Mutually increased risks for thyroid and breast cancer have been reported, but the contribution of etiologic factors versus increased medical surveillance to these associations is unknown.
Leveraging large-scale US population-based cancer registry data, we used standardized incidence ratios (SIRs) to investigate the reciprocal risks of thyroid and breast cancers among adult females diagnosed with a first primary invasive, non-metastatic breast cancer (N = 652,627) or papillary thyroid cancer (PTC) (N = 92,318) during 2000–2017 who survived ≥1-year.
PTC risk was increased 1.3-fold [N = 1434; SIR = 1.32; 95 % confidence interval (CI) = 1.25–1.39] after breast cancer compared to the general population. PTC risk declined significantly with time since breast cancer (Poisson regression = P trend <0.001) and was evident only for tumors ≤2 cm in size. The SIRs for PTC were higher after hormone-receptor (HR)+ (versus HR-) and stage II or III (versus stage 0-I) breast tumors. Breast cancer risk was increased 1.2-fold (N = 2038; SIR = 1.21; CI = 1.16–1.26) after PTC and was constant over time since PTC but was only increased for stage 0-II and HR + breast cancers.
Although some of the patterns by latency, stage and size are consistent with heightened surveillance contributing to the breast-thyroid association, we cannot exclude a role of shared etiology or treatment effects.
A number of studies have reported increased risk for thyroid cancer among breast cancer survivors and the reciprocal increased risk for breast cancer among thyroid cancer survivors [ , ]. Past studies have hypothesized that the association between breast and thyroid cancers may be explained by shared etiological factors (e.g., genetic factors [ PTEN -mutation] and obesity) or treatment received for initial cancer [ ]. Additionally, cancer survivors undergo heightened clinical surveillance, which may result in incidental detection of primarily early-stage, small-sized tumors that may not have become clinically apparent during the patient’s lifetime [ , ]. While it is established that this more intensive clinical surveillance immediately after diagnosis can result in spuriously inflated risks of subsequent malignancies compared with the general population [ ], prolonged heightened surveillance also has been suggested as a potential explanation for the observed increased risks of these second breast and thyroid cancers [ , , ]. However, previous studies have not systematically assessed breast and thyroid cancer risks by latency, stage, and tumor size.