ESTRO 2024 - Abstract Book

S4363

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Quantification of the targeting accuracy of the Synchrony modality on Radixact machine

Marie Fargier-Voiron, Anthony Nagy, Nicolas Perichon, Cedric De Marco, Mireille Conrad, Oscar Matzinger, Maud Jaccard

Swiss Radio Oncology Network, Clinique Générale-Beaulieu, Geneva, Switzerland

Purpose/Objective:

The Radixact Synchrony modality enables real-time correction for tumor motion during tomohelical treatments delivery by adjusting jaws and/or MLC positions [1]. The recent AAPM TG306 report recommends to perform End-to End (E2E) tests analyzed with the Gamma Index methodology [2], which prevents discrimination between dose and geometric errors. Besides, the Winston-Lutz test procedure cannot be easily applied to tomohelical treatment to characterize the machine precision. Thus, the aim of this study is to quantify the Synchrony targeting accuracy by adapting the Cyberknife E2E methodology to the Radixact machine.

Material/Methods:

Real-time tracking is implemented on the Radixact machine by adapting the jaws position in case of a displacement in the longitudinal direction, or by correcting the MLC aperture in case of a vertical or lateral displacement of the target. Target position is detected thanks to a 2D-kV image system mounted perpendicularly to the treatment beam, either directly (for lung tumors) or indirectly (using implanted fiducials as surrogates). All available tracking methods were evaluated in this study: fiducial (FID), fiducial with respiratory motion (FIDResp), and lung with respiratory motion (LResp). Plans were generated with RayStation12A, with jaws settings of 1 and 2.5 cm, on three different anthropomorphic phantoms according to the considered tracking methods. These phantoms allowed the insertion of two perpendicular radiochromic films centered on a sphere representing the target. The prescription dose of 4.2 Gy was set to the 70% isodose around the target. Calculated dose distributions were strictly identical between plans without and with Synchrony. Phantom setup was performed with a 3D-kVCT acquisition (ClearRT), using the finest resolution parameters. Previously, coordinate coincidence was checked between kV imaging and treatment beam, respecting the recommended tolerance of 1 mm. For each tracking method investigated, multiple irradiations were performed, as described below . The reference targeting accuracy was first determined (Synchrony off, no motion). Then different scenarios were compared to the reference one, from the simplest (Synchrony on, no target motion) to the most complex (Synchrony on, multiple simultaneous target motions in different directions). The targeting accuracy was determined by calculating the difference on each direction between the dose distribution center and the geometric center of the films, using Accuray’s E2E software. The total targeting error (TTE) was determined as the quadratic sum of the differences on each direction.

Results:

TTE values were <1 mm in the reference conditions (no Synchrony, no motion) on each phantom (FID: 0.3 mm; FIDResp: 0.5 mm; LResp: 0.8 mm). With the simple activation of Synchrony and no motion, TTE exceeded 1 mm, with