ESTRO 2020 Abstract Book

S395 ESTRO 2020

RRMM, eligibility criteria and the monitoring signal used to guide gating or tracking. Respondents were also asked if they wished 1) to change or expand their use of RRMM for a tumor site already treated with RRMM and 2) to implement RRMM for a new tumor site and to rank the barriers to implementation in order of importance. Results The questionnaire was filled out by 200 centres from 41 countries. 68% of respondents used RRMM in at least one tumor site (“users”). Inspiration BH was the dominant technique for breast and lymphoma, whereas the spread in technique was greater for other sites (Table 1). Within any given tumor site, users only applied RRMM in a subset of patients. The most frequently selected percentage range of patients treated using RRMM was <25% for lung, pancreas and lymphoma, 25-50% for breast and >75% for liver. However, for liver and pancreas, >50% of users applied RRMM in >50% of patients. The main selection criteria was “left breast” (76%) for breast and SBRT (~50%) for lung, liver and pancreas. Across all tumor sites, external marker was the main RRMM signal used by >60% of respondents. For breast and lymphoma this was followed by surface imaging and breathing volume. KV/MV imaging was frequently used for liver and pancreas (with markers) and for lung (with or without markers) (Fig 1a). Tracking was mainly done on robotic linacs with hybrid monitoring. For breast and lung, 36% and 49% of the centres respectively wish to expand or implement RRMM (Fig 1b). In contrast, for liver and pancreas >55% of centres do not use RRMM and do not wish to implement it. Overall 71% of centres wish to implement RRMM for any new treatment site (Fig 1c) but human/financial resources and capacity on machine were the main barriers (Fig 1d-e).

Conclusion Thirty-two percent of respondents do not use any form of RRMM. Although RRMM was common in the thorax, it was generally applied for less than half of the patients. There is an unmet need for RRMM solutions, particularly in lung cancer. The main barriers to implement RRMM are human/financial resources and capacity on the machine. OC-0704 Six degrees of freedom dynamic motion- including dose reconstruction in a treatment planning system S. Skouboe 1 , R. De Roover 2,3 , C.G. Muurholm 1 , W. Crijns 2,3 , T. Ravkilde 4 , R. Hansen 4 , T. Depuydt 2,3 , P.R. Poulsen 1,5 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark ; 2 Catholic University of Leuven, Department of Oncology, Leuven, Belgium ; 3 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium ; 4 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark ; 5 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark Purpose or Objective Intrafractional motion during radiotherapy delivery can deteriorate the delivered dose. Dynamic rotational motion exceeding 20º has been observed during prostate cancer radiotherapy, but methods to determine the dosimetric consequences of dynamic rotations are lacking. Here, we create and experimentally validate a dose reconstruction method that accounts for dynamic rotations and translations in a standard treatment planning system (TPS). Material and Methods The dose reconstruction accumulates the dose in points of interest while the points are moved in six degrees of freedom (6DoF) in a pre-calculated time-resolved 4D dose matrix to emulate dynamic motion in a patient. The required 4D dose matrix was generated with 0.4s time resolution by splitting the original treatment plan into multiple sub-beams and recalculating the dose of the split