Merkel cell carcinoma in the setting of hematologic disease is associated with unique features and potential pitfalls.

Merkel cell carcinoma (MCC) is a rare but aggressive neuroendocrine carcinoma of the skin, often associated with polyomavirus and ultra-violet light exposure. Immunosuppression is associated with increased risk of development of MCC, including that associated with hematolymphoid disorders such as chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). We sought to determine whether MCC arising in patients with hematologic disorders showed unique features. Searching archived material at three institutions, we identified 13 patients with MCC and at least one hematologic malignancy and 41 patients with MCC with no reported hematologic malignancy. CLL/SLL was the most common hematologic disorder in this setting (9/13 cases). Clinical history, variation in morphologic appearance, unusual site distribution and concern for progression of underlying hematologic disease all contributed to potential diagnostic challenges. Overlapping marker expression between MCC and hematologic neoplasms created potential diagnostic pitfalls (e.g. CD138, Pax5, TdT, Bcl2, CD56, and CD117). In addition, we newly identify expression of CD5 and LEF-1 in a subset of MCC, including in patients with CLL/SLL. MCC in patients with hematologic malignancy were more common in men (92% versus 59%, p < 0.05) and showed an unusual site predilection to non-sun exposed sites (3/13 on the buttocks) with none presenting on the face or scalp. By contrast, face or scalp lesions were common in MCC without an associated hematologic malignancy (17/41, p < 0.05). Our findings reaffirm the need for skin surveillance in the setting of immune deficiency and for vigilance to identify unusual presentations of MCC in patients with or without hematologic disorders.