No Increase in Breast Cancer Risk in Japanese Women Taking Oral Contraceptives: a Case-Control Study Investigating Reproductive, Menstrual and Familial Risk Factors for Breast Cancer

摘要

Background: Low-dose oral contraceptives (OC) were approved by the Japanese Ministry of Health, Laborand Welfare in 1999, yet despite their contraceptive and non-contraceptive health benefits, only 5% of the targetpopulation use them. Fear of increased cancer risk, particularly breast cancer, is one reason for this. Due tolow OC uptake and low screening participation, a paucity of data is available on the risk of OC use and breastcancer in Japanese women. The present study investigated OC use and breast cancer risk, as well as menstrual,reproductive and family factors. Materials and Methods: This was a clinic-based case-control study of womenaged 20-69yrs who had undergone breast screening between January 2007 and December 2013 in central Tokyo.In all, 28.8% of the participants had experience with OC use. Cases were 155 women with a pathologicallyconfirmed diagnosis of breast cancer. Controls were the remaining 12,333 women. Results: Increased age was asignificant risk factor for breast cancer (p<0.001). A lower risk was found in premenopausal women presentlytaking OC compared to never users (OR 0.45; 95% CI 0.22-0.90) after adjusting for age, parity and breastfeeding, and a family history of breast cancer. Conclusions: Increased age rather than OC use had a greatereffect on breast cancer risk. This risk may be decreased in premenopausal women with OC use, but furtherlong-term prospective studies are necessary.