Risk of second primary thyroid cancer in cancer survivors

A risk factor for thyroid cancer (TC) may be a history of former cancer and cancer therapy. The precise risk of a second primary thyroid carcinoma has not yet been revealed. In this study, we evaluated standardized incidence ratios (SIRs) of second primary thyroid cancer (SPTC) with consideration of different conditions and further analyzed the clinicopathological characteristics and survival of these patients. The cohort was selected from the US Surveillance, Epidemiology, and End Results (SEER) Program between 1975 and 2019. The standardized incidence ratios, morbidity risk, clinicopathological features, and survival of second primary thyroid carcinoma were analyzed. Propensity score matching (PSM) was used to balance covariates. Kaplan-Meier method was performed to assess the survival outcomes. Overall, 7066 patients with SPTC and 83,113 patients with primary TC were identified. The SIR of TC in tumor patients was 1.51/10,000, statistically higher than the natural population (0.94/10,000, P < 0.05). The most significant tumors contributing to the increased SIRs of SPTC were acute lymphocytic leukemia (3.49/10,000), Hodgkin's lymphoma-nodal (3.29/10,000), salivary gland cancer (3.23/10,000), and kidney and renal pelvis cancer (3.05/10,000). The incidence of TC increased significantly in tumor patients who received radiotherapy/chemotherapy before age 35. The age at diagnosis of the SPTC was much older than the primary TC (64.01 vs. 49.55 years, p < 0.001). The SPTC had a higher percentage of histological grades 3/4 (23.14% vs. 15.19%, p < 0.001). Survival analyses demonstrated a worse prognosis for the SPTC group compared to the primary TC group. But after PSM, the survival outcomes of the two groups tended to be equivalent (P = 0.584). The SIRs of TC are higher in tumor patients. The most significant factors contributing to the increased risk of SPTC were some specific former tumors and acceptance of radiotherapy/ chemotherapy before age 35. There was no significant difference in survival between SPTC and primary TC.