PATH-02. Angiomyxofibromatous tumor of the falx cerebelli: a rare and unclassified intracranial lesion mimicking posterior fossa pathologies

Abstract Low-grade fibromyxoid tumors are extremely rare in children and, to our knowledge, have not been previously reported in the posterior fossa. These lesions can mimic more common posterior fossa pathologies, posing diagnostic challenges. We report a unique case of a low-grade angiomyxofibromatous tumor arising from the falx cerebelli in a 10-year-old female, presenting with features resembling cystic posterior fossa lesions and resulting in obstructive hydrocephalus. Histopathological examination revealed stellate tumor cells within a myxoid and edematous stroma, along with a plexiform vascular pattern. Immunohistochemistry showed diffuse positivity for vimentin and focal positivity for S-100 protein. The tumor was negative for EMA, CD34, MIC2, Bcl-2, GFAP, cytokeratin, CAM 5.2, desmin, and SMA. A literature review identified only five previously reported cases of similar tumors, none involving the posterior fossa. Differential diagnoses considered included fourth ventricular arachnoid cyst, meningioma, neurocysticercosis, pilocytic astrocytoma, myxoma, solitary fibrous tumor, and angiomyxoma. Gross total resection was achieved, and postoperative follow-up revealed no recurrence. Reported cases, including ours, have shown no recurrence with follow-ups extending up to six years. This tumor is not currently classified by WHO or other CNS/soft tissue tumor classifications, underscoring the need for further study into the classification and pathogenesis of myxoid CNS tumors. This case highlights the importance of considering angiomyxofibromatous tumors in the differential diagnosis of pediatric posterior fossa lesions. Awareness of this entity and its favorable prognosis may aid in appropriate surgical management and improved outcomes.