1004 - Radiation-Induced Hypothyroidism in Patients with Breast Cancer after Hypofractionated Radiotherapy: Updated Results from a Prospective Cohort Study
Presenter(s)
X. Zhao1, H. Fang1, H. Jing1, Y. Tang1, Y. Song1, Y. Liu1, J. Jin2, B. Chen3, S. Qi4, Y. Tang5, N. Lu1, N. Li4, Y. LI2, and S. Wang1; 1Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China, 2State Key Laboratory of Molecular Oncology and Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences (CAMS) and Peking Union Medical College (PUMC), Beijing, China, 3Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, Beijing, China, 4State Key Laboratory of Molecular Oncology and Department of Radiation Oncology, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China, 5State Key Laboratory of Molecular Oncology and Department of Radiation Oncology, National Cancer Center/ National Clinical Research Center for Cancer/ Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China
Purpose/Objective(s):
To present updated long-term follow-up results from a prospective cohort study evaluating the incidence and risk factors of radiation-induced hypothyroidism (RHT) after adjuvant hypofractionated radiotherapy (RT) in breast cancer, and to identify an optimal dose constraint for the thyroid gland.Materials/Methods:
Consecutive eligible patients treated with adjuvant hypofractionated RT were prospectively enrolled. Thyroid function tests were obtained at baseline and regularly after RT (every 3–4 months for 2 years, then every 6 months up to 5 years). Endpoints included RHT and RHT requiring thyroid hormone replacement therapy. RHT was defined as two consecutive elevations of serum thyroid-stimulating hormone with decreased or normal free thyroxine. Cumulative incidence was estimated from the end of RT using the Kaplan–Meier method and compared by log-rank test. Multivariate analyses were performed using Cox proportional hazards regression. A Cox model with penalized splines assessed the dose–response relationship between thyroid mean dose (Dmean) and RHT. Two-sided P < 0.05 was considered significant.Results:
A total of 500 patients treated from 2017–2020 were analyzed. All received chest wall/breast irradiation with or without regional nodal irradiation (43.5 Gy in 15 fractions). Among them, 369 (73.8%) received supraclavicular nodal radiotherapy (SCRT). Elevated baseline TSH was present in 82 patients (16.4%). At a median follow-up of 50.9 months (range, 1.0-98.0), 147 patients (29.4%) developed RHT and 67 (13.4%) required hormone replacement therapy. The 5-year cumulative incidence of RHT and RHT requiring hormone replacement was 33.3% and 15.2%, respectively. SCRT was associated with higher 5-year incidence of RHT (38.9% vs. 14.5%, P < 0.001) and RHT requiring hormone replacement (19.0% vs. 6.2%, P < 0.001). On multivariate analysis, elevated baseline TSH and increasing thyroid Dmean (as a continuous variable) were independent predictors of both RHT and RHT requiring hormone replacement therapy. A thyroid Dmean > 21 Gy was identified as the threshold for predicting RHT (HR = 2.30, P < 0.001), with a significantly higher 5-year incidence compared with Dmean = 21 Gy (41.1% vs. 20.5%, P < 0.001). A thyroid Dmean > 28 Gy was identified as the threshold for predicting RHT requiring hormone replacement (HR = 2.40, P = 0.001), corresponding to a significantly higher 5-year incidence compared with Dmean = 28 Gy (27.6% vs. 13.1%, P < 0.001).Conclusion:
RHT is a common late toxicity after hypofractionated RT for breast cancer. Maintaining the thyroid mean dose below 21 Gy may reduce the risk of RHT, while a constraint below 28 Gy may decrease the likelihood of RHT requiring thyroid hormone replacement therapy.