Posterior Fossa Neoplasm, Pediatric

Susan I. Blaser, MD, FRCPC

DIFFERENTIAL DIAGNOSIS

Common

Pilocytic Astrocytoma
Medulloblastoma (PNET-MB)
Ependymoma
Brainstem Glioma, Pediatric

Less Common

Ganglioglioma
Schwannoma
Meningioma, CPA-IAC
Hemangioblastoma
Choroid Plexus Papilloma

Rare but Important

Anaplastic Astrocytoma
Atypical Teratoid-Rhabdoid Tumor
Choroid Plexus Carcinoma
Medulloblastoma Variants
Medulloepithelioma
Dysplastic Cerebellar Gangliocytoma

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Most common pediatric posterior fossa (PF) tumors
- Medulloblastoma (PNET-MB)
- Astrocytomas
  - Pilocytic astrocytoma (PA)
  - Infiltrating “glioma” (astrocytoma, WHO grade II)
- Ependymoma
Imaging
- Findings on conventional MR overlap
- Location helpful in differential diagnosis
  - Tectum, cerebellum: PA
  - Pons: Diffusely infiltrating astrocytomas
  - Midline (vermis, fourth ventricle): PNET-MB, PA
  - Fourth ventricle + lateral recess/CPA mass: Ependymoma
- DWI, MRS (normalized to water)
  - Can discriminate between pediatric PF tumors
  - PNET-MB, atypical teratoid-rhabdoid tumor (ATRT) show DWI restriction
- Examine entire neuraxis in child with PF tumor prior to surgery!
  - T1 C+ essential (look for CSF spread)
History, PE (e.g., cutaneous markers) important

Helpful Clues for Common Diagnoses

Pilocytic Astrocytoma
- Child with cystic cerebellar mass + mural nodule
- Solid component low density NECT, high signal T2
Medulloblastoma (PNET-MB)
- Early childhood: Solid vermis mass extends into, fills, &/or obstructs 4th ventricle
- Later onset: Lateral cerebellar mass
- Hypercellular: ↑ Density on NECT, ↓ T2
- DWI: Restricts
- 2-5% have nevoid basal cell carcinoma (Gorlin) syndrome (BCCS)
  - Typically seen with desmoplastic variant
  - Look for jaw cysts, bifid ribs, etc.
  - XRT can lead to induced basal cell carcinomas, other intracranial neoplasms within irradiated field
Ependymoma
- Extrudes through 4th V outlet foramina into cisterns
- Coarse calcifications
- Diffusion restriction uncommon, may predict anaplastic behavior
Brainstem Glioma, Pediatric
- Tectal plate glioma
  - NECT: Increased density progresses to Ca++
  - CECT/MR: Faint or no enhancement
- Pontine glioma
  - Enlarged pons engulfs basilar artery
  - Enhances late in course, rarely at diagnosis
- Dorsal exophytic glioma
  - Tumor protrudes into 4th ventricle
  - If large, may be difficult to differentiate from PA
  - Look for FLAIR signal change in dorsal brainstem or peduncles

Helpful Clues for Less Common Diagnoses

Ganglioglioma
- Brainstem most common PF site
- Look for expansion of nucleus cuneatus/gracilis
Schwannoma
- Vestibular schwannoma (ICA/CPA) looks like “ice cream on cone”
- T2 hyperintensity helps differentiate from meningioma
- Multiple in NF2
Meningioma, CPA-IAC
- Broad dural base, covers IAC
- Variable signal, but T2 hypointensity common
- Hyperostosis, tumoral calcifications
- May have intra- or juxtatumoral cyst(s)
Hemangioblastoma
- Late teen or adult
- Intra-axial (cerebellum > medulla, cord)
  - Cyst + nodule > solid
  - Solid component shows flow voids, enhances avidly
  - Multiple lesions diagnostic of von Hippel-Lindau (VHL)
- Avidly enhancing mural nodule abuts pia
- Look for visceral markers of VHL in any child/young adult with hemangioblastoma
Choroid Plexus Papilloma
- Frond-like 4th V or CPA tumor
- Avidly enhancing
- Hydrocephalus common

Helpful Clues for Rare Diagnoses

Anaplastic Astrocytoma
- Infiltrating mass involves predominantly white matter
- Enhancement none to sparse or patchy enhancement
- Ring enhancement suggests progression to GBM
Atypical Teratoid-Rhabdoid Tumor
- Imaging similar to PNET-MB plus
  - ATRT patients generally younger
  - Cysts, hemorrhages more common
  - CPA involvement more common
  - Frequent metastases at diagnosis
- Both ATRT, PNET-MB show diffusion restriction
Choroid Plexus Carcinoma
- Similar to CPP plus
  - Cysts, necrosis, bleeds
  - CSF/ependymal/parenchymal spread
Medulloblastoma Variants
- Desmoplastic medulloblastoma (MB)
  - 5-25% of all medulloblastomas
  - 55-60% of PNET-MBs in children < 3 y
  - PNET-MB in older children, young adults often also desmoplastic variant
  - Desmoplastic subtype of MB in children < 2 is major diagnostic criterion for basal cell nevus syndrome (Gorlin syndrome)
  - Nodular collections of neurocytic cells bounded by desmoplastic zones
  - Lateral (cerebellar) location
- MB with extensive nodularity (MBEN)
  - Formerly called “cerebellar neuroblastoma”
  - Usually occurs in infants
  - Gyriform or “grape-like” appearance
  - May mature → better prognosis
Medulloepithelioma
- Rare embryonal brain &/or ocular tumor
- Inhomogeneous signal, enhancement
Dysplastic Cerebellar Gangliocytoma
- Diffuse or focal hemispheric mass
- Thick cerebellar folia with “striated” appearance
- Evaluate for Cowden syndrome

Image Gallery

Sagittal T1 C+ MR shows a typical tumor cyst with enhancing mural nodule

. There is hydrocephalus and protrusion of the cerebellar tonsils

through the foramen magnum (acquired Chiari 1).

Axial T2WI MR shows increased signal of the solid component

of the mass. Interstitial edema

is present in the temporal lobes.

(Left) Sagittal T2WI MR shows a hyperintense mass

filling and expanding the 4th ventricle. The tumor does not extend through the 4th ventricular outlet foramina. There is hydrocephalus with acquired tonsillar herniation