Abstract
BACKGROUND: Malignant transformation of epidermoid cysts (ECs) into squamous cell carcinoma (SCC) is extremely rare, with associated leptomeningeal carcinomatosis being much less likely. Malignant transformation of ECs has a poor prognosis with a median overall survival of 10 months following transformation. OBSERVATIONS: A 59-year-old male with a history of a subtotally resected posterior fossa EC presented with worsening symptoms 14 years following the initial subtotal resection. Imaging showed obstructive hydrocephalus and extensive leptomeningeal carcinomatosis. Laboratory workup confirmed malignant transformation to SCC. LESSONS: Nonmalignant recurrence of benign ECs is relatively common, often due to the complexity of complete resection and adherence to adjacent neurovascular structures. On the other hand, malignant degeneration of an EC is extremely rare. Any rapid change in symptoms, failure to recover following surgery, and new areas of irregular thickening and enhancement on imaging should prompt further evaluation for this process. https://thejns.org/doi/10.3171/CASE24738.