ESTRO 2020 Abstract book

S1089 ESTRO 2020

check the positioning of the patients. The kV matches were analyzed by finding the standard deviation for the 3 cardinal axes. Results The recalculated treatment plans on the weekly CTs showed that the plans still comply with the criteria for target coverage. For patient 9 and 10, the target coverage of the worst case scenario for the primary treatment plan was lower due to involvement of the brainstem. This led to a compromise on the robustness of the target coverage. However, the nominal plans met the criteria (fig. 1). The results from the daily kV match showed a standard deviation of 0.5 mm in vert, 0.6 mm in long and 0.5 mm in lat. Deviations ranging between +3 mm and -2 mm. Conclusion The treatment plans were robust to setup inaccuracies and anatomical changes. None of the patients were replaned during treatment. The treatments have been delivered according to the primary treatment plans. Based on the results we may consider reducing the number of CT scans during treatment. The small standard deviation indicates that the setup margins might be reduced but more data are needed.

PO‐1858 Quality assurance of proton treatment plans by evaluating uncertainty doses in brain tumors. B. Ihsan 1 , J. Kallehauge 1 , M. Høyer 1 , O. Nørrevang 1 , K. Seiersen 1 , Y. Lassen 1 , B. Smulders 2 , P.W. Nyström 1 , A. Vestergaard 1 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus N, Denmark ; 2 Copenhagen University Hospital, Department of Onkology, Copenhagen, Denmark Purpose or Objective The goal in optimizing radiation therapy (RT) is to give the target the prescribed dose, while lowering the dose to normal tissue as much as possible. The criteria are 98% of the target volume must minimum has 95% of the prescribed dose. To ensure coverage of the target during treatment, a margin is added comprising setup and machine uncertainties. This enlarges the irradiated volume resulting in possible increase in normal tissue irradiation. Proton therapy (PT) is well suited to reduce the dose to normal tissue. However, PT is more sensitive to anatomical changes and setup inaccuracies, impacting on the range of the proton beam. Many proton treatment centers use CTV as planning target and evaluate the robustness of the plan according to setup and range uncertainties. The aim of this study is to assess the target coverage by evaluating the treatment plans on a weekly basis. The secondary aim is to investigate whether the setup margins can be reduced. Material and Methods The first 10 patients with brain tumors, treated at our center, were included in this study. The robustness of the treatment plans was evaluated with +/- 3 mm setup uncertainty and 3.5 % range uncertainty. Treatment plans of all patients were recalculated on the weekly CT without recalculating the beamline. The robustness of these plans was evaluated with 0 mm setup uncertainty and 3.5 % range uncertainty in order to verify the impact of anatomical changes on the dose distribution. The image guided RT strategy was one daily CBCT at the start of the treatment with 0º couch angle and 2D kV images after every couch rotation. The kV images from daily treatments were matched in long, lat and vert to

PO‐1859 Evaluation of atlas‐based synthetic CT for MRI radiotherapy planning in the head and neck region P.M.H. Lee 1 , R. Appleyard 2 , G. Chiu 3 1 Hong Kong Santorium and Hospital, Department of Radiotherapy, Hong Kong, Hong Kong SAR China ; 2 Sheffield Hallam University, Faulty of Health and Wellbeing, Sheffield, United Kingdom ; 3 Hong Kong Sanatorium and Hospital, Department of Radiotherapy, Hong Kong, Hong Kong SAR China Purpose or Objective MRI plays a major role in radiotherapy planning in the head and neck region due to its superb soft tissue contrast.