Exposure to radiotherapy for first primary cancer as a risk factor for second primary thyroid cancer: A case-control study.

Background: Second primary neoplasms are associated with high mortality and morbidity rates in cancer survivors successfully treated for the first malignancy. Studies suggested an association between the type of first neoplasm and risk of subsequent thyroid cancer, with part of this risk attributable to exposure to radiotherapy during treatment of the first primary tumor. This study aimed to determine whether radiotherapy is a risk factor for thyroid cancer in patients previously treated for another neoplasm.

Methods: This retrospective case-control study included patients diagnosed with their first cancer between 2007 and 2017. Patients who subsequently developed thyroid cancer as a second primary neoplasm were defined as "cases", and patients who did not develop a second cancer were defined as "controls". Exposure to radiotherapy was the primary risk factor of interest; other risk factors were the site to which radiotherapy was delivered and the first neoplasm type.

Results: Exposure to radiotherapy was associated with an increased risk of thyroid cancer (odds ratio [OR]=2.410, 95% confidence interval [CI]: 1.219-4.764), in particular, in women (OR=3.121, 95% CI: 1.232-7.907) and in patients receiving radiotherapy to the thorax (OR=6.298, 95% CI: 2.581-15.370). The median survival time from first cancer recovery to thyroid cancer occurrence was 63.80 months; there was no difference in survival between patients who did and did not receive radiotherapy (P=0.899).

Conclusion: Radiation to the thorax can increase the risk of thyroid cancer as a second neoplasm among patients with cancer successfully treated for their first cancer.