ESTRO 2024 - Abstract Book

S4336

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

ESTRO 2024

R. Levin-Epstein et al., “Clinical Assessment of Prostate Displacement and Planning Target Volume Margins for Stereotactic Body Radiotherapy of Prostate Cancer,” Front Oncol, vol. 10, Apr. 2020, doi: 10.3389/fonc.2020.00539.

N. Jornet et al., “Multicentre validation of IMRT pre-treatment verification: Comparison of in-house and external audit,” Radiotherapy and Oncology, vol. 112, no. 3, pp. 381–388, Sep. 2014, doi: 10.1016/j.radonc.2014.06.016.

V. Hernandez et al., “What is plan quality in radiotherapy? The importance of evaluating dose metrics, complexity, and robustness of treatment plans,” Radiotherapy and Oncology, vol. 153, pp. 26–33, Dec. 2020, doi: 10.1016/j.radonc.2020.09.038.

2401

Poster Discussion

Proton beam therapy vs online adaption with photons in the treatment of rectal cancer (NCT05883800)

Johanna Austrheim Hundvin 1,2 , Unn Hege Lilleøren 1 , John Alfred Brennsæter 1 , Sara Pilskog 1,2

1 Haukeland University Hospital, Deparment of Oncology and Medical Physics, Bergen, Norway. 2 University of Bergen, Department of Physics and Technology, Bergen, Norway

Purpose/Objective:

The RAPIDO trial in locally advanced rectal cancer (LARC) showed a reduction in distant metastases using short-course radiotherapy and intensified chemotherapy, at the cost of increased bowel toxicity [1]. Both image guided proton beam therapy (PBT) and cone beam CT (CBCT)-based online adaptive photon radiotherapy (ART) have potential to spare adjacent healthy tissue, which is currently being investigated in the PRORECT trial (PBT; NCT04525989) and the TNT-RECORD study (ART; NCT05883800). The goal of this study is to compare the two techniques to find the optimal choice for this treatment regime.

Material/Methods:

49 post-CBCT images from 10 patients (4-5 per patient) with LARC receiving pre-operative image guided volumetric modulated arc therapy (VMAT) were used as reference for actual dose received by the patient. The clinical target volume (CTV), bladder, pelvic bones, lumbosacral spine (LSS), sacral nerve and bowel loops were delineated on all images, except for areas where air-artefacts forced the bowel loop contouring closer to a bowel bag structure. As in the RAPIDO regimen, the prescribed dose was 25 Gy in five fractions. For PBT, margins were added to the gross tumour volumes (GTVs) to account for varying location. The primary GTV was expanded with radial and craniocaudal margins of 15 mm and 20 mm, respectively, while the margin of the nodal GTV was an isotropic expansion of 10 mm. Both structures were cropped towards muscles and bones. The smoothed combination of the GTVs with margins and the CTV (contoured according to international consensus guidelines [2]) comprised the internal target volume (ITV). Robustly optimized (3.5%, 0.6 cm) intensity modulated proton therapy (IMPT) plans were created for a Varian ProBeam machine with two posterior oblique fields in RayStation 2023B (RaySearch Laboratories, Inc.), optimizing on CT images with bowel gas overwritten as water. All plans maintained