Merkel cell carcinoma: A systematic review of the demographic and clinical characteristics of 847 cases in Japan.

Merkel cell carcinoma (MCC) is a rare but aggressive neuroendocrine carcinoma of the skin associated with Merkel cell polyomavirus and immunosuppression. Although MCC incidence is rising worldwide, MCC has not been sufficiently investigated in Japan. This study aimed to determine MCC demographics in Japan, including incidence, age, sex, location, spontaneous regression, and pure/combined MCC. Using PubMed and Igaku Chuo Zasshi, 847 MCC cases between 1985 and 2015 were extracted, and the main epidemiological characteristics were described. The mean age of all patients was 77.5 years. Regarding the characterized lesions, 63.0% were located on the head and neck, 5.2% on the trunk, 12.6% on the upper limb, 15.1% on the lower limb, 3.5% on the buttocks, and 0.6% on the genitals. Histopathological information regarding the presence of other malignancies could be retrieved in 611 cases, and a coexisting malignancy, mainly squamous cell carcinoma and Bowen's disease, was present in 14.2%. Subcutaneous MCC was observed in 31 patients with a male : female ratio of 1.07 (16 men/15 women). Nodal lesions with unknown primary tumor location were described in 19 patients with a male : female ratio of 0.9 (nine men/10 women) and a mean age of 77.7 years. Of 640 evaluable cases, spontaneous regression developed in 9.1%. Among those 58 patients, the male : female ratio was 1:2.1 in 56 evaluable cases (18 men/38 women). Merkel cell polyomavirus was assessed in 180 patients, and the virus was detected in 31.1% and not detected in 68.9% of the patients. MCC is a rare disease in Japan, with incidence rates and male : female ratios differing from those in the USA and European countries. Besides, this study reveals the high frequency of subcutaneous MCC and MCC with divergent differentiation patterns and spontaneous regression in Japan compared to other countries.