Objective:To evaluate the effect of radiotherapy on hypothyroidism (HT) in patients with breast cancer after radical surgery, and to establish a HT prediction model.Methods:Clinical data of 296 patients who received radiotherapy after radical mastectomy and followed up regularly in Affiliated Cancer Hospital of Xinjiang Medical University from January 2015 to May 2023 were retrospectively analyzed. Baseline clinicopathological information was extracted and thyroid dose volume histograms were reviewed to obtain dosimetric parameters such as thyroid volume, thyroid D mean, D min, D max and V 15 Gy-V 50 Gy, etc. The thyroid function levels of all patients were detected before radiotherapy and at 3, 6, 9, 12, 18, 24, 36, 48 and 60 months after radiotherapy, and the incidence of HT after treatment was calculated to determine the effect of radiotherapy on thyroid function. Cox regression model was used for univariate and multivariate analyses to identify the factors associated with HT and establish the predictive model. The optimal cut-off value of influencing factors was calculated by using the receiver operating characteristic curve. The cumulative incidence curve was drawn by Kaplan-Meier method. Inter-group differences were compared by using log-rank test. Results:At a median follow-up of 36 months, HT occurred in 130 patients (43.92%). The peak of HT occurred approximately 12 (9-18) months after radiotherapy. Univariate analysis showed that neoadjuvant chemotherapy ( HR=1.736, 95% CI=1.222-2.467, P=0.002), thyroid D min ( HR=1.001, 95% CI=1.001-1.001, P<0.001) and thyroid D mean ( HR =1.001, 95% CI=1.001-1.002, P<0.001) were the risk factors for HT. Multivariate analysis showed that thyroid D mean was correlated with the occurrence of HT ( HR=1.001, 95% CI=1.001-1.001, P=0.002). The optimal cut-off value of thyroid D mean was 2771.66 cGy, and the incidence of HT was statistically different between high-risk group (D mean>2771.66 cGy) and low-risk group (D mean≤2771.66 cGy) ( P<0.001). Conclusions:Radiotherapy after radical surgery is associated with the occurrence of HT in patients with breast cancer. Minimizing the thyroid D mean below 2771.66 cGy is an effective method to protect thyroid function. It is recommended to start monitoring thyroid function no later than 9 months after radiotherapy in patients receiving neoadjuvant chemotherapy.

Objective:To evaluate the effect of radiotherapy on hypothyroidism (HT) in patients with breast cancer after radical surgery, and to establish a HT prediction model.Methods:Clinical data of 296 patients who received radiotherapy after radical mastectomy and followed up regularly in Affiliated Cancer Hospital of Xinjiang Medical University from January 2015 to May 2023 were retrospectively analyzed. Baseline clinicopathological information was extracted and thyroid dose volume histograms were reviewed to obtain dosimetric parameters such as thyroid volume, thyroid D mean, D min, D max and V 15 Gy-V 50 Gy, etc. The thyroid function levels of all patients were detected before radiotherapy and at 3, 6, 9, 12, 18, 24, 36, 48 and 60 months after radiotherapy, and the incidence of HT after treatment was calculated to determine the effect of radiotherapy on thyroid function. Cox regression model was used for univariate and multivariate analyses to identify the factors associated with HT and establish the predictive model. The optimal cut-off value of influencing factors was calculated by using the receiver operating characteristic curve. The cumulative incidence curve was drawn by Kaplan-Meier method. Inter-group differences were compared by using log-rank test. Results:At a median follow-up of 36 months, HT occurred in 130 patients (43.92%). The peak of HT occurred approximately 12 (9-18) months after radiotherapy. Univariate analysis showed that neoadjuvant chemotherapy ( HR=1.736, 95% CI=1.222-2.467, P=0.002), thyroid D min ( HR=1.001, 95% CI=1.001-1.001, P<0.001) and thyroid D mean ( HR =1.001, 95% CI=1.001-1.002, P<0.001) were the risk factors for HT. Multivariate analysis showed that thyroid D mean was correlated with the occurrence of HT ( HR=1.001, 95% CI=1.001-1.001, P=0.002). The optimal cut-off value of thyroid D mean was 2771.66 cGy, and the incidence of HT was statistically different between high-risk group (D mean>2771.66 cGy) and low-risk group (D mean≤2771.66 cGy) ( P<0.001). Conclusions:Radiotherapy after radical surgery is associated with the occurrence of HT in patients with breast cancer. Minimizing the thyroid D mean below 2771.66 cGy is an effective method to protect thyroid function. It is recommended to start monitoring thyroid function no later than 9 months after radiotherapy in patients receiving neoadjuvant chemotherapy.