Answer: Merkel cell carcinoma

Histology: The lymph node is replaced by a small blue cell neoplasm. The cells have pleomorphic hyperchromatic nuclei, glassy nuclei, and a high mitotic rate. Immunohistochemical labeling revealed the expression of cytokeratin in a dot-like pattern.

Discussion: Merkel cell carcinoma is an aggressive form of skin cancer. The neoplasm has the histologic appearance of a small blue cell tumor. The cells are round-to-oval, the nuclei are hyperchromatic, and the cells have scant cytoplasm. The nuclei have evenly dispersed chromatin, inconspicuous nucleoli and a high mitotic rate. Merkel cell carcinomas often arises in the elderly and in the immunosuppressed. The latter finding has suggested to some that Merkel cell carcinoma may have an infectious origin. Recently Feng and colleagues sequenced the expressed genes in a series of Merkel cell carcinomas using a technique called digital transcriptome subtraction. Remarkably, Feng and colleagues found a fusion transcript between a previously undescribed virus T antigen and a human receptor tyrosine phosphatase. The viral sequences were from a previously unknown polyomavirus that Feng and colleagues call Merkel cell polyomavirus (MCV or MCPyV). Merkel cell polyomavirus sequences were detected in 8 of 10 (80%) Merkel cell carcinomas that they studied. This remarkable study suggests that a virus may contribute to the pathogenesis of Merkel cell carcinoma.

Reference(s):
– Feng et al, Science, 2008 Feb 22;319(5866):1096-100; and Bichakjian et al. Cancer, 2007 Jul 1;110(1):1-12