ESTRO 2024 - Abstract Book

S4337

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

ESTRO 2024

target coverage (ITV: D 98% ≥ 95%, GTV: D 98% ≥ 100%) and acceptable sparing of organs at risk (OARs) when the dose was recalculated on the not-overwritten CT and 45 robustness scenarios (3.5%, 0.6 cm). The PBT plans were then recalculated on corrected post-CBCTs for treatment evaluation. For ART, the planning target volume (PTV) was created by expanding the CTV with an isotropic margin of 4 mm. Using 9-12 equidistant fields in the Ethos research software (the Emulator, Varian, a Siemens Healthineers company) online ART was simulated on the respective pre-CBCTs. Doses from the simulated ART treatments were then overlaid on the six-degree bone-matched post-CBCT for dose evaluation in the Eclipse treatment planning system (Varian, a Siemens Healthineers company). The treatment was considered robust if average D 99% to the CTV exceeded 95% on the post-CBCTs. The treatment averaged bowel volume receiving 7.5 Gy (V 7.5Gy ) and 20 Gy (V 20Gy ) (assuming RBE=1.1 for PBT) on the post-CBCTs were analysed using Wilcoxon signed rank test, as both low and high dose levels can impact bowel toxicity [3]. The doses to the bladder, pelvic bones, lumbosacral spine (LSS), and sacral nerve were also compared.

Results:

Both techniques resulted in an average D 99% ≥ 96% to the CTV in all patients, except for patient 10 (Figure 1). In the case of the outlier patient, the target volume was clearly shifted caudally, which in a clinical setting would be discovered by a pre-CBCT and the treatment adjusted thereafter before irradiation. PBT showed significantly smaller low-dose volume (median PBT-V 7.5GyRBE =103 cc, ART-V 7.5Gy =226 cc, p<0.01), while ART resulted in the smallest high dose volume (median PBT-V 20GyRBE =75 cc, ART-V 20Gy =58 cc, p<0.05). The differences in dose to all OARs are shown in Figure 2.