Sex- and kindred-specific familial risk of non-Hodgkin's lymphoma.

A family history of non-Hodgkin's lymphoma (NHL) confers increased risk of NHL, but it is unknown whether the excess risk in males and females varies by the sex or kinship of the affected relative. We linked nationwide Swedish registries to identify parents and siblings of NHL patients who developed NHL between January 1, 1961 and December 31, 2002. In males, parental risks were approximately the same from fathers and mothers, whereas sibling risks were higher from brothers [standardized incidence ratio (SIR), 1.8; 95% confidence interval (95% CI), 1.0-2.9] than sisters (SIR, 0.9; 95% CI, 0.2-1.9). In females, parental and sibling risks were higher from same-sex relatives (SIR from mothers, 1.9; 95% CI, 1.2-2.7; SIR from sisters, 6.3; 95% CI, 4.0-9.3) than from opposite-sex relatives (SIR from fathers, 1.2; 95% CI, 0.7-1.9; SIR from brothers, 0.7; 95% CI, 0.2-1.6). These findings did not vary substantially by the age of diagnosis of the offspring. Risk of NHL in offspring was also increased among those with a parent diagnosed with multiple myeloma or leukemia. The relative risk of NHL among those with a parent diagnosed with any hematopoietic cancer was 1.5 (95% CI, 1.4-1.7) and that for having a sibling with any hematopoietic cancer was also 1.5 (95% CI, 1.2-1.9). Our results suggest that part of the familial risk of NHL may be attributable to shared environmental exposures, particularly between same-sex siblings.