Background: Cancer has long been recognized to have a familial component. Elevated risks for cancers at the same site for relatives of cancer probands have been reported for both common cancers and a number of the rarer cancer sites. For a particular cancer site, however, the estimated risks to relatives have varied considerably depending on criteria for selection of probands, how cancers were determined in relatives, and overall study design. Not surprisingly, the estimated risks of other cancers in relatives of probands with cancer at a given site have been subject to even more variation.

Purpose: The aim of this study was to use the Utah Population Database resource to systematically study familial clustering of 28 distinct cancer site definitions among first-degree relatives (parents, siblings, and off-spring) of cancer probands.

Methods: We estimated familial relative risks from the Utah Population Database by identifying all cases of cancer in these first-degree relatives. These observed values were compared with those expected based on cohort-specific internal rates calculated from 399,786 relatives of all individuals in the Utah Population Database known to have died in Utah.

Results: All sites showed an excess of cancers of the same site among relatives, with thyroid and colon cancers and lymphocytic leukemia showing the highest familial risks. When the analyses were restricted to cases with early ages at diagnosis, increased familial components for most cancer sites became evident. A significant difference in familial relative risk (FRR) between male (FRR = 4.04; 95% confidence interval [CI] = 3.13-5.07) and female (FRR = 2.24; 95% CI = 1.54-3.08) probands was found for colon cancer. Highly significant familial associations (one-sided; P < .001) were found among breast, colon, and prostate cancers and between breast and thyroid cancers. Statistically significant (one-sided, P < .01) associations were also found between tobacco-associated sites (lung, larynx, lip, and cervix).

Conclusions: This study represents a unique comprehensive population-based study of familial cancer. The familial associations reported here will be useful in generating hypotheses about specific genetic and environmental factors that can be tested in genetic linkage and case-control studies.