Invasive squamous cell carcinoma arising from asymptomatic choristomatous cysts of the orbit. Two cases and a review of the literature.

BACKGROUND: Epithelial choristomatous cysts are common orbital lesions, the most frequent of which are dermoid or epidermoid tumors. Massive enlargement or extraorbital extension of these benign lesions may occur. Malignant transformation of the epithelial lining of epidermoid cysts is rarely reported.
METHODS: Two patients are presented in whom the epithelial lining of a previously asymptomatic choristomatous cyst of the orbit underwent malignant transformation to produce invasive squamous cell carcinoma. The unusual origin and clinical presentation of the lesions caused a delay in the diagnosis and therapy in both patients. Metastatic workup was negative in both patients, who underwent resection of their tumors via orbital exenteration with craniofacial resection. Reported cases of malignant transformation in analogous choristomatous cysts elsewhere in the cranium are reviewed.
RESULTS: One patient is alive and well without recurrent disease 40 months postoperatively. The second patient died of a pulmonary embolus 2 months postoperatively. Autopsy showed no residual tumor. Overall, only 3 of 18 reported patients with epidermoid choristoma of the head and orbit with malignant transformation were alive when reported.
CONCLUSIONS: Malignant squamous metaplasia is believed to be a rare complication of orbital dermoid or epidermoid cysts, with only two previously reported cases. However, malignant transformation is relatively frequent in analogous epidermoid cysts found elsewhere in the cranial vault, especially after incomplete excision. These reports encourage the complete removal of epithelial choristomas of the orbit.