ESTRO 2021 Abstract Book

S733

ESTRO 2021

not well represent the actual GTV motion, and that the model generated probabilistic ITVs provide a flexible solution for defining more robust and suitable motion encompassing targets.

PD-0894 The impact of beam setup on robustness and organs-at-risk dose in IMPT for lung cancer patients S. Visser 1 , R. Wijsman 1 , D. Wagenaar 1 , C. O.Ribeiro 1 , A. Knopf 1 , J.A. Langendijk 1 , E.W. Korevaar 1 , S. Both 1 1 UMCG, Radiotherapy, Groningen, The Netherlands Purpose or Objective In the treatment of lung cancer with intensity modulated proton therapy (IMPT), the choice of beam arrangement is highly patient-specific, as locations of the target volume and anatomy can vary. Considerations have to be made concerning plan robustness and organs-at-risk (OAR). Current clinical practice tends to avoid lateral beams, as they are assumed to be less robust against anatomical variations. This study evaluates plan robustness using different beam setups, and their consequent impact on OAR dose. Materials and Methods For ten lung cancer patients, clinical 3D robustly optimised IMPT plans with no lateral beams were generated on the averaged 4DCT (IMPT clin ). A second IMPT plan was created, where one opposing beam was rotated to the lateral side as much as possible (IMPT lat ) (Fig. 1), aiming to avoid OAR. Both plans were clinically acceptable in terms of target dose coverage and OAR doses after robustness evaluation. OAR doses (lung, heart and oesophagus) were evaluated on the planning dose distribution with a constant relative biological effectiveness (RBE) of 1.1, and with a variable RBE using the McNamara model, assuming an α/β of 2 Gy. Subsequently, both plans were recalculated (with a constant RBE of 1.1) on weekly repeated averaged 4DCTs to evaluate OAR doses, and robustly evaluated for target dose coverage on the resulting voxel-wise minimum dose distribution. During the course of treatment, target and OAR doses evaluations were performed using dose accumulation. Results The accumulated target dose coverage was sufficient (D98% ≥ 95%) in 8/10 patients for both beam setups, with (mean±SD[%]) 95.7±0.9 and 95.9±0.8 over all patients for IMPT clin and IMPT lat , respectively. In case of inter-fractional beam path variations, such as deviating skin folds and scapula positions, target coverage was still adequate for both plans. OAR doses were similar or lower for IMPT lat , compared to IMPT clin (Fig. 2). For specific cases, when the target volume was located either more anteriorly or posteriorly, a large reduction in heart and lung dose was observed for the IMPT lat plans. This is shown for a patient in Figure 1, where a reduction of 0.74 Gy (mean lung dose) and 4.47 Gy (mean heart dose) was observed. The OAR doses differences were also observed when using a variable RBE or accumulated dose (Fig. 2). However, shrinking of the target volume caused overshoots in different anatomical regions of the patient. For the IMPT lat plans, this resulted in increased dose at the distal end of the lateral beam path, where the contralateral lung, and potentially the heart and spinal cord are located. Conclusion If large OAR doses reduction can be achieved when changing the beam setup, one should not avoid employing lateral beams at the expense of target robustness. However, daily soft-tissue monitoring via CBCT and/or CT remain compulsory to evaluate changing patient anatomy and its effect on the dose distributions during the entire course of treatment.