ESTRO 2023 - Abstract Book

S177

Saturday 13 May

ESTRO 2023

conflicting evidence regarding the effectiveness of SABR in OMD. An important difference between breast cancer and other entities (such as prostate and lung cancer) is the fact that PET/CT imaging is used less frequently during staging. In the current study, we aim to investigate the impact of 18F-FDG-PET/CT on the pattern of OMD as compared to conventional CT staging. Materials and Methods All patients in the current study underwent 18F-FDG-PET/CT imaging for breast cancer between 12/2006 and 01/2022 at the Technical University of Munich (TUM). Inclusion criteria were OMD (defined as ≤ 5 distant metastases) in 18F-FDG-PET/CT and/or CT imaging alone. 81 patients were included in the analyses with 158 18F-FDG-PET/CT images. For all patients, CT imaging findings were reported separately from 18F-FDG-PET findings, allowing comparison of conventional CT imaging and 18F-FDG-PET/CT. For each lesion suspicious of metastasis, we assessed the congruence between CT only and 18F-FDG- PET/CT. Data was analyzed using SPSS. The study was approved by the local Ethics Commission (2022-432-S-NP). Results According to the ESTRO/EORTC OMD classification 20% (n = 32) of the cases were De-novo OMD, 43% (n = 68) repeat OMD and 37% (n = 58) induced OMD. In only 32% (n = 50) of the cases, an identical OMD pattern was found in conventional CT imaging and 18F-FDG-PET/CT. In the remaining 108 cases (68 %) there were either differences regarding the number or the localization of the metastases (or both). In 44 % (n = 70) of the cases, the definition of OMD was met only in one of the two imaging modalities (18F-FDG-PET/CT or CT only). In 35% (n = 55) of the cases 18F-FDG-PET/CT revealed additional metastases which were not detected in CT imaging. Conclusion Our data demonstrate that the use of 18F-FDG-PET/CT has an important impact on the detected pattern of OMD. This should be considered during interpretation of existing evidence of “OMD breast cancer” and in future trials on the topic. PD-0242 Comparison of static and dynamic proton arc treatment planning algorithms in oropharyngeal cancer B.A. de Jong 1 , L. Zhao 2 , V. Wase 3 , O. Marthin 3 , X. Ding 2 , E. Engwall 3 , E.W. Korevaar 1 , J.A. Langendijk 1 , S. Both 1 1 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands; 2 Beaumont Health System, Rose Cancer Center, Royal Oak, Department of Radiation Oncology, Michigan, USA; 3 RaySearch Laboratories AB, Research and Development, Stockholm, Sweden Purpose or Objective Compared with IMPT, proton arc treatment (PAT) plan optimization benefits from more degrees of freedom, due to an increased number of employed gantry angles. This results in a potential dose reduction to healthy tissues, especially for complex target geometries as found in oropharyngeal cancer (OPC) treatment. Multiple algorithms have been proposed to optimize PAT using approaches with “static” gantry position or “dynamic” gantry movement during dose delivery. The PAT delivery time for clinically emerging dynamic approaches, are expected to be shorter relative to static approaches. We compared expected plan toxicity and inter-fraction robustness, between IMPT plans and PAT plans generated using one static and two dynamic treatment planning algorithms for OPC patients. Materials and Methods Five OPC patients were selected that qualified for PT based on model-based selection, with IMPT plans superior to the VMAT plans in terms of toxicities. Expected plan toxicity relative to VMAT ( Δ NTCP) for the proton plans was determined using NTCP models for grade ≥ 2 dysphagia and patient-rated xerostomia 6 months after treatment. The static PAT plans were produced using an energy layer reduction (ELR) algorithm. The dynamic PAT plans were produced both with a Spot scanning Proton Arc (SPArc) algorithm including energy sequencing and with an Early Layer and Spot Assignment (ELSA) algorithm. All plans were robustly optimized using 3mm/3% setup/density uncertainty settings, with identical optimization objectives in a research version of RayStation. Inter-fraction robustness was evaluated on a fraction-wise (weekly repeated CT) and course-wise (accumulated over all weekly repeated CTs) basis, by evaluating target coverage in a voxel-wise minimum distribution based on 28 scenarios with 1mm/3% setup/density uncertainty settings. Fraction-wise target coverage was compared to the clinical threshold for adaptive replanning in IMPT: D98% < 93.5% of the prescribed dose. Results Figure 1 shows the energy layer and relative monitor unit distribution over gantry angles in the PAT plans. Δ NTCP for dysphagia and xerostomia was greater for all PAT plans compared to IMPT (figure 2). Dysphagia average Δ NTCP: 9.9% (IMPT), 12.3% (ELR), 12.1% (SPArc) and 11.5% (ELSA). Xerostomia average Δ NTCP showed similar trends: 4.5% (IMPT), 10.0% (ELR), 9.8% (SPArc) and 8.8% (ELSA). The fraction-wise CTV7000 coverage fell below the replanning threshold in at least 3 out of 5 patients in the PAT plans, while this occurred just once in the IMPT plans. The course-wise accumulation showed to be less impacted by inter-fraction changes. Conclusion All PAT planning algorithms showed potential to reduce expected toxicity compared to IMPT. However, fraction-wise robustness suffered more than course-wise robustness from changes found in repeated CTs, indicating the need to improve PAT robustness and/or increased necessity for adaptive strategies, especially in the context of hypo fractionated treatments. Poster Discussion: Current challenges in proton therapy