Familial aggregation of early-onset cancers.

This registry-linkage study evaluates familial aggregation of cancer among relatives of a population-based series of early-onset (≤40 years) cancer patients in Finland. A cohort of 376,762 relatives of early-onset cancer patients diagnosed between 1970 and 2012 in 40,538 families was identified. Familial aggregation of early-onset breast, colorectal, brain and other central nervous system (CNS) cancer and melanoma was explored by standardized incidence ratios (SIR), stratified by relatedness. Gender-, age- and period-specific population cancer incidences were used as reference. Cumulative risks for siblings and offspring of the proband up to age ≤40 years were also estimated. Almost all early-onset cancers were sporadic (98% or more). Among first-degree relatives, SIR was largest in colorectal cancer (14, 95% confidence interval 9.72-18), and lowest in melanoma (1.93, 1.05-3.23). Highest relative-specific SIRs were observed for siblings in families, where also parent had concordant cancer, 90 (43-165) for colorectal cancer and 29 (11-64) for CNS cancer. In spouses, all SIRs were at population level. Cumulative risk of colorectal cancer by age 41 was 0.98% in siblings and 0.10% in population, while in breast cancer the corresponding risks were 2.05% and 0.56%. In conclusion, early-onset cancers are mainly sporadic. Findings support high familial aggregation in early-onset colorectal and CNS cancers. Familial aggregation in multiplex families with CNS cancers was mainly attributed to neurofibromatosis and in colorectal cancer to FAP- and HNPCC-syndromes. The pattern of familial aggregation of early-onset breast cancer could be seen to support very early exposure to environmental factors and/or rare genetic factors.