Background: The epidemiology of large-bowel cancer suggests a role for endocrine factors in its development. Although analytic studies have not consistently provided evidence for an association between reproductive history and large-bowel cancer, some relatively small studies have observed a reduced risk among women using postmenopausal hormone replacement therapy (HRT).
Purpose: This study was planned to evaluate more precisely the relationship between HRT and the risk of colon and rectal cancers.
Methods: Female residents of Wisconsin aged 30-74 years with a diagnosis of colon or rectal cancer within 2 years were identified through a statewide tumor registry. Control subjects were randomly selected from lists of licensed drivers if the case subjects were less than 65 years old and from lists of Medicare beneficiaries if they were 65-74 years old. Information on post-menopausal hormone replacement use, medical history, and family history was obtained in telephone interviews. After premenopausal women were excluded, 694 case subjects and 1622 control subjects remained for analysis. The odds ratios and 95% confidence intervals (CIs) obtained from conditional logistic regression models were used to estimate relative risks (RRs). All RRs were adjusted for age, family history of large-bowel cancer, use of screening sigmoidoscopy, and recent alcohol consumption.
Results: Compared with postmenopausal women who never used HRT, recent users had an RR of 0.54 (95% CI = 0.36-0.81) for colon cancer and an RR of 0.91 (95% CI = 0.54-1.55) for rectal cancer. This inverse association was observed among users of both estrogen only and combined estrogen and progestin preparations. Decreasing time since last use was inversely associated with colon cancer risk (P for trend < .001). The effect of HRT appeared to be stronger among women at lower absolute risk of colon cancer, particularly among women with lean body mass.
Conclusions: Use of HRT was associated with a statistically significant reduced risk of colon cancer. In contrast, no statistically significant relationship was observed for rectal cancer. Given the widespread use of postmenopausal hormones and the morbidity and mortality from adenocarcinoma of the bowel in women, these findings may have potentially important public health implications.