Merkel Cell Carcinoma Concurrent with Bowen's Disease.

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous malignancy of the elderly and immunocompromised patients. It is occasionally found coexisting with other diseases, such as squamous cell carcinoma, basal cell carcinoma, actinic keratosis, miscellaneous adnexal tumors, and rarely Bowen disease. A 75-year-old woman presented with a 6-month history of an irregularly shaped erythematous patch on the left mandibular angle. Three months later, a 1.5×1.0 cm sized painless and rapidly growing erythematous nodule developed on the patch. Microscopically, the patch lesion was consistent with that of Bowen disease. The nodular lesion showed a number of small uniform hyperchromatic cells with scanty cytoplasm. It showed dense small-cell like nodular infiltration in the dermis. Immunohistochemical staining for cytokeratin 20 showed a positive result with a dot-like perinuclear pattern. Additionally, the result for thyroid transcription factor-1 was negative, which is positive in small cell neuroendocrine carcinoma. From these findings, we diagnosed this lesion as MCC concurrent with Bowen disease.