Incidence and risk factors associated with bilateral breast cancer in area with early age diagnosis but low incidence of primary breast cancer: analysis of 10-year longitudinal cohort in Taiwan.

This study aims to examine the incidence and risk factors of bilateral breast cancer in area with low incidence rate. A total of 120 and 1902 women with bilateral and unilateral breast cancers were enrolled; various factors, including those concerning their medical history and life style, were extracted. Using Kaplan-Meier method, we calculate the cumulative incidence of contralateral breast cancer. The results show as follows. The cumulative incidences of contralateral breast cancer at 1, 3, 5 years after diagnosis of first breast cancer were 1.15, 1.94, and 2.97%, respectively. The statistically significant risk factors included menopause (Hazard Ratio (HR) =1.56, (1.00-2.42)), invasive lobular carcinoma (HR=2.98, (1.35-6.56)), receiving chemotherapy (HR=2.21, (1.43-3.42)) and/or radiotherapy (HR=3.32, (2.19-5.05) and a protective factor was tamoxifen therapy (HR=0.5 (0.34-0.74). Size of the second occurred tumour (2.97 cm) tended to be smaller than the first one (3.58 cm) with borderline statistical significance (p=0.0731). Comparing to the existing data on Western countries, we find a higher risk for developing contralateral breast cancer in Taiwan where a low incidence of first breast cancer rate with early age diagnosis is noted. It suggests that first primary breast tumour with early age of onset and lobular carcinoma are found more likely to develop bilateral breast cancers.