Association between a History of Breast Cancer and Decreased Thyroid Cancer-specific Mortality

Abstract Context The clinical relevance of the well-known association between thyroid cancer (TC) and breast cancer (BC) remains to be further defined. Objective To investigate the effect of a history of BC on the prognosis of TC. Design, setting, and patients This was a comparative cohort study of tumor behaviors and TC-specific mortality in 5,598 patients with papillary thyroid cancer (PTC) and 604 patients with follicular thyroid cancer (FTC), all with a history of BC (TC-BC patients), and their propensity score-matched TC patients without a history of BC (TCnoBC patients) in Surveillance, Epidemiology and End Results (SEER) 18. Main Outcome Measure(s) TC-specific mortality. Results Lower TC distant metastasis rates of 2.4% versus 3.0% in PTC and 6.1% versus 9.1% in FTC and TC-specific mortality rates of 1.3% versus 2.6% in PTC and 5.8% versus 8.4% in FTC were found in TC-BC patients versus matched TCnoBC patients (all P < 0.05). Comparing TC-BC patients with matched TCnoBC patients, hazard ratios (HRs) for mortality were 0.472 (95% CI 0.370 - 0.601) in PTC and 0.656 (95% CI 0.461 - 0.934) in FTC (all P < 0.05). Such HRs for mortality in PTC were 0.397 (95% CI 0.268 -0.588, P < 0.001) when TC occurred before BC versus 0.607 (95% CI 0.445 - 0.827, P = 0.002) when BC occurred before TC. Conclusions This study demonstrates a robust protective effect of a history of BC on TC-specific patient survival, which has strong implications for more precise prognostication of TC in such patients.