[Intraepidermal Merkel cell carcinoma with INSM1 expression. A case report]

We report the case of a 90-year-old male who presented with an erythematous desquamative plaque on his left cheek. Histopathology demonstrated an epidermal lesion with multifocal epithelial growth. The cells were small, with scant cytoplasm and hyperchromatic nuclei with molding and a high mitotic and apopototic rate. Immunohistochemistry showed positivity for CK20, CK7, synaptophysin and INMS1. These findings are consistent with a Merkel cell carcinoma in situ. This tumor corresponds to a primary neuroendocrine neoplasm of the skin, which usually affects elderly people with sun-exposed skin. Usually, it presents as a dermal tumor but intraepidermal involvement alone is extremely rare. In this scenario, a broad differential diagnosis should be considered, excluding all neoplasms that may present intraepidermal forms. The evolution of this entity is unknown.