Effect of breast cancer as the first or second primary cancer on the prognosis of women with thyroid cancer: a SEER database analysis

Background: Although many studies have evaluated the association between breast cancer (BC) and thyroid cancer (TC), this association has not been completely studied. In this study, we focused on the effect of BC as the first or second primary cancer on survival among women with TC. We present the following article in accordance with the STROBE reporting checklist. Methods: Using the Surveillance, Epidemiology, and End Results (SEER) database, we performed a retrospective cohort analysis of 134,785 women with BC and TC between 1974 and 2016. The patients were divided into three subgroups, namely, TC only (TC group, n=128,888), BC after TC (TC2BC group, n=3,000), and BC before TC (BC2TC group, n=2,897). The curves for overall survival (OS) and cancer-specific survival (CSS) were evaluated, and Cox regression analyses were performed to examine the effect of BC as the first and second primary cancer on the CSS and OS of TC patients. Results: The CSS rates in the TC, TC2BC, and BC2TC groups were 96.96%, 98.70%, and 96.70%, respectively, and the OS rates were 91.17%, 77.13%, and 78.63%, respectively. Using the TC group as a reference, the hazard ratios (HRs) for the CSS of the TC2BC and BC2TC groups were 0.333 [95% confidence interval (CI): 0.243-0.456, P<0.001] and 1.299 (95% CI: 1.062-1.589, P=0.011), respectively. The corresponding HRs for the OS were 1.544 (95% CI: 1.428-1.668, P<0.001) and 3.429 (95% CI: 3.158-3.724, P<0.001), respectively. Conclusions: Among the female patients with TC, BC, as the second primary cancer, has a significant protective effect on the CSS of TC patients; however, when BC is the first primary cancer, it is a risk factor. Besides, BC is a risk factor for the OS of TC patients, especially for patients who develop TC after BC.