Trends in Invasive Melanoma Thickness in Norway, 1983-2019

Monitoring melanoma incidence time trends by tumour thickness is essential to understanding the evolution of melanoma occurrence and guiding prevention strategies. To assess long-term incidence trends, tumour thickness was extracted from pathology reports in the Cancer Registry of Norway (1983-2007) and the Norwegian Melanoma Registry (2008-2019), n = 45,635 patients. Across all anatomic sites, T1 (≤ 1 mm) incidence increased most (men annual percentage change [AAPC] = 4.6, 95% confidence interval [95% CI] 4.2-5.0; women AAPC = 3.2, 95% CI 2.8-3.6); the increase was steep until 1989/90, followed by a plateau, and a further steep increase from 2004/05. Increased incidence was also observed for T2 (>1.0-2.0) melanoma (men AAPC = 2.8, 95% CI 2.4-3.2; women AAPC = 1.5, 95% CI 1.1-1.9), and T3 (>2.0-4.0) in men (AAPC = 1.4, 95% CI 0.9-1.9). T4 (>4.0) melanoma followed a similar overall pattern (men AAPC = 1.3, 95% CI 0.9-1.7, head/neck, upper limbs, and trunk; women AAPC = 0.9, 95% CI 0.4-1.4, upper limbs and trunk). Men had the highest T3 and T4 incidence and the sex difference increased with age. Regarding birth cohorts, age-specific incidence increased in all T categories in the oldest age groups, while stabilizing in younger patients born after 1950. Overall, the steep increase in T1 melanoma was not accompanied by a decrease in thick melanoma.