Objective: To determine incidences and time trends of second cancers among female breast cancer patients.
Methods: Using data of the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program, we designed three inception cohorts: 1975-1977 (n=25,920), 1983-1985 (n=32,722) and 1991-1993 (n=40,819), and analyzed their incidences of second cancers during the first 8 years after initial breast cancer diagnosis.
Results: Between the 1970s and the 1990s, the incidence rate of malignant second cancer significantly increased among female breast cancer patients, of which second non-Hodgkin's lymphoma and kidney cancer increased by about 150%, while second cancers of the thyroid, uterine corpus and skin melanoma increased 80%, and cancer of the lung increased 50%. The patterns of trend of second cancers were somewhat similar to those of the general population except for second endometrial cancer at all ages and second leukemia and skin melanoma among young patients aged 20-49. In the 1990s, the risk ratios (RR) of all sites cancer were found to be 5.5 (95% CI=5.0-6.1) for age 20-49, 1.3 (1.3-1.4) for age 50-64, and 1.2 (1.1-1.2) for age 65 and over, comparing breast cancer patients to general population. Additionally, radiotherapy slightly increased the risks of second leukemia (RR=1.8, 1.2-2.8), and second endometrial (RR=1.3, 1.0-1.6) and breast (RR=1.2, 1.1-1.3) cancers.
Conclusions: The fixed inception cohort method is valid for analyzing cancer registry-based second cancer data. By this method, we found that the incidence of second cancer has substantially increased among female breast cancer patients over the past 25 years. Observed changes in incidence may partially reflect the effect of treatments. Because the absolute number of affected patients is small, however, the breast cancer treatments have remained safe for most patients.