Basal cell carcinoma arising from epidermoid cyst: a case report. 
Cutaneous epidermoid cysts are very common lesions, but neoplastic transformation of their epithelium is extremely rare.  We describe a patient with basal cell carcinoma arising in the wall of an epidermoid cyst present on the back for 20 years.  We emphasize the advisability of a close histologic examination of all cutaneous cysts that are seemingly benign in order to avoid incorrect diagnosis.