ESTRO 2020 Abstract book

S435 ESTRO 2020

cancer. The main barriers to implement RRMM are human/financial resources and capacity on the machine.

D2%), -1.4% (-17.4-7.1%) (GTV D95%), -1.2% (-17.1-5.6%) (GTV D99%), and -0.1% (-3.2-7.6%) (GTV mean dose). Thus, dose reconstructions with dynamic motion revealed large interplay effects (cold and hot spots).

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 plan in the TPS (Eclipse). The dose accumulation was performed via TPS scripting by querying the dose of each sub-beam in dynamically moving points, allowing dose reconstruction with any dynamic motion. The dose reconstruction was validated with film dosimetry for two prostate dual arc VMAT plans with intra-prostatic lesion boosts. The plans were delivered to a pelvis phantom with internal dynamic rotational motion of a film stack (21 films with 2.5mm separation). Each plan was delivered without motion and with three prostate motion traces. Motion-including dose reconstruction was performed for each experiment. The 3%/2mm γ pass rate (γPR) was calculated for each motion experiment, with the static treatment being the reference, and compared between film measurements and TPS dose reconstruction. DVH metrics for RT structures fully contained in the film volume were also compared between film and TPS. Finally, the dynamic dose reconstruction was compared with a dose reconstruction with a static rotation equal to the mean rotation during each experiment. Results Figure 1A shows an example of the dose in a single film plane with and without motion. The γPR comparing motion and static doses was 37.4% in the TPS dose reconstruction and 36.9% with film. The TPS γPR in general agreed well with film with a mean (range) difference of 2.2% (0.5-4.6%) (Table 1). Comparing TPS with film, γPR was 95.4% (88.0- 99.8%). Dose reconstruction with constant mean rotation differed markedly from the dynamic dose reconstruction (Table 1, Figure 1). The mean (range) difference between dynamic and constant rotation was 4.3% (-0.3-10.6%) (urethra D2%), -0.6% (-5.6%-2.5%) (urethra D99%), 1.1% (-7.1-7.7%) (GTV

Conclusion A method to perform dose reconstructions for dynamic 6DoF was developed and experimentally validated. It proved large differences in dose distribution between dynamic and static rotations not identifiable through dose reconstruction with constant mean rotation.