ESTRO 2024 - Abstract Book

S5612

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

Barts Health, Radiotherapy, London, United Kingdom

Purpose/Objective:

HyperArc is a stereotactic radiosurgery (SRS) solution from Varian™, utilising the Encompass immobilisation device and 6 degrees of freedom (6DOF) couch, with a single button delivery and, up to 4, dynamic couch rotations. The recommended Varian workflow consists of cone-beam CT (CBCT) imaging and optional MV planar imaging for patient localisation.

Current literature exploring patient positional uncertainty is limited – only comparing averages pre- and post treatment, with no comment on extremes of position error (A. Williamson, 2019).

Due to this institution’s extensive experience with Accuray’s CyberKnife system to treat intracranial lesions, which uses frequent intra-fractional imaging, along with the limited data demonstrating patient positional uncertainty during HyperArc treatments, the decision was taken to implement a novel imaging protocol within this department. This decision was also influenced by the employment of the new Encompass immobilisation. The novel imaging approach consisted of 2D/3D stereoscopic kV imaging at each couch angle (0, 315, 45, 90, degrees), prior to each arc delivery, thus quantifying patient setup error, throughout treatment - which is not possible to ascertain when imaging pre- and post-treatment.

This study aimed to determine the safe removal of the intra-fractional imaging, in the context of small PTV margins. Thus, allowing for increased positional assurances for the new technique while reducing concomitant dose.

Material/Methods:

34 sequential patients treated with HyperArc from May 2022 to August 2023 were included in this study. All were immobilised supine in the Encompass™ SRS masks (QFix). Prescription doses ranged from 5 to 24 Gy per fraction - all GTVs had a 0.1cm margin to PTV. Following online CBCT imaging at couch zero, 6DOF couch corrections were made with a zero action-level. The novel on-line positional verification was performed using orthogonal-paired stereoscopic kV images for each couch angle using Varian 2D/3D matching and all residual 6DOF couch corrections applied with a zero action-level. This study evaluated the magnitude of the residual patient position translational error determined by 2D/3D matching at each couch position and compared with the departmental tolerance of +/-0.08cm to assess the frequency of the patient breaching a positional tolerance.

Results:

In total, 460 stereoscopic kV image pairs taken. 441 (95.9%) showed positional errors within departmental tolerance. 19 (4.1%) would have exceeded tolerance, had subsequent couch corrections not been applied. Tolerance breaches incidences occurrance: Arc 1 (couch 0): 2

Arc 2 (couch 315): 3 Arc 3 (couch 45): 11 Arc 4 (couch 90): 3 It should be noted that most incidences occurred for the largest couch rotation (between arcs 2 and 3). A limitation of this study is that the effect of contributory rotations was