Women previously diagnosed with breast cancer had a 60% increased risk of colorectal cancer, according to analysis of Swedish cancer registries, and breast cancer therapy had no effect on risk.
Women previously diagnosed with breast cancer had a higher risk of colorectal adenocarcinoma, compared with the general population (standardized incidence ratio, 1.59; 95% confidence interval, 1.53-1.65). Higher risk was observed for adenocarcinoma of the proximal colon, compared with the distal colon (SIR, 1.72; 95% CI, 1.61-1.82 vs. SIR, 1.46; 95% CI, 1.34-1.58).
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Risk for adenocarcinoma of the proximal colon, the predominant subtype in women, increased with age at breast cancer diagnosis, from an SIR of 1.60 for women diagnosed between the ages of 15 and 49 years, to an SIR of 1.51 for 50-59 years, to an SIR of 1.82 for age 60 years or greater. By contrast, the SIR for general colorectal adenocarcinoma dipped in the 50-59 age group, a finding that may point to the role of estrogen in colorectal carcinoma (Cancer Epidemiol. 2016 Feb 15. doi: 10.1016/j.canep.2016.01.006).
“It is well known that estrogen levels are elevated in breast cancer patients, which may contribute to the increased risk of colorectal cancer in this population. Interestingly, our results showed that the SIRs of colorectal adenocarcinoma changed in different age groups, compared with the general population. Specifically, the SIRs showed a drop in the 50-59 age group but increased again after the age of 60,” wrote Dr. Yunxia Lu, associate professor in the department of molecular medicine and surgery, Karolinska Institute, Stockholm, and colleagues. The researchers noted previous reports of endogenous estrogen levels decreasing after menopause for a short period, while exogenous hormones may take effect some time after menopause.
The study found no association between different types of breast cancer treatment, including antihormonal therapy, and the risk of colorectal cancer.
The retrospective analysis used data from the Swedish Cancer Register, including 179,733 patients diagnosed with breast cancer from 1961 to 2010, and 2,571 colorectal cancers identified. Analysis of treatment effects on colorectal cancer risk used data from the Stockholm-Gotland Breast Cancer Register of 20,171 patients with breast cancer and relevant treatment information.
Dr. Lu and coauthors reported having no disclosures.