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1. Pediatric Intracranial Tumors: Posterior Fossa MODERATOR:DR.RAMAKANTH M.D PRESENTOR:DR.A.PRANAVI

2. Posterior Fossa Tumors

3. Pilocytic Astrocytoma Cerebellar astrocytomas account for 30% of all posterior fossa tumors in children, with the most common histologic subtype being JPA. The majority of JPAs, 60%, arise from the cerebellum. patients with neurofibromatosis type 1 (NF1) will develop a cerebellar JPA, although the most common location for pilocytic astrocytoma in NF1 patients is the optic nerve or optic chiasm.

4. The classic imaging appearance of a JPA, is of a   large cyst with a solid mural nodule within one of the cerebellar hemispheres. large cystic component with a brightly enhancing mural nodule on ct with enhancing cyst wall. On MRI, the cystic portion is hypointense relative to gray matter on T1-weighted images and hyperintense relative to gray matter on T2-weighted images. low-grade  neoplasm (GRADE-1)

5. Enhancement patterns may vary, but JPA most commonly (46%) appears as a cyst with an enhancing wall and an intensely enhancing mural nodule . Diffusion-weighted imaging (DWI ) shows no restricted diffusion,. MR spectroscopy (MRS) performed on the solid portion of pilocytic astrocytomas shows elevated choline-to– N -acetyl aspartate (NAA) ratios and elevated lactate levels, which is an aggressive metabolite pattern. 

6. The elevated lactate levels in JPAs do not reflect necrosis, which is rare in pilocytic astrocytomas and, rather, reflect aberrant glucose utilization . DTI has been shown to be a useful adjunct in differentiating thalamopeduncular pilocytic astrocytomas  from infiltrating tumors in the posterior fossa because pilocytic astrocytomas displace corticospinal tracts, whereas other tumors may encase them or disrupt them. JPAs may mimic hemangioblastomas.

7. Medulloblastoma Medulloblastoma accounts for 35–40% of all posterior fossa tumors in children with peak occurrence at approximately 4 years old. Classic medulloblastoma typically arises from the roof of the fourth ventricle and is midline in location in 75–90% of cases. Desmoplastic medulloblastoma is a rare histologic variant that typically occurs off midline in the cerebellar hemisphere.

8. Classic medulloblastoma is a highly cellular , densely packed tumor , it appears hyperdense relative to brain on CT (89% of cases) and shows restricted diffusion on DWI.   This feature of medulloblastoma allows differentiation from JPA, ependymoma, and brainstem glioma. Medulloblastoma is a high-grade tumor with increased   rCBV on perfusion MRI.

9. T2-weighted imaging shows heterogeneous signal: The solid components appear hypointense relative to gray matter the cystic components, which are seen in 59% of cases, appear hyperintense. Calcifications can be found in up to 20% of cases and hemorrhage is rare. Enhancement may be variable

10. Medulloblastomas generally have the characteristic spectrographic signature for a neuroectodermal tumor with  high taurine , depleted NAA, and prominent choline and lipid peaks. Because treatment of patients with medulloblastoma involves craniospinal radiation, DTI and DWI are potentially useful in early detection and monitoring of radiation-induced white matter injury through the measure of fractional anisotropy (FA) and ADC values.

11. medulloblastoma are reported to have seeding of the subarachnoid  space; therefore, at the time of diagnosis, an MRI examination of the entire spine should be performed to determine if there is leptomeningeal dissemination.

12. Atypical Teratoid-Rhabdoid Tumor ATRT constitutes 1–2% of pediatric brain tumors and has a predilection for infants; it most commonly occurs in children younger than 3 years old. Within the CNS, ATRT most commonly occurs infratentorial and off midline ATRT mimics medulloblastoma.

13. ATRTs can now be differentiated from medulloblastomas using specific immunohistochemical markers and by detecting certain gene mutations or deletions, such as the lack of  INI1  expression on immunohistochemical stains. Conventional MRI shows heterogeneous signal intensity on T1-  and because the mass commonly contains cysts, hemorrhage, and calcifications. Eccentrically located cysts may favor the diagnosis of ATRT over primitive neuroectodermal tumor and medulloblastoma.

14. A highly aggressive appearance of a tumor with skull invasion may favor ATRT over other cystic masses such as JPA or desmoplastic infantile ganglioglioma. The enhancement pattern of ATRTs is most commonly heterogeneous and is rarely homogeneous, reflecting the complex histopathology of this tumor. Restricted diffusion is typical. MRS shows an aggressive metabolite pattern with elevated choline, decreased or absent NAA, and prominent lipid and lactate peaks.

15. Ependymoma Ependymoma is the third most common posterior fossa tumor in children. Incidence peaks in patients 0–4 years old.  Approximately 70% of intracranial ependymomas are infratentorial and arise from ependymal cells lining the floor of the fourth ventricle and foramen of Luschka . Neurofibromatosis type 2 (NF2) is the only known genetic disorder associated with a predisposition for ependymomas.

16. These features are reflected on conventional MRI as high signal intensity relative to uninvolved gray matter on T2-weighted and FLAIR pulse sequences. Areas of low signal intensity relative to gray matter on T2-weighted images and FLAIR images may represent calcifications or hemorrhage.

17. Sagittal images may be the key to the diagnosis used to identify the point of origin as the floor of the fourth ventricle, as seen in ependymoma, versus the roof, as seen in medulloblastoma. Calcification is a common feature seen in 50% of ependymoma cases and contrast enhancement is heterogeneous. Although not pathognomonic, the plastic nature of ependymoma results in the classic presentation of a fourth ventricle mass extending through the foramen of Luschka (15%) or foramen of Magendie (60%).

18. Some ependymomas may show restricted diffusion.  In general, perfusion MRI of ependymomas shows markedly elevated cerebral blood volume (CBV ) MRS  generally shows depleted NAA and elevated choline and lactate levels, but the primary application of MRS in the setting of ependymoma is to evaluate for tumor recurrence versus posttreatment change.

19. Brainstem Glioma Brainstem gliomas comprise approximately 10–20% of all intracranial tumors in children and 75% of brainstem gliomas occur in patients younger than 10 years. Brain stem gliomas are not designated as a specific pathologic category in the WHO classification of CNS tumors and are classified by location rather than histology. They are classified broadly as diffuse intrinsic gliomas or as nondiffuse brainstem tumors.

20. The diffuse intrinsic tumor type is the most common NF1 patients with brainstem gliomas have a more favorable prognosis than non-NF1 patients. On MRI, diffuse pontine gliomas characteristically expand the pons and are usually hypointense relative to gray matter on T1-weighted images and hyperintense relative to gray matter on T2-weighted and FLAIR images. Most diffuse brainstem gliomas do not enhance; however if they do enhance,enhancement is very little and heterogeneous.

21. Most diffuse gliomas do not show restricted  diffusion Most diffuse brainstem gliomas are histologically low grade, but a subset rapidly evolves into highgrade neoplasms; on advanced imaging techniques, high-grade neoplasms are suggested by focal areas of restricted diffusion and increased  rCBV .  These findings likely correspond to areas of anaplasia.

22. Malignant degeneration is suggested by increased lipids and reduced NAA-to-choline, creatine-to-choline, and myoinositol-to-choline ratios.

23. Diffuse intrinsic brainstem gliomas had higher mean concentrations of citrate than ependymomas, medulloblastomas, and JPAs. With respect to brainstem gliomas, DTI plays an essential role in diagnosis and surgical planning. Demyelinating diseases may mimic diffuse intrinsic brainstem gliomas however , tractography clearly distinguishes between the two because brainstem gliomas deflect white matter tracts whereas demyelinating diseases result in truncated fibers.

24. Hemangioblastoma Hemangioblastomas account for 1–3% of all intracranial neoplasms, and most occur in  middle-aged adults. In children younger than 18 years old, these tumors are extremely rare, with an incidence of less than 1 per 1 million. One of the most common manifestations of von Hippel–Lindau (VHL) syndrome is multiple CNS hemangioblastomas, with the most common site of presentation being in the cerebellum.

25. Patients with cerebellar hemangioblastomas typically present with headache, vertigo, ataxia, and ninth cranial nerve palsy; in some cases, polycythemia has been noted given that up to 40% have been reported to secrete erythropoietin. Hemangioblastomas are highly vascular tumors and may present as a mural nodule within a large cyst cavity (45%) or a purely solid tumor (45%). Typical hemangioblastomas are hypo- to isointense relative to gray matter on T1 and hyperintense relative to gray matter on T2 with enhancement of the mural nodule.

26. The cyst wall most commonly does not enhance unless lined by neoplasm. Large feeding and draining vessels in the periphery and within the solid component appear as tubular flow voids on T2-weighted imaging. Hemangioblastomas mimic pilocytic astrocytomas and pleomorphic xanthoastrocytomas in their imaging appearance, but because of their intrinsic high vascularity, hemangioblastomas have the highest rCBV  thus, perfusion MRI may be a useful diagnostic adjunct.

27. Intracranial teratomas Intra-axial teratomas typically present either antenatally or in the newborn period, Extra-axial teratomas usually present in childhood. They usually have cystic and solid components, contributing to an irregular outline. Solid components demonstrate variable enhancement. demonstrate at least some fat and some calcification T1 hyperintense components due to fat and proteinaceous/lipid-rich fluid intermediate components of soft tissue T1 C+ (Gd): solid soft tissue components show enhancement T2: mixed signal from differing components

28. Brain stem glioma  Cerebellar tumors Restricted diffusion ;lower ADC values No restricted diffusion ; higher ADC values Age >2yrs medulloblastoma Age <2yrs ATRT Extends to outlets/4th ventricle EPENDYMOMA No extension; cystic component with  enhancing mural nodule  Flow voids + hemangioblastoma Posterior fossa lesions  Flow voids(-) JPA

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