Radiation-induced hypothyroidism in patients with breast cancer: a retrospective analysis of 243 cases.

This study aims to estimate the incidence of hypothyroidism (HT) and to evaluate the predictors affecting the development of HT after radiotherapy (RT) for breast cancer, with a focus on radiation dose-volumetric parameters. Between 2009 and 2015, 243 patients undergoing RT for breast cancer were retrospectively analyzed. Free triiodothyronine (FT3), free thyroxin (FT4), and thyrotropin (TSH) were monitored before and after RT. The relation between the doses to thyroid gland (D, D, D), percentage of thyroid volume receiving > 10 Gy, 20 Gy, 30 Gy, 40 Gy, and 50 Gy (V10 to V50), absolute thyroid volume, and HT were analyzed. The risk of HT according to radiation fields and the other clinic factors were also evaluated. The median follow-up was 41 (range; 6 to 130) months. Sixty-seven percent of the patients received RT to the breast/chest wall and ipsilateral supraclavicular fossa. Of 243 patients, 51 (21%) were diagnosed with HT. The median time to the onset of HT was 27 (range; 5 to 64) months. There were no significant relationships observed between D or V10 to V50 and HT. The surgery type, clinical stage, nodal status, RT field, D, and D were statistically significant predictors for HT in univariate analysis. The D was the only prognostic factor predicting HT in multivariate analysis, and D > 21 Gy was a threshold value for the evolvement of HT. In this study, we present evidence that postoperative irradiation of patients with breast cancer may frequently lead to HT. Patients who have received RT for breast cancer, especially irradiation on the supraclavicular region, may require thyroid function screening after RT.