A history of breast cancer (BC) appears protective against mortality in patients with thyroid cancer (TC), suggests a study. This benefit is most pronounced with papillary TC (PTC) in patients who had TC first before BC (TC-1st patients), especially in those with a history of ER-positive BC.
“Occurrence of both TC and BC in the same patient may represent a special disease entity with a unique underlying biological or, likely, genetic background that needs to be identified,” said the investigators led by Dr Shuhuang Lin from the Thyroid Research Institute, School of Medicine, Southern University of Science and Technology, Shenzhen, Guangdong, China.
“Clinically, the findings in this study may be helpful for more precise prognostication of TC in such patients,” they added.
Lin and colleagues conducted this comparative cohort study of tumour behaviours and TC-specific mortality. In total, 5,598 patients with PTC and 604 with follicular TC (FTC), all of whom had a history of BC (TC-BC patients), were compared with propensity score-matched TC patients with no BC history (TCnoBC patients) in Surveillance, Epidemiology and End Results (SEER).
TC-BC patients had lower TC distant metastasis rates (PTC: 2.4 percent vs 3.0 percent; FTC: 6.1 percent vs 9.1 percent) and TC-specific mortality rates (PTC: 1.3 percent vs 2.6 percent; FTC: 5.8 percent vs 8.4 percent) than TCnoBC patients (p<0.05 for all). [J Clin Endoc Metab 2024;109:1222-1230]
Compared with matched TCnoBC patients, those with TC-BC had significantly reduced mortality risks (PTC: hazard ratio [HR], 0.472, 95 percent confidence interval [CI], 0.370‑0.601; FTC: HR, 0.656, 95 percent CI, 0.461‑0.934; p<0.05 for all).
Notably, the survival benefit in PTC patients was more robust when TC occurred before BC (HR, 0.397, 95 percent CI, 0.268‑0.588; p<0.001) than when BC occurred before TC (HR, 0.607, 95 percent CI, 0.445‑0.827; p=0.002).
Mechanism
“Previous studies have established an intrinsic association between TC and BC (4, 7, 8) and also an association between a history of TC and lower BC-specific mortality (12),” Lin said. [Surgery 2016;159:23‐29; Thyroid 2015;25:1330‐1338; Thyroid 2007;17:1277‐1288; PLoS One 2019;14:e0221093]
“These studies suggest that occurrence both of TC and BC in the same patient may represent a special disease entity with unique clinical outcomes. It has not yet been defined whether a history of BC may affect the prognosis of TC,” Lin added.
The study was not powered to explain mechanistically why TC-1st patients had a better prognosis. The investigators posited that younger age could have driven the protective effect of a history of BC on the prognosis of TC, since TC-1st patients were younger than those who had BC first at TC diagnosis (age, 55 vs 67 years).
“The bidirectional relationship between TC and BC in their effects on each other's clinical outcomes likely represents a special disease entity with a specific biological basis,” Lin said. “A unique genetic background in such patients may be one mechanism for the occurrence of the two cancers in the same patient.”