Interactions of familial and hormonal risk factors for large bowel cancer in women.
BACKGROUND
Family history of colorectal cancer has been consistently associated with an increased personal risk of this disease. Since evidence suggests that hormones are related to colon cancer risk in women, the effect of family history on large bowel incidence may be modified according to endogenous and exogenous hormone levels.


METHODS
We analysed data from a population-based case-control study of female colorectal cancer to evaluate family history and cancer risk. Cases (n = 702) were female residents of Wisconsin with a new diagnosis of colorectal cancer, identified through a statewide tumour registry. Controls (n = 2274) were randomly selected from lists of licensed drivers and from rosters of Medicare beneficiaries. All relative risks (RR) were adjusted for age, body mass index, smoking and alcohol history, education, and use of hormone replacement therapy.


RESULTS
Compared with women who reported no history of cancer in a first degree relative, women with a family history had an RR of 2.07 (95% confidence interval [CI]: 1.60-2.68). Regardless of which parent was affected, risks were increased about twofold, while sibling history was associated with about a 50% increase in risk. Risk was greater if more than one family member was affected (RR 3.65, 95% CI: 1.81-7.37). The association between family history and risk was stronger for colon cancer than for rectal cancer. There were no indications that exogenous hormonal factors, notably hormone replacement use, modified these risks. There was a suggestion that high parity attenuated the risks associated with family history (P = 0.07).


CONCLUSIONS
These results confirm that family history of colorectal cancer is associated with a doubling of risk for large bowel cancer in women; some histories were associated with greater risk. This relation was not substantially different among subgroups of women with varying exogenous and endogenous hormone exposures.