EXTERNAL RADIOTHERAPY PRIOR TO THYROID-CANCER - A CASE-CONTROL STUDY

Purpose: The aim of this investigation was to study previous radiotherapy of malignant diseases as a risk factor for thyroid cancer. Methods and Materials: By using the Swedish Cancer Registry all cases of thyroid cancer with another malignant disease at least one year previously and living within the catchment area of our hospital were traced. During 1959-1989 a total of 1056 cases of thyroid cancer were identified. Of these 37 had had another previous malignant disease and they constituted the cases in this study. As controls four persons with at least two malignant diseases, thyroid cancer excluded, were selected for each case from the same cancer registry. Results: Ten (27.0%) of the 37 patients with thyroid cancer as a second tumor had earlier been irradiated with the treatment dose including the thyroid gland as compared with 34 (24.5%) of the 139 control patients. Eight of the ten cases with previous irradiation of the thyroid gland had papillary cancer. The median latency was 13 years. The estimated radiation dose in the thyroid varied between 3 and 40 Gy. External radiotherapy gave a crude odds ratio of 1.1 with 95% confidence interval = 0.5-2.8 for thyroid cancer. The weighted odds ratio was calculated to 2.3 with confidence interval = 0.5-8.9. Conclusion: This case-control study gave a non significantly increased odds ratio for thyroid cancer in patients with external radiotherapy including the thyroid gland.