Primary Cutaneous Neuroendocrine Cell Tumor (Merkel Cell Carcinoma) with Prominent Tubulo‐Glandular Features, Mimicking Eccrine Carcinoma

Abstract

Although cutaneous Merkel cell carcinoma (MCC) may show divergent differentiation including tubular/glandular, squamous, eccrine, and rhabdomyoblastic differentiation, a diffuse glandular pattern is exceedingly rare. Herein, we present a case of MCC with prominent tubulo‐glandular differentiation, which precluded a definitive diagnosis in the initial punch biopsy. The patient was a 43 year‐old male with 5–6 weeks history of a rapidly growing plaque at his back. A punch biopsy revealed an infiltrating lesion consisting of tubulo‐glandular structures lined by hyperchromatic basaloid cells, along with some angulated, basaloid tumor nests embedded in a desmoplastic stroma. Immunostains revealed that the tumor cells were strongly positive for chromogranin and BerEP4, variably positive for CK7 and CK5/6, but negative for CK20, synaptophysin, S‐100, EMA, GCDFP‐15, ER/PR, TTF1 and CEA. The reexcision specimen showed multifocal epidermotropism and typical finely stippled nuclear chromatin. Although the lack of CK20 staining is unusual, the histologic characteristics along with the remaining immunohistochemical studies favor the diagnosis of a primary cutaneous neuroendocrine cell carcinoma (MCC) over the variants of microcystic adnexal carcinoma or basal cell carcinoma with neuroendocrine differentiation. Our case illustrates that prominent glandular differentiation may occur in MCC, and sometime create difficulty in definitive diagnosis in small biopsy specimen.