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Background/aim: We evaluated the incidence of radiation-induced hypothyroidism and its risk factors in patients with head and neck cancer who underwent radiotherapy using simultaneous integrated boost-volumetric-modulated arc therapy (SIB-VMAT)., Patients and Methods: This retrospective study included 86 patients who received definitive radiotherapy using SIB-VMAT for head and neck cancer. The incidence of ≥ grade 2 hypothyroidism was evaluated. We also evaluated the relationships between hypothyroidism development and clinical factors and thyroid dose-volume parameters., Results: During a median follow-up period of 17 months (range=3-65 months), 31 patients (36.0%, 31/86) developed grade 2 hypothyroidism requiring hormone replacement therapy. No patients experienced ≥ grade 3 hypothyroidism. The cumulative incidences of hypothyroidism at 1 and 2 years after radiation therapy were 24.5% and 38.7%, respectively, with a median onset time of 10.0 months (range=3.0-35.0 months). Thyroid volume (p=0.003), volume of the thyroid spared at 60 Gy (VS60; cut-off value, 5.16 ml; p=0.009), VS70 (cut-off value, 8.0 ml; p=0.007), VS60 equivalent dose in 2 Gy fraction (EQD2; cut-off value, 7.78 ml; p=0.001), and VS70EQD2 (cut-off value, 10.59 ml; p=0.008) were significantly associated with the development of radiation-induced hypothyroidism., Conclusion: Radiation-induced hypothyroidism is not rare in patients with head and neck cancer undergoing radiotherapy using SIB-VMAT. Radiation dose-volume parameters detected in this study may be useful indicators to prevent this complication., (Copyright © 2024 International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved.)