Cancer risks for relatives of children with cancer.

We determined the extent and distribution of cancers in relatives of 379 children newly diagnosed with cancer. Family history was collected from 1,337 first-degree and 3,399 second-degree relatives and incidence compared with national age- and gender-specific rates. Overall, 14 children (3.7%) had a relative with a history of childhood cancer and 26 children (6.9%) had a first-degree relative with a history of cancer, with only one of these having an identifiable familial cancer syndrome. There was a higher than expected incidence of childhood cancer among first-degree relatives (parents and siblings) (standardized incidence ratio (SIR) 1.43; 95% CI 0.54-5.08). There was also a higher than expected incidence of adult cancers among first-degree relatives (SIR 1.45; 95% CI 0.93-2.21), particularly in females (SIR 1.82; 95% CI 1.26-3.39). The increased family cancer history in first-degree females was largely attributable to an effect in mothers (SIR 1.78; 95% CI 1.27-3.33). The gender-specific association was reflected in higher than expected incidence rates of breast cancer in both mothers (SIR 1.92; 95% CI 0.72-6.83) and aunts (SIR 1.64; 95% CI 0.98-2.94). These findings support the hypothesis that previously undetected familial cancer syndromes contribute to childhood cancer.