Spatial density of primary malignant melanoma in sun-shielded body sites: A potential guide to melanoma genesis

UV radiation is a major factor in melanoma genesis, but non-UV linked factors are also operational, since primary malignant melanomas can emerge in body sites that never see the sun. The scarcity of melanomas in sun-shielded body sites reflects only the absolute number of melanomas, not the number of tumours per square unit of the surface in which they emerge. Studies on melanoma density conducted by us and others are here briefly reviewed. The access to reliable numbers along with measurable anatomical areas directed our choice of melanomas at the sun-shielded locations described here. Melanomas at the body surface. Calculations of surface areas bearing melanomas relative to the total body surface included sites on the vulva, subungual tissues, volar and palmar skin, and, for comparison melanomas of the face during the same period of time. The density of vulvar melanomas was identical to that in chronically sun-exposed facial skin. Subungual melanomas were almost nine times denser than expected whereas melanomas of palms and soles showed a lower density than expected. Melanomas beneath the body surface. The densities of melanomas in the vagina, anal canal and uvea, were calculated separately and compared to the average density of cutaneous melanomas (CMMs) during the same period of time. Melanomas of the anal canal displayed a density almost twice the average for CMMs, whereas the vaginal melanomas were similar in density to CMMs. In contrast, the density of the uveal melanomas was calculated as 50 and 41 times (men and women, respectively) the average density of CMMs.

The high density of some melanomas in sun-shielded body areas indicates the presence of factors underlying the origins of these tumours that seem to be equivalent in strength to UV radiation and also implies that specific anatomical sites favour the emergence and proliferation of melanomas, independent of UV radiation.