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

ABSTRACT

BACKGROUND: 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. PATIENTS AND METHODS: 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). RESULTS: 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). CONCLUSION: 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.