Tomotherapy planning station

For VMAT, treatment planning was performed by Pinnacle TPS using the SmartArc optimization algorithm. Treatment machine, dose calculation algorithm, dose grid size and PTV margin setting were the same as in the aforementioned 3DCRT conditions. Beam energy was 10 MV, and one full arc (181 to 179° clockwise) with a collimator angle of 15° was used. All patients received a dose covering 50% volume (D50%) prescription of 76 Gy for PTV in 38 Fr. The dose constraints of the rectum were D2% < 74.4 Gy, D10% < 66.5 Gy and D50% < 20.0 Gy. The dose constraints of the bladder were D15% < 73 Gy, D25% < 70 Gy, D35% < 67 Gy and D50% < 57 Gy.
For HT, treatment planning was performed by Tomotherapy Planning Station (Accuray, Sunnyvale, CA) and TomoHD (Accuray, Sunnyvale, CA) was used as the treatment machine. The plans for HT were created with a dynamic jaw mode using a jaw size of 2.51 cm and pitch size of 0.287 cm in all cases. The convolutional/superposition algorithm with the fine dose grid was used as the dose calculation algorithm. The dose prescription and dose constraints were the same as those in VMAT.

Treatment planning was performed using native and contrast enhanced CT/MRI. Patients were immobilized with individualized thermoplastic head masks. Technical details of CIRT are described elsewhere (24 (link), 25 (link)). Treatment planning for CIRT was performed using Syngo PT Planning, Version 13 (Siemens, Erlangen, Germany) and TomoTherapy^®-Planning Station (Accuray, Sunnyvale, CA, USA) for photon radiotherapy planning. Patients were treated with a fixed horizontal beam/gantry for CIRT utilizing 1-2 coplanar/non-coplanar beams.
All patients received combined IMRT and CIRT. The base plan was performed using a helical intensity-modulated radiotherapy (IMRT) with daily image guidance (TomoTherapy^®, Accuray, Sunnyvale, CA, USA), with 5 daily fractions per week (Figure 1).

For treatment planning Syngo PT PlanningVersion 13 (Siemens, Erlangen, Germany) was applied for CIRT and TomoTherapy^®-Planning Station (Accuray, Sunnyvale, CA, USA) for photon RT (24 (link)). Carbon ion boost to the PTV1 was performed with 18–24 Gy in 3.0 Gy (RBE, relative biological effectiveness) fractions (five to six fractions per week) and photon RT to the PTV2 was performed with 46 Gy to 56 Gy in 2.0 Gy fractions (five fractions per week) for both postoperative and definitive RT due to the local aggressiveness and radioresistance of ACC. Median total equivalent dose to 2.0 Gy fractions (EQD2) prescribed to the median PTV and covering the 95% prescription isodose was 80 Gy (range 71–82 Gy).
Delivered doses to critical structures were limited according to current guidelines as low as possible in order to reduce RT-related toxicity (Supplementary Table 1) (25 (link), 26 (link)). In Table 1, treatment characteristics are shown in detail. In Figure 1, the combined treatment plan for a patient with a pT3pN0 (0/34) ACC of the left parotid gland is shown. CTV2 involved the left parotid space and the left cervical lymphatic drainage (Figures 1A–C) and CTV1 involved the left parotid space only (Figures 1D–F).