Analysis of hereditary component of cancer by use of a familial index by site

To quantify hereditary factors in the risk of cancer, we matched 1 283 047 cancer patients listed in the Swedish Cancer Registry with healthy controls from a national database, and identified the number of individuals who were first-degree relatives of other indivduals in the same cohort. Division of the number of relatives in the patient cohort by the number in the control cohort yielded a “familial index”. The following cancers had high familial indices: eye 16·5 (95% Cl 2·5–666), testis 9·0 (3·2–35), Hodgkinʹs disease 6·5 (2·3–26·0), and thyroid 6·2 (3·7–12). Overall, however, familial factors made only a minor contribution to susceptibility to cancer.