Despite the long-term increases in melanoma incidence rates, there has been only one recent trend analysis by anatomic site at a single large academic center in the United States (13
). Though it is well-established that the majority of melanomas occur on the trunk and lower extremities for men and women, respectively (4
),our results further demonstrated that melanoma of the trunk in women increased between the time periods 1975-1982 and 1999-2006 over 9 SEER Registries in the United States at a greater rate than for other sites in women, while the opposite was observed for men. The increase in trunk melanoma in women could be secondary to changes in sun exposure from various cultural, environmental, or behavioral factors. As described earlier, changes in the net drift suggest differential secular trends, whereas differences in longitudinal age trends suggest differential age-related biological or natural history effects. Hence, the combination of the high net drift and low longitudinal age trend of female trunk melanoma in demonstrate that melanomas are rising preferentially on the trunk among young women.
Past studies in the United States (13
) and other countries have also shown the increasing prevalence of trunk melanoma in women (14
) and younger patients (15
). Melanomas developing at different body sites are thought to be associated with distinct patterns of sun exposure, and the different anatomical distribution of lesions in men and women has been attributed to gender-specific patterns of sun-exposure (15
). Melanomas of the head and neck are associated with chronic sun exposure, whereas trunk melanomas are thought to be associated with intermittent patterns of sun exposure, supporting the hypothesis that melanomas may arise through divergent causal pathways (8
). Some authors have suggested that susceptibility of melanocytes to malignant transformation might be site dependent, which could help explain why the relevance of known risk factors is not uniform by body site (18
).The variable age distribution of risk by gender and anatomic site, particularly for trunk melanoma, might also support the hypothesis of a possible modulator role of sex hormones in female trunk melanoma (16
). Estrogens are known to increase the number of melanocytes and modify their melanin content (19
). However, one would need to hypothesize that sex hormones are changing over time; hence, the relationship between sex hormones and trunk melanoma should be analyzed further.
The rising melanoma incidence rates of the trunk in young women could also be partially caused by changes in clothing patterns. Young females are frequently wearing bikinis on the beach, and summer female casual clothes often leave part of the back and front of the trunk exposed to the sun (16
). Finally, the increase in melanomas of the trunk in women could also be secondary to changes in sun exposure or a result of increased tanning behavior. It is noteworthy that usage of tanning beds, recently classified by the International Agency of Research on Cancer as a cause of melanoma (20
) is most prevalent among young women (21
This analysis has important limitations. First, like most population-based registry studies, our analyses lacked information on individual risk factors, particularly a history of ultraviolet radiation exposure and characterization of nevus pattern, and had the potential of incomplete data collection. It is also widely recognized that trends in melanoma incidence are affected by underreporting (23
) and surveillance issues. Melanoma has a longer reporting delay (up to 7 to 10 years) than other cancer sites because of the difficulties associated with reporting a cancer that is increasingly diagnosed in a non-hospital setting (24
). Since there is a delay, the melanoma estimates for recent years are minimal levels of risk and may rise as data collection for recent years is completed. Case finding is therefore, an important aspect of data quality control for the SEER program, and SEER has adjusted data collection activities for melanoma to try to minimize errors. Age period cohort models share in all of the intrinsic limitations of standard descriptive studies. Additionally, since age, period, cohort are collinear (cohort = period – age), it is impossible to determine the independent effects for age, period, and cohort, giving rise to the so-called non-identifiability issue. Nonetheless, the parameters net drift and longitudinal age trend can be identified in the restricted age period cohort model, following the method of Holford (25
). Thus, using these two estimable parameters, we have demonstrated statistically significant evidence for rising trunk melanomas in young women in the United States.
In conclusion, we present evidence for changing melanoma anatomic trends in the United States. Trunk melanomas among younger women in the United States are increasing relative to all other anatomic body sites, suggesting different trends in carcinogenic exposures or etiologic differences by melanoma anatomic site. Future surveillance and analytic studies on melanoma should include gender and anatomic site.