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

Purpose: The aim of this investigation was to study previous radiother apy of malignant diseases as a risk factor for thyroid cancer. Methods and Materials: By using the Swedish Cancer Registry all cases of thyr oid cancer with another malignant disease at least one year previously and living within the catchment area of our hospital were traced. Dur ing 1959-1989 a total of 1056 cases of thyroid cancer were identified. Of these 37 had had another previous malignant disease and they const ituted the cases in this study. As controls four persons with at least two malignant diseases, thyroid cancer excluded, were selected for ea ch case from the same cancer registry. Results: Ten (27.0%) of the 37 patients with thyroid cancer as a second tumor had earlier been irradi ated with the treatment dose including the thyroid gland as compared w ith 34 (24.5%) of the 139 control patients. Eight of the ten cases wit h previous irradiation of the thyroid gland had papillary cancer. The median latency was 13 years. The estimated radiation dose in the thyro id 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 cance r. The weighted odds ratio was calculated to 2.3 with confidence inter val = 0.5-8.9. Conclusion: This case-control study gave a non signific antly increased odds ratio for thyroid cancer in patients with externa l radiotherapy including the thyroid gland.