Radiotherapy for primary thyroid cancer as a risk factor for second primary cancers.

Although radiation is considered a risk factor for thyroid cancer, the potential relationship between radiation therapy and the risk of second primary cancer among patients with first primary thyroid cancer has not been evaluated. We identified 26,639 patients with first primary thyroid cancer in the Surveillance, Epidemiology, and End Results (SEER) database from 1973 to 2000. Information on radiation therapy as well as second primary cancers was recorded in SEER. The proportional hazards model was utilized to estimate adjusted risk ratios (RRs) and their 95% confidence intervals (CIs) to assess the potential association between radiation therapy for thyroid cancer and the risk of second primary cancers. With 270,674.33 person-years of follow-up, 1,896 (7.1%) of the 26,639 patients with first primary thyroid cancer developed second primary cancers. Among the second primaries, 35 occurred in the thyroid. No obvious association was observed between radiation therapy and the overall risk of second primary cancer after ten years of follow-up (RR=1.07, 95% CI=0.88-1.30). However, an increased risk was seen for several cancers, including upper digestive system cancers (RR=1.66, 95% CI=1.07-2.57) and myeloid malignancies (RR=3.26, 95% CI=1.39-7.67). Radiation therapy was associated with reduced second cancer risks for thyroid cancer (RR=0.18, 95% CI=0.04-0.76). Beam radiation might be important to the digestive system, radioactive implants might be associated with the male genital system, radioisotopes might have an effect on myeloid malignancies, and combined beam radiation with radioactive implants or radioisotopes might be related to the increased risk of respiratory system cancers. This study suggests that radiation therapy for patients with first primary thyroid cancer might be associated with an increased risk of developing a second primary cancer in the upper digestive system and second primary myeloid malignancies. Radiation therapy for adult patients with thyroid cancer might be associated with a reduced risk of second primary thyroid cancer.