Cancer risk in the siblings of individuals with major birth defects: a large Nordic population-based case-control study.

Background: Individuals with major birth defects are at increased risk of developing cancer, indicating a common aetiology. However, whether the siblings of individuals with birth defects are also at an increased risk of cancer is unclear.

Methods: We used nationwide health registries in four Nordic countries and conducted a nested case-control study. We included 40 538 cancer cases (aged 0-46 years) and 481 945 population controls (matched by birth year and country), born between 1967 and 2014. The relative risk of cancer among individuals whose siblings had birth defects was computed with odds ratios (OR) and 95% confidence intervals (CIs), using logistic regression models.

Results: In the total study population (aged 0-46 years), we observed no overall difference in cancer risk between individuals whose siblings had birth defects and those who had unaffected siblings (OR 1.02; 95% CI 0.97-1.08); however, the risk of lymphoid and haematopoietic malignancies was elevated (1.16; 1.05-1.28). The overall risk of childhood cancer (0-19 years) was increased for siblings of individuals who had birth defects (1.09; 1.00-1.19), which was mainly driven by lymphoma (1.35; 1.09-1.66), neuroblastoma (1.51; 1.11-2.05) and renal carcinoma (5.03; 1.73-14.6). The risk of cancer also increased with the number of siblings with birth defects (Ptrend = 0.008).

Conclusion: Overall risk of cancer among individuals (aged 0-46 years) whose siblings had birth defects was not elevated, but the risk of childhood cancer (ages 0-19 years) was increased. Our novel findings are consistent with the common aetiologies of birth defects and cancer, such as shared genetic predisposition and environmental factors.