Superficial-spreading and nodular melanomas in Norway: a comparison by body site distribution and latitude gradients

Superficial-spreading melanoma (SSM) and nodular melanoma (NM) are the most common histological types of melanoma. These are seemingly distinct entities, on the basis of epidemiology, clinical behavior, tumor biological, and histological aspects as well as molecular characteristics. Intermittent sun exposure seems to play a major role in SSM, which has an increasing incidence rate during the last decades. However, relationship with sun exposure is more complex in the case of NM, as the latter may arise on any body part and has a more stable incidence rate. To obtain more information on the role of sun exposure in these two types of melanoma, we have compared body site distributions and latitude gradients for a period of 30 years in Norway. The study was based on official reports from the Cancer Registry of Norway, using melanoma incidence rates for a period of 30 years (1978-2007), by age, sex, anatomic site, and region of Norway. Our results show that in Norway, SSM is more strongly related to intermittent sun exposure than NM, as it arises mostly on the trunk as compared with the head and neck. Moreover, SSM has a higher incidence in the Southern regions of Norway, whereas for NM, the north-south gradient is not statistically significant. Differences in sun exposure at northern latitudes have a major impact on different types of melanoma. It seems that NM is less sun-induced than SSM, for which other etiological and pathogenetic factors may play important roles.