dark signal foci likely represent calcification and hemorrhage. Sagittal postcontrast Tl -weighted image (right) shows fairly homogeneous enhancement. (Click to magnify figure)

In general, both GCT and PCT appear hyperdense on CT and isointense on T2- weighted images as a result of the cellularity and high nuclear-to-cytoplasm ratios. On CT, most germinomas are well-circumscribed, hyperdense or, less often, isodense, enhancing masses without tumor calcification (although the tumor often displaces or engulfs a calcified pineal gland). 88 On MR imaging germinomas are well circumscribed, slightly hypointense to isointense on Tl-weighted images, and isointense to hyperintense on T2-weighted images. Variable cystic areas and hemorrhage may be seen.

Pineal teratomas occur almost exclusively in males with a peak incidence around puberty. 47,57 Teratomas contain all three germ layers. Mature teratomas (benign teratoma) contain fully differentiated tissue, and immature teratomas (malignant teratoma) contain some primitive tissue. 57 Intracranially, immature teratomas are more common than mature teratomas. Teratomas appear heterogeneous as a result of hemorrhage, cysts, fat, calcification, and even bone. They are hypointense to isointense on Tl-weighted images and heterogeneously hyperintense on T2-weighted images. Ringlike enhancement may be seen . 88

Figure 17. For comment see figure [18] (Click to magnify figure)

Figure 18. Pineocytoma. Axial CT scan (figure 17 ) shows heavily calcified hypodense pineal mass. Sagittal precontrast Tl -weighted image (left) shows large heterogeneous pineal mass. The dark signal within the inferior portion represents calcification. A shunt tube is present. Sagittal postcontrast Tl -weighted image (middle) shows enhancement within the superior portion of the mass. Axial T2- weighted image (right) shows heterogeneous signal pineal mass. (Click to magnify figure)

Embryonal carcinoma, endodermal sinus tumor (yolk sac tumor), and choriocarcinoma are rare, highly malignant pineal GCT with a poor prognosis. There is a male predominance with an age peak between 10 and 20 years. 47 These tumors are radiographically indistinguishable from the other pineal GCT. The more malignant tumors have a greater tendency toward necrosis, hemorrhage (especially choriocarcinoma), local invasion, and cerebrospinal fluid spread.

Endodermal sinus tumor secretes a-fetoprotein, choriocarcinoma secretes P-human chorionic gonadotropin, embryonal carcinoma and teratoma secrete both, teratomas also secrete carcinoembryonic antigen, and germinomas secrete placental alkaline phosphatase. 14,33,88 The prognosis is significantly worse for tumors that secrete both a-fetoprotein and human chorionic gonadotropin. 14 Elevation of tumor markers precedes radiographic tumor recurrence. 14

PCT are rare and consist of pineocytoma, pineoblastoma, and two intermediate or transitional forms. 65 The median age for pineoblastomas is 18 years and for pineocytomas is 36 years. 65 There is no sex predilection. 65 About 50% of PCT calcify, and 85% to 90% have hydrocephalus. 65 PCT are isodense to hyperdense on CT and strongly enhance. On MR imaging, PCT are hypointense to isointense on Tl-weighted images.

Pineoblastomas tend to be more isointense and pineocytomas tend to be more hyperintense on T2-weighted images. Pineocytomas are more well defined and homogeneous and do not show cerebrospinal fluid spread. Pineoblastomas are more heterogeneous because of hemorrhage, necrosis and cyst formation and show cerebrospinal fluid spread (10%). 65 The 5-year survival for pineocytomas is 67% and for pineoblastomas (including intermediate grades) is 58%. 65 There is the rare association of pineoblastoma with bilateral retinoblastoma (trilateral retinoblastoma), which, in most cases, is diagnosed before 2 years of age, is usually inherited, and is seen in about 3% of cases of bilateral retinoblastoma (25% to 40% of all retinoblastoma cases are bilateral). 58

HYPOTHALAMIC HAMARTOMA

Hypothalamic hamartomas (HH) are congenital, nonneoplastic heterotopias. 6 They contain ectopic rests of gray and white matter of varying degrees of differentiation that are usually attached to the tuber cinereum or mamillary bodies. 29,83 They usually present with isosexual precocious puberty (often before 2 years of age) and less commonly seizures, in particular, early onset of gelastic seizures. 29,46,71 In addition, patients are often mentally impaired. 46 Associated abnormalities include microgyria, heterotopia, callosal agenesis or dysgenesis, polydactyly, facial anomalies, heart defects, and Laurence-Moon-Biedl syndrome. 29,83 The association of immature hypothalamic hamartoma (hamartoblastoma) with extracranial anomalies is known as Pallister-Hall syndrome. 83 HH are also associated with diabetes insipidus, obesity, and growth hormone-secreting pituitary adenomas. 29 There is no sex predilection. 6,29,46,53,71,83 HH may be sessile to pedunculated and extend downward into the interpeduncular cistern or upward into the floor of the third ventricle. The sessile HH are more commonly associated with gelastic seizures, and the pedunculated HH are more commonly associated with precocious puberty. 6,29 Of 50 histologically proven HH, 74% were associated with precocious puberty, and 48% were associated with gelastic seizures, generalized seizures, and behavioral disturbances. 29 The pathogenesis of precocious puberty is unclear. It has been speculated that local pressure, abnormal neural connections, abnormal endocrine activity, or a combination of these may play important roles. 29 The proven efficacy of gonadotropin-releasing hormone analogues in suppressing puberty and retarding skeletal maturation favors medical management over surgery for the initial treatment of precocious puberty. 71 Possible causes for the seizures include an interconnection of the hamartomatous neurons to the limbic system or association of the seizures with the midline abnormalities or hemispheric malformations. 6 Epilepsy in some patients with HH may be difficult to control, and surgery may become necessary. Overall, surgery has a beneficial effect on seizures in approximately 30% of patients. 29

On MR imaging, HH presents as a well-circumscribed nonenhancing mass that compared with gray matter is isointense on Tl- weighted images and isointense to slightly hyperintense on T2-weighted images. The signal characteristics are usually homogeneous but may be heterogeneous and occasionally have a peripheral rim of Tl or T2 isointensity with a central focus of Tl hypointensity and T2 hyperintensity. 83 On CT, the lesion is usually well circumscribed, is isodense to brain, and does not enhance. The mass is usually homogeneous but uncommonly may have large cystic components that extend into the temporal fossa on either side and appear identical to arachnoid cysts. 53 Calcification has not been reported. The main differential diagnosis is hypothalamic glioma. The lack of enhancement, similar signal intensity or density to gray matter, and stability over time help differentiate HH from other tumors.

Figure 19. A, Hypothalamic neuronal hamartoma (frontal section). Whitish mass in the hypothalamic region compressing the inferior portion of the third ventricle. B, Hypothalamic neuronal hamartoma (MT x100). Coexistence of well-differentiated nerve cells and glial cells, giving the appearance of normal neural parenchyma (Click to magnify figure)

Figure 20. Hypothalamic hamartoma. Sagittal T1 -weighted image (left) and Coronal T2-weighted image (right) show hypothalamic mass that is isointense to gray matter on both sequences (arrow). (Click to magnify figure)

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