Abstract Book

S1123

ESTRO 37

EP-2052 On-line dose-guided proton therapy to account for inter-fractional motion: a proof of concept K. Busch 1 , L. Muren 1 , A. Andersen 1 , J. Pedersen 1 , L. Dong 2 , S. Thörnqvist 3 , J. Petersen 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 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).

are for pod 1 and 2: lat -/+174cm, vert +150cm, long +142cm and for pod 3: lat 0cm, vert +190cm, long +230cm from the radiation isocenter. The optimal position shifts the pods ±46cm longitudinally from the current setup. Monitoring the patient in the bore is theoretically feasible according to our results, but we suspect inclusion of a minimal angle between the surface ROI and a camera ray will prohibit monitoring during treatment. Further refinement of the camera-model, such as the inclusion of two stereovision sensors, will be Based on a geometric simulation study and ceiling camera position optimization, surface tracking systems shows promise for positioning of thorax patients for O-ring gantry linac systems. performed. Conclusion