Basal cell carcinoma showing connections with epidermal cysts.

Basal cell carcinomas arising from epidermal cysts are rare. A 76-year-old Japanese man had had a blackish nodule on his right knee for 15 years, under which he later noticed the development of a subcutaneous nodule. On histological examination masses of tumour cells showed the feature of adenoid and solid patterns of basal cell carcinoma that were connected to the wall of epidermal cysts in many places as well as with the overlying epidermis.