Studies conducted in Western countries have reported excess risks for second primary malignancies after breast cancer. However, there is little documentation of ethnic differences in these excess risks. Asian women are characterized by younger age at diagnosis of breast cancer, but very few reports are available on the incidences and risks for second primary cancers in this region. Using population-based data from the Taiwan National Cancer Registry (TNCR) for the period 1979 to 2003, we quantified standardized incidence ratios and cumulative incidence of second cancers among 53,783 women with initial diagnoses of breast cancer. Age-specific incidences showed peaks among women in their 40s, and 1,085 cases (2.02%) developed nonbreast second primary cancers. The risk for second cancers differs significantly according to age at diagnosis of breast cancer. In comparison with women diagnosed when ≥50 years (standardized incidence ratio, 0.96; 95% confidence interval; 0.89-1.04), there were significantly greater risks for bone, corpus uterine, ovarian, thyroid, esophageal, kidney and lung cancers, nonmelanoma skin cancer, and leukemia or lymphoma in women diagnosed when <50 years (standardized incidence ratio, 1.43; 95% confidence interval, 1.29-1.58). The survival probabilities differed between breast cancer patients with and without second cancers (P < 0.001). After diagnosis of the second cancer, the median survival time was only 2.87 years. In conclusion, we confirmed that young age at diagnosis of breast cancer predicted a subsequently increased risk for second malignancies, and the second cancers indeed worsen survivorship in patients who survived breast cancer.
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