ESTRO 37 Abstract book

S1126

ESTRO 37

2 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA 3 Haukeland University Hospital, Section of Medical Physics, Bergen, Norway Purpose or Objective Proton therapy (PT) is sensitive to inter-fractional motion and density variations due to the finite range of protons. We propose the concept of online dose-guided PT (DGPT) where online dose re-calculations of possible isocenter shifts are performed to find the superior re-positioning of the patient without re-planning. The aim of this study was to investigate the potential of DGPT to restore target coverage while sparing normal tissue (NT), exemplified by inter-fractional target motion in the pelvis. Material and Methods Five patient models were created from a patient with locally advanced prostate cancer using the planning CT (pCT) and four repeat CTs (rCTs), with the Hounsfield Unit values overwritten to water, air and the average of bone. Intensity-modulated PT (IMPT) plans were created on the pCT geometry (Eclipse, Varian Medical Systems) using two lateral opposed beams. The prostate, seminal vesicle and lymph node clinical target volumes (CTVs) were expanded with anisotropic margins of 5-9 mm to create the corresponding planning target volumes (PTVs). To simulate inter-fractional motion, we moved the combined prostate and seminal vesicle PTV with 3, 5, 10 and 15 mm in all six directions along the three cardinal axes in both the pCT and the rCTs, resulting in 24 and 96 scenarios, respectively. Initially, conventional image- guided PT (IGPT) was investigated by moving the plan isocenter according to the simulated inter-fractional motion, and re-calculating the plan. The potential of DGPT was subsequently explored by calculating multiple dose distributions with the isocenter shifted an additional 1-15 mm along the three cardinal axes away from the field position used for the IGPT re-calculation. The resulting DGPT and IGPT distributions were evaluated on CTV coverage using the volume receiving 98% of the prescribed dose (V98%), while NT was evaluated based on the maximum dose to 1 cc (D1cc). We particularly investigated whether the re-calculated DGPT dose distributions had both better coverage of all three targets and a lower NT dose compared to the IGPT dose distribution. Results DGPT improved the dose distribution in all three targets and NT in half of the simulated target motion scenarios compared to IGPT (13 of 24 scenarios in the pCT and 47 of 96 scenarios in the rCTs). The largest benefits of DGPT were seen for large motion and most notable for anterior motion (Fig. 1). For simulated motion of 10 and 15 mm DGPT led to better dose distributions for 8 of 12 scenarios in the pCT and 32 of 48 in the rCTs. We found a clear pattern that the best DGPT strategy was to move the fields back towards their original position relative to bony anatomy (Fig. 2).

Conclusion Using a simplified patient model of inter-fractional pelvic motion we showed that DGPT resulted in better dose distribution in half of the scenarios. DGPT has potential to improve dose coverage and spare NT compared to IGPT, with the greatest benefits for large organ motion. EP-2053 Treating moving targets with scanned proton therapy: Is 5 mm initial tumour motion a safe threshold? L.A. Den Otter 1 , R.M. Anakotta 1 , M. Dieters 1 , C.T. Muijs 1 , S. Both 1 , A.C. Knopf 1 1 UMCG University Medical Center Groningen, Radiotherapy Oncology, Groningen, The Netherlands Purpose or Objective Pencil beam scanning (PBS) is a highly conformal technology to treat cancer. The time structure of PBS makes the treatment of moving tumours challenging due to the interplay effect. According to literature, PBS in combination with rescanning can be safely applied without motion mitigation strategies to lung tumours that move 5 mm or less. However, the question is whether the motion measured during treatment simulation will remain below 5 mm during the course of treatment. We investigated the inter-fractional lung tumour motion variation in a unique data set providing five repeated 4DCTs per patient, to evaluate if a 5 mm threshold is a reliable indicator for considering PBS treatments. Material and Methods For 19 NSCLC patients (11 male, 8 female, age: 47-89, stage: II-IV) weekly 4DCT imaging was performed during treatment simulation before and during the treatment course to monitor the anatomical changes and differences in motion. Gross tumour volumes (GTV) were delineated on the maximum inspiration and expiration phases of the planning 4DCT and on the weekly repeat 4DCTs. GTV volume changes were acquired and the weekly inter-fraction motion variation was evaluated by measuring the GTV centroid translations in all three directions. Results The patients showed a median initial tumour motion of 1.3 mm (range: 0.0 – 5.1 mm) for a median initial GTV volume of 28.7 cm 3 (range: 1.9 – 430.0 cm 3 ). Figure 1 shows the measured 3D-vector motion for week 0 (before start treatment) and week 1-5 (treatment course). The maximum deviation from the initial measured motion for the patients was on average 2.1 mm (range: 0.3 – 7.9 mm). Centroid displacements remained under 5 mm for 16 out of 19 patients over the entire course of treatment. For patients number 3, 4 and 11, an increase in motion up to a maximum of respectively 9.1, 11.2, and 6.5 mm was observed (Figure 2). GTV volumes for 15 out of 19 patients shrank during treatment with a median decrease of 35.4% (range: 10.8% - 63.7%), and a median absolute volume change of 8.3 cm 3 (range: 0.5 - 105.9 cm 3 ).