Variation in incidence and fatality of melanoma by season of diagnosis in new South Wales, Australia

Abstract

Seasonal variation in cutaneous melanoma incidence with a summer peak is poorly understood. It has been hypothesized to be due to increased diagnosis in summer or a late-promoting effect of sun exposure. We analyzed the characteristics of incident cases of cutaneous melanoma and their outcome by season of diagnosis in the population of New South Wales, Australia. Cases of melanoma (25,845 cases; 10,869 females and 14,976 males) were registered by the New South Wales Central Cancer Registry in 1989 to 1998. There was significant seasonal variation in incidence (P < 0.0001, Nam test). The summer to winter ratio was greater for women, younger people, lesions on the limbs, and superficial spreading melanoma. Melanomas were thicker in winter than in summer (medians 0.75 and 0.70 mm, respectively; P < 0.0001, Kruskal-Wallis test). Cases were followed for a median of 63 months and 2,710 (10.5%) died from their melanoma. Fatality from melanoma was lower for melanomas diagnosed in summer than winter (relative fatality = 0.72; 95% confidence interval, 0.65-0.81); the 5-year survival rate was 92.1% for diagnosis in summer and 89.0% for diagnosis in winter. This result remained significant after adjustment for year of diagnosis, age, sex, Breslow thickness, anatomic location, and histologic type (relative fatality = 0.82; 95% confidence interval, 0.72-0.94). Seasonality in melanoma incidence is probably caused mainly by increased and earlier diagnosis in summer, although a late-stage promotional effect of sun exposure cannot be excluded completely. Earlier diagnosis may also reduce fatality when melanoma is diagnosed in summer. Independence of variation in fatality with season from seasonal variation in thickness, however, suggests that sun exposure around the time of diagnosis decreases fatality of melanoma.