Risk of subsequent primary thyroid cancer after another malignancy: latency trends in a population-based study.

Purpose: To evaluate the risk of a subsequent primary thyroid cancer (SPTC) in patients with common invasive cancers, with attention to latency trends and histology associations.
Methods: Patients with one of 10 common invasive cancers were followed from 1975 to 2008 in 9 registries participating in the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database. Standardized incidence ratios (SIRs) for SPTC were determined by the multiple primary-SIR program in SEER*Stat.
Results: A total of 2502 SPTCs were observed. Greatly elevated SIRs for SPTC were noted for 9 of 10 evaluated cancers in the 12 months after initial diagnosis. The SIRs remained elevated 12-59 months after diagnosis for all cancers except leukemia, uterine, and bladder cancers. Increased risks persisted 60-119 months beyond diagnosis for renal (SIR 2.56) and breast cancer (SIR 1.16); and 120+ months for renal cancer (SIR 2.46). Increased SPTC risk after renal and female breast cancers was mostly seen in nonirradiated patients. The principal histology association was between papillary thyroid cancer and renal cell carcinomas.
Conclusions: Many common cancers are associated with increased risk of SPTC beyond 12 months of initial diagnosis. Although this can be explained partly by continued surveillance bias, radiation effects, and known rare familial associations for some tumors, these factors alone are unlikely to explain the persistent, significant two-way association with renal and breast cancers. Additional research is needed to further define the biological and environmental mechanisms underlying these associations.