Non-thyroidal second primary malignancy in papillary thyroid cancer patients

The occurrence of non-thyroidal second primary malignancy (NTSPM) in patients with papillary thyroid cancer (PTC) is well documented, but epidemiological data are conflicting.

The aim of this study was to evaluate the incidence of NTSPM in a large series of patients with PTC and to assess its potential risk factors.

Single-center cohort study with retrospective data collection conducted on consecutive PTC patients diagnosed from 1988 to 2018 with a minimum follow-up time of 2 years. NTSPM was defined as any primary malignancy with histological confirmation occurring in an anatomical site other than the thyroid. According to the timing of occurrence, NTSPM were subdivided into anachronous, synchronous or metachronous (diagnosed >6 months before, within 6 months and >6 months after PTC diagnosis, respectively).

We included 773 individuals (83.3% females), median age at PTC diagnosis was 47.0 (IQR: 37.0-58.0) years and median follow-up time was 9.9 (6.2-16.3) years. Incidence of NTSPM was 15.5% (n = 120) and its standard incidence ratio (SIR) was higher when compared to the general population (SIR: 2.70). Family history of malignancy and younger age at diagnosis were associated respectively with 206 and 4% increased risk of developing metachronous neoplasia (HR: 2.06 (95% CI: 1.10-3.86) and 1.04 (95% CI: 1.02-1.05), respectively).

In our series, the occurrence of NTSPM was not uncommon and its incidence was higher compared to the general population. First-degree family history of malignancy was a strong risk factor for multiple primary malignancies.