Does in situ melanoma really come before invasive melanoma? Descriptive epidemiology questions this relationship.

Abstract

While the incidence rates for skin melanoma have in creased worldwide in the last decades, Breslow tumor thickness has decreased 1 . The incidence of in situ melanoma has also increased, and it doubled in the USA between 1988 and 2006 2 .These recent trends are in part due to the diffusion of early diagnosis 1,2 , which is based on the assumption that the detection of less inva sive lesions (i.e., those susceptible to more effective treatment ) will prevent the occurrence of the more in vasive and deadly ones. If the pattern for melanoma in dicated a worsening progression from in situ to invasive lesions -thin, first, then thick -the mean age at diagnosis for such lesions should increase accordingly. We retrieved melanoma cases incident between 2000 and2005fromtheTuscanyCancerRegistryarchive.This is a population-based cancer registry active in central Italy since 1985 on a population of about 1.2 million in habitants. Between 2000 and 2005, 1513 skin melanomas were newly diagnosed, 318 (21.0%) in situ and 1195 (79.0%) invasive. Among the latter, 607 (50.8%) were ≤1 mm , 410 (34.3%) >1 mm , while the Breslow thickness of 178 lesions (14.9%) could not be assessed. The mean ages of patients at diagnosis of melanomasin situ (57.69 years) and invasive melanomas (57.52) were similar (Student’s t-test, P = 0.87) . However, among in vasive melanomas, thin lesions (≤1 mm) were diag nosed at a younger age (54.03 years) than among in situ melanomas (P = 0.0014). By contrast, patients with > 1 mm thick melanomas were older at the time of diagno sis (61.95 years) than patients with in situ melanomas (Table 1 ). These results are mainly due to lentigo malig nant melanomas , which occur at a rather old age (mean age at diagnosis 71.3 years) and are more frequent among in situ (69/318, 21.6%) than among invasive melanomas (36/1195; 3.0%). According to the present data, in situ melanoma does not seem an obligate precursor of thin invasive melanoma, as it is diagnosed at an older age than inva sive melanoma ≤1 mm. Although most of the results are driven by lentigo maligna melanomas, descriptive epi demiology suggests different pathways for in situ and invasive melanomas, at least for thin ones. There may be 2 different in situ melanomas, some with an indolent behavior and others which are more aggressive ; micro melanomas invasive at diagnosis 3 may belong to the lat ter group.