Merkel cell carcinoma and melanoma: etiological similarities and differences.

Merkel cell carcinoma (MCC) of the skin and cutaneous malignant melanoma can now be compared epidemiologically through the use of population-based data not previously available for MCC. The results may provide new clues to etiology. In this study, United States data covered by the Surveillance, Epidemiology, and End Results (SEER) Program were from nine areas of the United States (approximately 10% of the population). In 1986-1994, 425 cases of MCC were registered. The annual age-adjusted incidence per 100,000 of MCC was 0.23 for whites and 0.01 for blacks; among whites, the ratio of melanoma to MCC was approximately 65 to 1. Only 5% of MCC occurred before age 50, unlike the lifelong risk of nodular and superficial spreading melanoma. Regional incidence rates of both cancers increased similarly with increasing sun exposure as measured by the UVB solar index. The most sun-exposed anatomical site, the face, was the location of 36% of MCC but only 14% of melanoma. Both cancers increased in frequency and aggressiveness after immunosuppression and organ transplantation (36 cases from the Cincinnati Transplant Tumor registry and 12 from published case reports) and after B-cell neoplasia (5 cases in this study; 13 from case series in the literature). The SEER data contained reports of six patients with both types of cancer; 5 melanomas before the diagnosis of MCC and 1 after diagnosis. MCC and melanoma are similarly related to sun exposure and immunosuppression, but they differ markedly from one another in their distributions by age, race, and anatomical site, especially the face.