Incidence of contralateral breast cancer in Japanese patients with unilateral minimum-risk primary breast cancer, and the benefits of endocrine therapy and radiotherapy

Abstract

Tamoxifen is recommended as adjuvant endocrine therapy for patients with minimum-risk breast cancer. It is primarily effective at prevention of contralateral and ipsilateral breast cancer recurrence after breast-conserving surgery. The incidence of contralateral breast cancer and the absolute benefit of endocrine therapy among patients with unilateral minimum-risk breast cancer in Japan, where the incidence of breast cancer is low, are unknown. We retrospectively studied the incidence of contralateral breast cancer, and the efficacy of endocrine therapy, in a cohort of 2074 Japanese women with unilateral breast cancer whose primary tumor was pTis (n=1905) or pT1mic (n=169) (unknown for endocrine therapy, n=4; unknown for radiotherapy, n=2). We also assessed the efficacy of endocrine therapy and radiotherapy for prevention of ipsilateral and contralateral breast cancer recurrence in 1205 patients who underwent breast-conserving surgery (unknown for endocrine therapy, n=2; unknown for radiotherapy, n=2). The incidence of contralateral breast cancer per 1000 person-years was 5.1 (95% confidence interval (CI), 3.7-7.1) among patients without endocrine therapy (n=1364) and 3.6 (95% CI 2.1-6.1) among those with endocrine therapy (n=706). The incidence of ipsilateral breast cancer recurrence after breast-conserving surgery per 1000 person-years was 9.2 (95% CI 6.5-13) among patients without endocrine therapy (n=753) and 4.2 (95% CI 2.2-8.1) among those with endocrine therapy (n=450). The incidence of ipsilateral breast cancer recurrence after breast-conserving surgery per 1000 person-years was 9.9 (95% CI 6.3-15.6) among patients without radiotherapy (n=380) and 5.9 (95% CI 3.9-9.0) among those with radiotherapy (n=823). The incidence of contralateral breast cancer among minimum-risk breast cancer patients in Japan, where the incidence of breast cancer is low, was similar to that in Western countries. Endocrine therapy is indicated for this population.