As1200

Figure 1a shows the experimental setup for the evaluation of beam‐positioning accuracy. TrueBeam STx (Varian Medical Systems), with a 2.5 mm leaf‐width HD120 multileaf collimator (MLC) and carbon fiber couch (Brainlab, Munich, Germany), was used to evaluate the beam‐positioning accuracy. A WL test‐dedicated phantom with a 3 mm diameter steel ball (TM‐WINS: R‐Tech Inc., Tokyo, Japan) was placed on the couch top and aligned with the room laser beam. This study evaluated the beam‐positioning accuracy using the WL test and EPID‐based measurements.¹⁶ The EPID (aS‐1200, Varian Medical Systems) had a pixel pitch of 0.336 mm and image size of 1190 × 1190 pixels. Here, the EPID was set with a source‐to‐detector distance of 1500 mm and pixel size of 0.244 mm at the isocenter plane. MLC‐shaped square fields of 10 × 10 mm² were created separately at the isocenter and off‐axis positions. Figure 1b shows an example of an MLC‐shaped square field. Here, MLC‐shaped square fields were created at each off‐axis position (distances of 0, 25, 50, 75, and 100 mm away from the isocenter position) in the Y direction in the beam's eye view (BEV). For each gantry angle, 17 patterns of MLC‐shaped fields were observed at positions of 0, 25, 50, 75, and 100 mm in the X and Y directions.

Varian's ring‐shaped Halcyon linac (Varian Medical Systems, Palo Alto, CA, version 2.0) is designed for image‐guided radiation therapy (IGRT). The treatment unit is equipped with a double stack multi‐leaf collimator (SX2 MLC) with individual leaf widths of 1.0 cm when projected at isocenter. Both layers, which possess a relative offset of 0.5 cm, can independently contribute to beam modulation. The preconfigured system supports only a single flattening‐filter‐free (FFF) beam at nominal beam energy of 6MV. For fluence measurement the Halcyon linac is equipped with a light sensitive a‐Si photodiode based EPID (aS1200, Varian Medical Systems, Palo Alto, CA). Its active area covers 43 × 43 cm² with a 154 cm source‐to‐imager distance (SID) and a 1280 × 1280 pixel matrix, translating into an imaging area of 28 × 28 cm² and a resolution of about 45 pixels/cm at isoplane.