Where the sun does not shine: is sunshine protective against melanoma of the vulva?

Intermittent sun exposure and sunburn are strong predictors of cutaneous malignant melanoma (CMM). On the other hand, melanomas may arise also in non-sun-exposed areas such as the vulva. However, little is known about a possible relationship between sun exposure and vulvar melanoma. Temporal and latitudinal dependencies of the incidence rates of vulvar melanoma were studied in comparison with those of CMM among Caucasians in Sweden, East Germany, USA and Victoria (Australia). The ratios of vulvar melanoma incidence rates to those of CMM tend to decrease with increasing CMM rates. The incidence rates of CMM have increased with time until recently, while those of vulvar melanoma have either decreased or remain constant. In USA vulvar melanoma incidence rates seem to increase from south to north, while for CMM incidence rates on sun exposed skin areas decrease from south to north. Comparison of latitudinal trends of the incidence rates of vulvar melanomas and CMM show opposite trends. Whenever CMM rates increase, either with time or with decreasing latitude (indicating increased sun exposure) the ratio of vulvar melanoma rates to CMM rates on exposed skin, seem to decrease. Thus, latitudinal trends seem to support the assumption that vulvar melanomas are not generated by UV radiation, and the possibility exists that solar UV radiation, probably via its role in vitamin D photosynthesis in exposed skin, may have a protective effect against vulvar melanoma and should be further investigated.