Abstract: ZHANG Gao-lei *, WANG Yang, GU Xiao-guang, ZHANG Jun-ge, ZHANG Zhen, TU Ping. *Department of Dermatology and Venereology, Peking University First Hospital, Beijing 100034, China Corresponding author: TU Ping, Email: tup0207@sina.com 【Abstract】 Objective To estimate the value of detection of Merkel cell carcinoma polyomavirus （MCPyV） in the diagnosis of Merkel cell carcinoma （MCC）. Methods Two cases of MCC were studied using light microscopy and immunohistochemistry. PCR was performed to detect DNA sequences encoding MCPyV large T antigen（LT）and viral protein 1 （VP1） in paraffin-embedded tissue specimens from the two patients with MCC, five patients with T cell lymphoma, two normal human controls, as well as in two T cell lymphoma cell lines MAC1 and MAC2. DNA sequencing was also carried out. Results Both of the patients with MCC were male. The patient 1 presented with a mass in the right anterior shin for more than one year, and the patient 2 had a mass in the left knee for more than six months. Skin examination revealed densely distributed pink nodules in the right anterior shin, with confluence into an indurated plaque which measured 10 cm × 8 cm with superficial erosion, exudates and crusts and was surrounded by multiple irregularly sized erythematous nodules with limited mobility in the patient 1, as well as a royal blue, hard, poorly marginated nodular mass measuring 5 cm × 4 cm in the left medial knee with limited mobility in the patient 2. Pathological manifestations were similar in the two patients. Tumor cells were uniform with large hyperchromatic nuclei, eosinophilic and sparse cytoplasm, and fine chromatin. Mitotic figures were easily seen. Immunohistochemistry revealed that the tumor cells stained positive for pan-cytokeratin, synuclein （Syn）, neuron-specific enolase （NSE）, chromogranin （CgA）, CK20, and Ki-67 （> or = 60%）, but negative for S100 protein, HMB45, CD34, thyroid transcription factor 1 （TTF-1）, CK7 and leukocyte common antigen （LCA）. MCPyV DNA was detected in both MCC specimens, but absent in the other skin specimens or T cell lymphoma cell lines. Conclusions MCC has distinctive clinical and pathological appearance. Immunohistochemistry and detection of MCPyV DNA sequences using PCR may be beneficial to the definitive diagnosis of MCC. 【Key words】 Carcinoma, Merkel cell; Polyomavirus

Key words: Merkel cell carcinoma