Radiotherapy
Helical tomotherapy (HT) was delivered once daily, 5 days per week, as previously described [5]. In brief, the planning dose at D95 (dose received by 95% of the target volume) was set at 67.5 grays (Gy) for the planning gross target volume of the primary tumor (pGTVnx) and the planning gross target volume of the metastatic lymph node (pGTVnd). The planning target volume (PTV) was set at 60 Gy and the PTV2 was set at 54 Gy in 30-33 fractions. No more than 5% of PTV volume received more than 110% of the prescribed dose. Dose-volume constraints for organs at risk (OARs) were set as follows: (1) parotid gland V30 <50% or Dmean ≤28 Gy; (2) brainstem Dmax ≤54 Gy; (3) spinal cord Dmax ≤45 Gy; (4) optic nerve Dmax ≤54 Gy; (5) temporo-mandibular joint Dmax ≤60 Gy; and (6) lens Dmax ≤5 Gy. HT plans were developed for a field width of 2.5 cm, a pitch of 0.30–0.38, and a modulation factor equal to 2.0–3.0. During radiation therapy, patients underwent megavoltage computed tomography (MVCT) imaging at least once each week to verify patient setup. The imaging frequency was determined by the magnitude of setup errors from initial daily scans. Carotid radiotherapy dose was the same among different groups.