Merkel Cell Carcinoma in Situ Arising in Association with Squamous Cell Carcinoma in Situ

Summary: An 85-year-old Caucasian man with a history of multiple skin cancers presented with a lesion on the left nasal ala. Biopsy revealed a combined squamous cell carcinoma in situ (SCCis) and Merkel cell carcinoma in situ (MCCis). MCCis is a rare intraepidermal form of Merkel cell carcinoma (MCC), distinguished by its confinement to the epidermis and a significantly better prognosis compared to invasive MCC. To date, only 31 documented cases of MCCis have been reported in the English literature, commonly on the head and neck of older men, with many cases demonstrating coexisting keratinocyte dysplasia. Immunohistochemical staining of the MCCis demonstrated positivity for CK7 in hyperchromatic cells, along with focal positivity for chromogranin and CK20. INSM1 staining revealed rare, weakly positive cells. The MCCis was negative for SV40/polyoma, P63, and synaptophysin. An outside report demonstrated negative staining for SOX10, adipophilin, and androgen receptor. The P63 highlights full-thickness atypical keratinocytes. The patient underwent Mohs surgery, and a sentinel lymph node biopsy was negative for carcinoma. This case represents an exceedingly rare presentation of predominately CK7 positive MCCis coexisting with SCCis and contributes to the body of knowledge of the development of Merkel cell polyoma virus negative MCC.