INNV-22. CEREBELLOPONTINE EPIDERMOID CYST TO SQUAMOUS CELL CARCINOMA: MALIGNANT TRANSFORMATION, LEPTOMENINGEAL CARCINOMATOSIS, AND MANAGEMENT

Abstract BACKGROUND Intracranial epidermoid cysts (EDC) are rare, slow-growing, congenital tumors, that generally follow a benign course. EDCs have a rare potential for transformation to squamous cell carcinoma (SCC). We present a case of a malignant EDC transformation to SCC with leptomeningeal (LM) dissemination. METHODS Case report and review of literature. CASE PRESENTATION A 44-year-old female presented for evaluation of left-sided tinnitus and sensorineural hearing loss, which revealed a left cerebellopontine angle (CPA) tumor. She underwent a subtotal resection at age 47. One year later, she had rapid symptomatic recurrence of the cyst, as well as LM deposits of the right internal acoustic canal bundle, the left trigeminal nerve, and along the lumbar and sacral spine. She underwent repeat resection of the CPA lesion, with pathology now consistent with SCC, with CDKN2A loss. She received focal radiation therapy at the posterior fossa and the lumbar spine. SCC-directed chemotherapy with pembrolizumab and carboplatin followed by 5 cycles of pembrolizumab monotherapy was given as adjuvant therapy, prior to her developing symptomatic LM disease. CONCLUSION Malignant transformation of intracranial EDC is a rare occurrence in these typically benign tumours. Prognosis is especially poor when there is associated LM involvement, with median survival noted to be 10 months. Awareness of this phenomenon amongst clinicians is needed to ensure appropriate surveillance and aggressive treatment if it remains within patient wishes, though there is no consensus on recommendations for treatment. The patient presented has maintained disease stability with LM dissemination for over 10 months and ongoing, with a regimen of chemoradiotherapy informed by therapeutic approach to systemic SCC. To our knowledge, this is the first reported case of EDC to SCC transformation and LM dissemination stabilized with chemoradiotherapy using an immune-checkpoint inhibitor.