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

The clinical relevance of the well-known association between thyroid cancer (TC) and breast cancer (BC) remains to be further defined.

This work aimed to investigate the effect of history of BC on the prognosis of TC.

This was a comparative cohort study of tumor behaviors and TC-specific mortality in 5598 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. The main outcome measure was TC-specific mortality.

Lower TC distant metastasis rates of 2.4% vs 3.0% in PTC and 6.1% vs 9.1% in FTC and TC-specific mortality rates of 1.3% vs 2.6% in PTC and 5.8% vs 8.4% in FTC were found in TC-BC patients vs matched TCnoBC patients (all P < .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 < .05). Such HRs for mortality in PTC were 0.397 (95% CI, 0.268-0.588; P < .001) when TC occurred before BC vs 0.607 (95% CI, 0.445-0.827; P = .002) when BC occurred before TC.

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.