Trial design and procedure
The trial was conducted between March 2018 and September 2022. Participants were recruited between March 2018 and December 2019. A total of 92 patients met our criteria were enrolled and assigned to the thyroid dose-volume constraint and non-restricted groups (1:1) using simple randomization. (1) The serial numbers of 92 subjects were proposed; (2) Numbers were randomly generated; (3) Subjects with odd random numbers were assigned to group A and even numbers to group B; (4) The thyroid dose volume was restricted in group A and not in group B.Patients with stage T1N0M0/T2N0M0 received radical radiotherapy alone. Patients with T1N1M0/T2N1M0 were treated with single agent chemotherapy combined with radiotherapy. Stage III-IVB patients were treated with a combination regimen that included concurrent and/or sequential radiotherapy and cisplatin-based chemotherapy. Radiotherapy was performed using 6 MV photons via IMRT. The thyroid dose-volume constraint group was defined as the percentage of thyroid volume receiving more than 40 Gy (V40) less than 85%. The non-restricted group was not restricted. Furthermore, CT and MRICT fusion planning was performed for all patients with co-registration software (Oncentra MasterPlan version 1.5, Nucletron B.V., Veenendaal, The Netherlands).Sample size was calculated based on historical data suggesting an incidence of 21.4% and 50.4% in the dose-constraint and unrestricted group, respectively. Assuming a dropout rate of 10%, a sample size of 90 patients (45 in each group) was required with 80% power and two-sided α of 0.05.