A Possible Association Between Thyroid Cancer and Breast Cancer

Several lines of evidence suggest that breast cancer (BC) and thyroid cancer (TC) occur together in the same female patients more frequently than would be expected by chance. This study investigated the prevalence and clinicopathological characteristics of second primary BC in TC patients and second primary TC in BC patients.

A retrospective case-controlled study was performed in 4243 patients with differentiated TC and 6833 patients with BC. Age-matched control groups without second malignancies were selected.

Of the 4243 patients with TC, 55 patients developed subsequent BC during a five-year follow-up (range 2-40 years); the standardized incidence ratio (SIR) was 2.45 [confidence interval (CI) 1.83-2.96]. Among the 6833 patients with BC, 81 patients developed subsequent TC during a 6.2-year follow-up (range 2-40 years); the SIR was 2.18 [CI 1.43-2.82]. Subsequent second BC or TC diagnosed within five years of the initial primary malignancy showed more clinical characteristics consistent with early-stage cancer than did control BC or TC patients. Notably, the expression of both the estrogen and progesterone receptors was significantly higher in the tissues of BC patients with coexisting TC compared with those with BC alone.

The overall risk of second primary TC or BC is increased in patients with prior BC or TC, respectively. The early detection of second cancer might have contributed to these findings. However, BC that coexisted with TC had a higher expression of hormone receptors, suggesting an association between the molecular pathogenesis of TC and BC.