Secondary Salivary Gland Malignancy in Thyroid Cancer: A United States Population Based Study

BACKGROUND: There is an increased risk of second primary malignancies with thyroid cancer. However, the risk and characters of secondary salivary gland malignancy (sSGM) in patients with thyroid cancer have not been evaluated before. METHODS: We used the Surveillance Epidemiology and End Results (SEER) 18 registry to identify thyroid cancer patients from 1973 to 2014. We then calculated the risk of sSGM using standardized incidence ratio and excess risk. Separately, all cases of primary salivary gland malignancy (pSGM) diagnosed between 1973 - 2014 were extracted from the SEER 18 registry, and their characteristics compared with sSGM using independent samples t-test for continuous variables and Chi-square tests for categorical variables. RESULTS: There were a total of 68,339 cases of primary thyroid cancer. Of these, 18 patients developed sSGM with the observed to expected ratio being 3.58 (95% CI: 2.12 to 5.65; P < 0.05) and excess risk being 0.48 per 10,000 population. The incidence of sSGM remained higher between 6 months to 10 years from the time of diagnosis of thyroid carcinoma. The risk of developing sSGM was significantly higher if they were below 60 years of age (O/E: 4.51; 95% CI: 2.33 - 7.88; P < 0.05), were females (O/E: 4.91; 95% CI: 2.80 - 7.97; P < 0.05), were whites (O/E: 3.04; 95% CI: 1.62 - 5.1 9; P < 0.05), had well-differentiated thyroid carcinoma (O/E: 9.70; 95% CI: 3.90 - 19.98; P < 0.05) or were treated with radioactive iodine (O/E: 5.26; 95% CI: 2.72 - 9.19; P < 0.05). While the proportion of females developing sSGM was significantly greater than those developing pSGM (88.9% vs. 44%; P < 0.05), there was no statistical difference between pSGM and sSGM in terms of the age at diagnosis, the proportion of patients diagnosed before 60 years of age, anatomic site of origin or the histological grade of tumor. CONCLUSIONS: Patients with thyroid cancers are at an increased risk of developing sSGM than the general population. This risk is greater if the person is below 60 years of age, female, white, with well-differentiated thyroid carcinoma or is treated with radioactive iodine.