Merkel cell carcinoma (MCC) was originally described by Toker in 1972 as trabecular carcinoma of the skin. Other names include Toker tumor, primary small cell carcinoma of the skin, primary cutaneous neuroendocrine tumor, and malignant trichodiscoma.
MCC is an aggressive neuroendocrine carcinoma arising in the dermoepidermal junction. (See Figure 1.) Although the exact origin and function of the Merkel cell remains under investigation, it is thought to have features of both epithelial and neuroendocrine origin and arise in cells with touch-sensitivity function (mechanoreceptors).[3,4,5,6,7,8,9]
Figure 1. Merkel Cell Anatomy.
In Surveillance, Epidemiology and End Results (SEER) Program data from 1986 to 2001, the age-adjusted U.S. annual incidence of MCC tripled from 0.15 to 0.44 per 100,000, an increase of 8.08% per year. Although this rate of increase is faster than any other skin cancer including melanoma, the absolute number of U.S. cases per year is small. About 1,500 new cases of MCC were expected in the United States in 2007.[10,11,12,13,14,15]
MCC incidence increases progressively with age. There are few cases in patients younger than 50 years, and the median age at diagnosis is about 65 years (see Figure 2). Incidence is considerably greater in whites than blacks and slightly greater in males than females.[10,11,12,13,15]
Figure 2. . Frequency of MCC by age and sex of men (square) and women (circle). Reprinted from Journal of the American Academy of Dermatology, 49 (5), Agelli M and Clegg L, Epidemiology of primary Merkel cell carcinoma in the United States, pp. 832–41, Copyright (2003), with permission from Elsevier.
The apparent increase in incidence may reflect an actual increase and/or more accurate diagnostic pathology tools, improved clinical awareness of MCC, an aging population, increased sun exposure in susceptible populations, and improved registry tools.
MCC occurs most frequently in sun-exposed areas of skin, particularly the head and neck, followed by the extremities, and then the trunk.[3,13,16] Incidence has been reported to be greater in geographic regions with higher levels of ultraviolet B sunlight.
A 2009 review of 3,804 MCC cases from the SEER Program database from 1973–2000 tabulated the ten most common sites of MCC (see Table 1).