Posterior fossa tumors

Posterior fossa tumors have a very different differential in an adult as opposed to a child.

Adult

• cerebellar metastases (most common)

  • especially lung cancer and breast cancer

  • also melanoma, thyroid malignancies, and renal cell cancer

  • gastrointestinal stromal tumor (very rare) ⁵

• hemangioblastoma: most common posterior fossa primary brain tumor in adults

• astrocytomas, medulloblastomas, and ependymomas ⁶ are encountered in the posterior fossa of younger adults but are rare in older adults, accounting for <1% of all tumors

• lymphoma ⁴

• lipoma ⁴

• Lhermitte-Duclos disease​

• subependymoma: most frequently near the obex

• choroid plexus papilloma: most frequently near the obex

• rosette forming glioneuronal tumor (RGNT): frequently in the superior vermis

• schwannoma: most commonly of the vestibular nerve

An important space-occupying lesion (the most common in fact) to remember is that of a stroke, which when subacute can mimic a tumor. 

Child

• medulloblastoma: most common (30-40%) ⁷

  • hypercellular tumor (diffusion restriction with low ADC values ~550 x 10^-6 mm²/s, hyperdense on non-contrast CT)

  • they do not usually extend into the basal cisterns through the foramen of Luschka and foramina of Magendie

  • drop metastases and leptomeningeal spread are common at presentation

• posterior fossa astrocytoma

  • pilocytic astrocytoma: second most common (25-35%) ⁷

    • large cystic component

    • brightly enhancing mural nodule

• brainstem glioma (20-25%) ⁷

• posterior fossa ependymoma (10-15%) ⁷

  • usually arises from the floor of the 4^th ventricle

  • protrudes out of the foramen of Luschka and foramina of Magendie

  • does not usually cause significant diffusion restriction

• atypical teratoid/rhabdoid tumor (AT/RT) (1-2%) ⁷

  • young children - median age is less than 2-3 years ⁸

  • very agressive

• hemangioblastoma (uncommon except in patients with vHL)

• teratoma (in infants)

A quick and handy mnemonic for posterior fossa tumors in children is BEAM.

Although it is true that posterior fossa tumors are much more common in children than in adults the distribution does vary with age ²:

• 0 to 3 years of age: supratentorial > infratentorial

• 4 to 10 years of age: infratentorial > supratentorial

• 10 to early adulthood: infratentorial = supratentorial

• adults: supratentorial > infratentorial

Overall 50-55% of all brain tumors in children are found in the posterior fossa ³.