Merkel cell carcinoma of the anorectum: a case report and review of the literature.

Merkel cell carcinoma is a highly aggressive neuroendocrine carcinoma that most commonly arises in sun-exposed skin. Established risk factors include advanced age, fair skin, chronic ultraviolet light exposure and immunosuppression. Merkel cell polyomavirus infection is implicated in a subset of cases. Primary extracutaneous Merkel cell carcinoma is exceedingly rare. We report a case of Merkel cell carcinoma of the anorectum in a 44-year-old woman of Indian descent with no apparent risk factors. She presented with rectal bleeding and worsening perineal discomfort. A polypoid tumor was identified in the anal canal. No cutaneous lesions were detected. Histopathology revealed a small round blue cell tumor with neuroendocrine features. Immunohistochemistry for CK20, synaptophysin, CD56, neurofilament and SATB2 were positive while CK7, CDX2 and TTF1 were negative. Polymerase chain reaction and immunohistochemistry were negative for Merkel cell polyomavirus. Merkel cell carcinoma should be considered in the differential diagnosis of a high grade neuroendocrine carcinoma at an unusual site such as the anorectum in this case. Correct diagnosis of Merkel cell carcinoma at an extracutaneous site is crucial because it should prompt careful evaluation for a skin primary and it may make the patient eligible for immune checkpoint inhibitor therapy.