International patterns and trends in the incidence of melanoma and cutaneous squamous cell carcinoma, 1989-2020.

BACKGROUND: Cutaneous squamous cell carcinoma (cSCC) and melanoma have different associations with sun exposure, so comparing the trends in incidence rates of the two different cancers may provide insights into changing patterns of exposure to ultraviolet radiation.

METHODS: We compared trends in the incidence of cSCC and melanoma in seven susceptible populations residing at mid-to-high latitudes: Finland, Norway, Sweden, Denmark, Scotland, the Netherlands and Tasmania (Australia). We fitted Joinpoint models to describe trends in age-standardized incidence rates for melanoma and cSCC and calculated the average annual percentage rate of change for the period 1989-2020 (1989-2018 for Tasmania). We calculated the incident rate ratio as the ratio of the age-standardized rates (European Standard Population) for cSCC to melanoma and conducted age-period-cohort modelling to compare age, period, and cohort effects.

RESULTS: The ratio of cSCC-to-melanoma incidence increased with proximity to the equator and over time. In the most recent time period, the incidence of cSCC was higher than the incidence of melanoma for men and women in all seven populations. While the ratio of cSCC-to-melanoma incidence was higher for men compared with women, in most countries the cSCC-to-melanoma IRR increased over time to a greater extent in women compared with men. Melanoma incidence was higher among younger people and cSCC incidence was higher among older people; the age at which the incidence of cSCC overtook incidence of melanoma was progressively younger with proximity to the equator.

CONCLUSIONS: Despite concerted international efforts to preserve the ozone layer over the past four decades resulting in significant reductions in surface UV-B at mid-latitudes, the incidence of skin cancer, particularly cSCC, continues to rise in those regions. Our findings are consistent with a stronger association with age-associated cumulative sun exposure for cSCC compared with melanoma, and suggest that women are receiving greater UV radiation exposure in recent decades than in the past.