A history of breast cancer has been reported as a risk factor for colorectal cancer in women. However, in view of the ambiguous nature of existing evidence, and the growing interest in targeted colorectal cancer prevention, Craig Newschaffer, from Johns Hopkins School of Hygiene and Public Health, Baltimore, USA, and colleagues sought to quantify this risk.
The investigators estimated the risk of colorectal cancer after breast-cancer diagnosis in around 227,000 women with first incident breast cancer between 1974 and 1995.
Observed colon and rectal cancer risk was compared with that expected in the general population.
Comparisons were stratified by age at breast-cancer diagnosis, stage of cancer, ethnic origin of patient, and follow-up time.
|Risk of disease in women with previous breast cancer:|
Colon cancer: 5% less
Rectal cancer: 13% less
Women with previous breast cancer were 5% less likely to develop colon cancer and 13% less likely to develop rectal cancer than women in the general population.
Stratification suggested that the risk reductions observed for colon and rectal cancer were most pronounced for women with breast cancer diagnosed after 65 years of age, in Caucasian women, women with local-stage breast cancer, and women diagnosed in the later study years (1990-94).
Craig Newschaffer comments, "Our results suggest that breast-cancer history should not be thought of as a risk factor for colorectal cancer.
"We cannot explain the reduced risk for colorectal cancer that we noted in some groups of breast cancer patients; however, protection is unlikely to stem from endogenous biological factors.
"Patients or their physicians should not, however, misconstrue these findings - colorectal cancer is the third most common non-skin malignant disorder among women in the USA, and about 40% of those diagnosed die from the disease."
"Women with breast cancer - just as those without - should be encouraged to participate in colorectal-cancer screening and to make lifestyle choices likely to minimize colorectal-cancer risk," he concludes.