Edge accelerator

Throughout this paper we use the IEC 61217 coordinate convention. For an observer standing at the foot of the treatment table and facing the gantry the positive x‐, y‐, and z‐ directions extend, respectively, left to right in the crossplane, toward the gantry along the caudal‐cephalad direction in the in‐plane, and toward the ceiling. The couch angle is 270° when the couch is in the 9 o'clock position, and increases with counterclockwise (CCW) rotation to 360°/0° as the home position is approached, and further increases from 0° to 90° when the couch reaches the 3 o'clock position.
All measurements were done on an Edge accelerator equipped with a 120‐leaf MLC (Varian Medical Systems, Palo Alto, CA). Dose distributions were computed on the Eclipse, Version 13.6.26 TPS with AAA algorithm provided by the same manufacturer.

To characterize how the Mevion S250i proton out‐of‐field dose compares to a modern (6 MV) VMAT treatment plan for a similar target volume, a dosimetrically similar plan was created using the Eclipse Treatment Planning System (Varian Medical Systems, Palo Alto, CA) employing a Varian Edge accelerator. Out‐of‐field contribution was measured using optically stimulated luminescent detectors (OSLDs). As in the proton plan, the prescription dose was 66 Gy in 33 fractions (2 Gy per fraction) to the 6 cm target volume and the plan was optimized to provide desired coverage of the PTV (95% of the PTV received the full prescription dose). The photon plans were delivered on a Varian Edge linear accelerator. OSLDs were placed at the same positions as the edge of the WENDI (47, 39.5, and 32 cm from isocenter). The OSLDs were annealed prior to treatment to ensure that the residual dose was minimized and the fields were repeated five times to ensure an adequate measurement signal.