Malignant melanoma of the skin: risk, tumour characteristics and mortality in adult twins born in Norway between 1905 and 1945 - a cohort study.

In this cohort study, data on the observed reduced incidence of malignant melanoma (MM) of the skin in twins compared with a control group were related to tumour characteristics and all-cause mortality. The twin cohort consisted of 20 926 individuals and the control group included individuals picked at random and matched to the twin cohort with respect to age, sex and place of birth. The cohorts were linked to the Cancer Registry of Norway and all cases of MM diagnosed from 1960 to 1999 were followed until October 2003. The mean observation time from diagnosis to death or censoring was 10.9 years (SD, +/-8.5 years). The relative risk of MM for the twins was 0.68 [95% confidence interval (CI): 0.55, 0.83]. No statistically significant difference was found in the distribution of histopathological subtypes or pathological stage between the cohorts. The crude all-cause mortality risk for the twin cases was 0.79 (95% CI: 0.58, 1.07). Stratified analysis indicated effect modification by gender on all-cause mortality risk (Breslow-Day test of homogeneity P=0.02). Multivariate analysis was performed separately for men and women. The adjusted mortality risk ratio for male twin cases was 1.11 (95% CI: 0.75, 1.65) and for female twin cases 1.05 (95% CI: 0.61, 1.8). The twins had a lower relative risk of MM compared with the control group, but similar tumour characteristics and all-cause mortality.