ESTRO 37 Abstract book

S207

ESTRO 37

map by deformation of the CT-SIM. This synthetic CT is used for dose calculation and re-optimization using the same number and directions of beams. The phantom’s bladder and rectum are equipped with small pockets, which can hold 9 pieces of radiochromic film (GafChromic EBT3) with a size of 1x2 cm. After each fraction (of 2 Gy) the films were replaced, digitized and calibrated to dose. The integrated treatment planning system of the MRIdian was used for dose calculation. The calculated dose distributions in the planes of the film pieces were exported and registered to the film measurements. Per fraction, the total gamma pass rate over all films was calculated using 3%/3 mm criteria, and the mean dose difference between calculation and measurement over all fractions was calculated per film location.

Conclusion Performing DO to improve treatment quality appeared to be unsuitable for MR-Linacs with perpendicular magnetic fields. The dose conformity in the GTV deteriorates due to electron return effects which were underestimated in overridden densities during treatment planning. Since MR-Linacs are expected to enable reduced planning margins, the ratio of lung tissue inside the PTV is likely to decrease. The results of this study, however, show that density overrides need to be considered with great care for MR-Linacs. OC-0409 A film-based end-to-end test for MR-guided online adaptive radiotherapy D. Hoffmans 1 , O. Bohoudi 1 , N. Niebuhr 2,3,4 , A. Pfaffenberger 2,3 , L. Battum 1 , B. Slotman 1 , M. Palacios 1 , A. Bruynzeel 1 , F. Lagerwaard 1 1 VU University Medical Center, Radiation Oncology, Amsterdam, The Netherlands 2 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany 3 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiooncology, Heidelberg, Germany 4 Heidelberg University, Department of Physics and Astronomy, Heidelberg, Germany Purpose or Objective The implementation of a MRIdian system (Viewray Inc., Mountain View) allows for Stereotactic MR-guided Adaptive Radiation Therapy (SMART), including online re- planning based on the daily anatomy. As a consequence, existing methodologies for end-to-end testing need to be revised in order to perform QA for daily adaptation. The purpose of this study is to present the first results of a film-based end-to-end test for MR-guided adaptive radiotherapy. Material and Methods Given the availability of the anthropomorphic, deformable and multimodal ADAM-pelvis phantom (Figure 1), the end-to-end test was simulated for prostate cancer patients. Two such patients, treated with SMART in 5 fractions have been selected as reference (TXSIM1 and TXSIM2). From these patients, the bladder and rectum filling was determined. At simulation the bladder filling was 320 and 220 cc for respectively TXSIM1 and TXSIM2, the rectum was air filled for P2. During treatments the average bladder filling was 150 cc (range 98 – 232 cc) for TXSIM1 and 195 cc (range 150-235 cc) for TXSIM2. Air filled rectum was observed during 1 respectively 5 fractions for TXSIM1 and TXSIM2. During this phantom study, these rectum and bladder filling volumes were recreated in the silicone bladder and rectum of the phantom. The clinical SMART workflow consists of generating a simulation CT- and MR (MR-SIM) scan, which are co-registered and contoured and were used to create an IMRT base plan of 10 Gy in 5 fractions. Prior to each fraction an MRI is acquired, which is deformable co- registered to MR-SIM and re-contoured. The transformation from the MR-SIM to the fraction MRI forms the basis for the creation of a synthetic electron density

Results The averaged dose differences per location over all re- optimized fractions are shown in Figure 2. The overall mean relative dose difference was 1.7% (SD = 3.2%). The average gamma pass rate for all fractions was 95.1% (SD = 3.1%) and 92.9% (SD = 3.4%) for TXSIM1 and TXSIM2, respectively.

Conclusion End-to-end testing of online daily adaptive treatment, including daily variation of patient’s geometry is shown to be feasible. Film measurements showed good agreement with dose calculation. OC-0410 Cone-beam CT based dose calculation in the thorax region L.P. Kaplan 1 , L. Hoffmann 2 , D.S. Møller 2 , U.V. Elstrøm 2 1 Aarhus University, Dept. of Physics and Astronomy, Aarhus C, Denmark 2 Aarhus University Hospital, Dept. of Medical Physics, Aarhus C, Denmark Purpose or Objective Anatomical changes during radiotherapy (RT) of lung cancer can lead to target under-dosage or over-dosage of organs at risk. Cone-beam CT scans (CBCT) are taken daily for patient setup and are used in adaptive RT to visually identify anatomical changes. The purpose of this study was to validate dose calculation based on CBCT images to make daily assessment of delivered dose possible. To overcome known limitations of CBCT dose calculation stemming primarily from scattered radiation, a stoichiometric calibration in the correct scatter conditions is made.