Increased risk of certain second primary malignancies in patients treated for well-differentiated thyroid cancer.

BACKGROUND: The objective was to evaluate the incidence of second primary malignancies (SPMs) in thyroid cancer patients compared to age- and gender-matched controls without thyroid cancer from the general population of the same region. METHODS: Tampere and Oulu University Hospitals treated 910 patients with well-differentiated thyroid cancer during 1981-2002. The Finnish cancer registry provided follow-up data for patients and controls (n = 4542) for an average of 16 years. The incidence of invasive malignancies per 10 000 person-years was calculated and compared between patients and controls. The follow-up period ended December 31st, 2011. RESULTS: Young patients <40 years [Rate Ratio (RR) 1.73, p = 0.037] and patients diagnosed since 1996 (RR 1.51, p = 0.029) had an increased incidence of SPMs. Patients had an increased risk of sarcomas and soft tissue tumours (RR 4.37, p = 0.004) and haematological and lymphatic malignancies (RR 1.87, p = 0.035), especially non-Hodgkin lymphomas (RR 2.78, p = 0.035). The overall incidence of SPMs was not statistically higher in patients (109 SPMs/910 patients vs. 500 SPMs/4542 controls, RR 1.12, p = 0.269). Most patients were radioiodine-treated (81 %). The risk of SPMs with low cumulative radioiodine doses was RR 0.94 (≤ 3.7 GBq, p = 0.650) and with high doses RR 1.37 (>3.7 GBq, p = 0.143). Cumulative radioiodine dose increased during the study period. CONCLUSIONS: The overall incidence of SPMs was not higher in patients than in controls. The incidence of SPMs in thyroid carcinoma patients was higher in patients <40 years old and patients diagnosed since 1996. The incidence of sarcomas and lymphomas was higher in patients than in controls.