ESTRO 2024 - Abstract Book

S4733

Physics - Quality assurance and auditing

ESTRO 2024

22

Digital Poster

Assessing the targeting accuracy of a linac using an off-axis Winston-Lutz end-to-end procedure

Juan-Francisco Calvo-Ortega 1,2 , María-del-Carmen Aranda Quintero 1

1 Hospital Quirónsalud Málaga, Radiation Oncology, Málaga, Spain. 2 Hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain

Purpose/Objective:

To investigate the accuracy of a linac for treating targets located far (~ 8 cm) from the isocenter.

Material/Methods:

A Varian TrueBeam linac equipped with a high-definition multileaf collimator (HD-MLC) was used. The following off axis Winston-Lutz end-to-end (WL-E2E) test was performed:

1) A BB cube (QUASAR™ IsoCenter Cube, 6 mm-diameter tungsten ball) was randomly placed in the head area of an AnatGe® BrainVacc inmobilization device (Figure 1), and CT scanned using a thickness slice of 1.25 mm.

2) CT images were retrieved in the Eclipse treatment planning system, and the ball was outlined.

3) A non-coplanar off-axis Winston-Lutz (WL) plan was designed in the Eclipse, following the method described by Eagle et al. (J Appl Clin Med Phys. 2023 Feb;24(2):e13816). The WL plan consisted of 12 fields (6 MV FFF) of 2 × 2 cm2 defined by the MLC such that the distance between the planned isocenter and the ball center was ~ 8 cm. The combinations of the gantry (G) and couch (T) angles (IEC scale) were: G270T0, G0T0, G90T0, G180T0, G0T45, G35T45, G135T45, G0T90. G135T90, G0T315, G325T315 and G225T315. 4) The WL plan was ‟ treated” on the linac after performing CBCT-guided alignment. To avoid inter-observer variability, a single technologist performed the CBCT-based alignment in all WL tests. Each field of the WL plan was delivered onto the EPID of the linac. 5) The collected megavoltage images were analyzed using the Pylinac v2.3.2 software, and the following metrics were registered: 1) Maximum ( ‟ maximum deviation”) and medium 2D CAX-BB distances obtained over the 12 images collected for each WL plan. The 2D CAX-BB distance is the radiation center-ball center deviation measured on each portal image. 2) X (Left-Right)-Y (Anterior-Posterior)-Z (Superior-Inferior) shifts (absolute values) in 3D space of the ball center with respect to the isocenter center. Thirty-five off-axis WL-E2E tests were performed by randomly placing the BB cube in the BrainVacc device. In addition, a subset of 10 CT scans was used to designe ten isocentric WL-E2E tests by placing the treatment isocenter at the ball center. To avoid inter-observer variability, a single technologist performed the CBCT-based alignment in all WL-E2E tests. One test was repeated three times to assess intra-observer variability