pilocytic astrocytoma

**Epidemiology/Location:** Most common primary CNS tumor in children (peak 5-15 years). Classically in the posterior fossa (cerebellar hemispheres), but also optic pathway (optic glioma), hypothalamus, and brainstem. Cerebellar lesions can compress the 4th ventricle and cause obstructive hydrocephalus (headache, vomiting, ataxia, papilledema).

**Pathology:** - WHO grade 1, GFAP-positive (astrocytic origin). - Biphasic pattern: dense areas of bipolar "hairlike" (piloid) cells with Rosenthal fibers (eosinophilic, corkscrew-shaped cytoplasmic inclusions) alternating with loose, microcystic areas with eosinophilic granular bodies. - Gross: well-circumscribed, often a cyst with a mural nodule (the nodule is the tumor, so it must be resected). - Low mitotic activity. Microvascular proliferation and even some pleomorphism do not imply a higher grade here.

**Genetics/Associations:** - MAPK pathway activation, most often from a KIAA1593-BRAF fusion (the most common alteration in sporadic cerebellar tumors). BRAF V600E is less common. - Neurofibromatosis type 1 (NF1): optic pathway gliomas are typically pilocytic astrocytomas. Look for café-au-lait spots, Lisch nodules, and neurofibromas. - IDH1 and TP53 mutations are typically absent. This helps distinguish it from diffuse astrocytomas.

**Imaging:** Cystic mass with an enhancing mural nodule. Solid variants also occur.

**Treatment/Prognosis:** Gross total surgical resection is often curative. Prognosis is excellent (10-year survival above 90%). Malignant transformation is rare. Residual or unresectable tumors, such as hypothalamic or optic pathway lesions, may need chemotherapy or targeted MAPK inhibitors.

**High-yield comparisons:** - Medulloblastoma: a malignant posterior fossa tumor in children, with small blue cells and Homer-Wright rosettes. It arises in the midline vermis. - Hemangioblastoma: a cerebellar tumor in adults with von Hippel-Lindau disease. It has a cyst with a mural nodule and can cause polycythemia (EPO). - Ependymoma: arises from the 4th ventricle, with perivascular pseudorosettes and blepharoplasts. - Glioblastoma: a grade 4 adult supratentorial tumor with "butterfly" spread, pseudopalisading necrosis, and GFAP positivity.

**Pearl:** A child with a cerebellar cyst plus mural nodule, and Rosenthal fibers on histology, has a pilocytic astrocytoma.