Trends of Second Primary Malignancy in Patients with Thyroid Cancer: A Population-Based Cohort Study in Israel.

Background: Thyroid cancer (TC) is the most common endocrine malignancy. TC patients have a good prognosis and a low disease-related mortality rate. Since such patients are often young, they may be at a higher risk for a second primary malignancy (SPM). This study sought to determine the incidence, risk, and types of SPM between 1980 and 2011, and to assess SPM trends over time among Israeli TC patients.
Methods: Data were derived from the Israel National Cancer Registry. Primary TC patients diagnosed during 1980-2009 were followed up for SPM incidence until December 31, 2011. Standardized incidence ratios (SIRs) of observed to expected SPM (based on the general population rates) were calculated using Poisson regression. Analyses were stratified by time period of initial TC diagnosis (1980-1995 and 1996-2009).
Results: A total of 11,538 TC patients were identified. After exclusion of 107 duplicate cases, records of 1032 patients with SPM were analyzed (an SPM incidence of 8.9%). SIRs for all-site SPMs were 1.23 [confidence interval 1.08-1.35] for males and 1.19 [confidence interval 1.10-1.27] for females. SIRs for tumors of the urinary system and prostate were significantly elevated in males, as were SIRs for tumors of the brain, urinary system, breast, and lung in females. Variables associated with increased risk of developing SPMs included a younger age at TC diagnosis, a shorter latency period, being born in Asia/Africa for both sexes, and being born in Israel for females. Compared with the general population, a subanalysis by TC diagnosis during 1980-1995 and 1996-2009 disclosed a higher SPM incidence for the latter time period in males and for both time periods, with a slightly higher SIR for the latter time period in females.
Conclusions: The overall risk of SPM in Israeli TC patients was significantly greater for both sexes compared with the general population, thus identifying TC patients as a high-risk group and calling for caretakers to apply specific follow-up guidelines.