ESTRO 2023 - Abstract Book

S1442

Digital Posters

ESTRO 2023

Purpose or Objective The isocentricity(C iso ) of a machine is defined, and QA’d by using only the 6MV beam. This study investigated the C iso variations between 6 MV and flattening filter-free (FFF) treatment beams on C-arm Linacs. If C iso discrepancies persist between beams, OAR doses could increase and be clinically significant for stereotactic ablative radiotherapy (SABR). Materials and Methods Isocentricity was measured on 6MV, 6FFF, and 10FFF beams of five Varian TrueBeams using a Winton-Lutz (W-L) type test. A phantom with a 6.35mm steel ball bearing was placed approximately at the isocenter(ISO) using the lasers, ISOL (Fig 1a). A 2.0x2.0cm MLC-defined aperture was used for the irradiation (Fig 1b). The C iso of the radiation ISO (ISOR) of each beam was measured by exposing 27 fields at different gantry, collimator, and couch angles as listed in Fig 1c. The ISOR was determined as the position of the average aperture centers of the images taken at the collimator angles of 90 and 270 at the four cardinal angles. Thus the displacement of the sphere’s placement (ISOL) from the true radiation isocenter, ISOR, can be determined. Fixing the gantry and collimator at 0 degrees, the couch was rotated from 270 to 90 degrees, with W L images acquired every 15 degrees. Assuming the couch rotates about the projection of the ISOR, the arc through which the ball bearing should move is determined and indicated in blue in Fig 1d and e. The actual trajectory, captured from the W-L images, is indicated in orange. The couch C iso was then determined by the maximum deviation of the ball bearings from the expected trajectory. The overall C iso error, ∈ iso, was determined by the maximum value of the vector sum of the collimator, gantry, and couch C iso as per MPPG 9a. An in-house program, WINZLOCK, was used to analyze the images and generate these values.

Results A total of 14 energy-specific ∈ iso were identified from the five treatment machines (Table 1). Four out of nine (44.4%) non-6x ∈ iso differed by 0.2 to 0.4mm from the 6x values. Three (marked in red in Table 1) were more than 0.3mm and close to or exceeded the recommended tolerance of 1.0mm. Fig 1d & e show examples of the C iso difference between the 6x and 6FFF of 245, resulting in a 0.3mm difference in ∈ iso . A similar difference was observed for the 10FFF. These differences were independently verified with the vendor's IsoLock measurement. Since 6FFF beam on 245 is clinically used, the beam position of that machine was subsequently tuned, reducing ∈ ISO to 0.7 mm.