ESTRO 2020 Abstract Book

S396 ESTRO 2020

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 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).

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. OC-0705 Dose restoration: an online adaptive strategy to fight against range uncertainties in proton therapy E. Borderías 1 , X. Geets 1,2 , E. Sterpin 1,3 1 UCLouvain, MIRO Molecular Imaging Radiotherapy and Oncology, Brussels, Belgium ; 2 Cliniques Universitaires Saint Luc, Radiotherapy, Brussels, Belgium ; 3 KULeuven, Oncology, Leuven, Belgium Purpose or Objective Proton therapy is more sensitive to uncertainties in treatment planning and dose delivery than photon therapy. Density changes in the patient may alter planned proton ranges and compromise plan quality. In this work, we study the clinical benefit of an on-line adaptation strategy, dose restoration (DR), in a challenging location such as lung. DR aims to compensate for density changes by accurately reproducing the planned dose in repeated CT images, without considering re- contouring modifications. We also investigate to what extent stabilizing dose distributions through DR can reduce the need of full adaptation. Material and Methods Our database included a planning CT and two repeated 4D- CTs (1rCT, 2rCT) for 14 lung cancer patients. 4D-Robust optimization on CTV was performed in RayStation 8A, with 5 mm setup error, 3% of range uncertainty, and three phases of the respiratory cycle (end-exhale, end-inhale and MidPosition). DR uses isodose contours generated from the initial dose and patient specific weighted objectives to reoptimize the plan and mimic the initial dose in repeated CTs. Fully automatic robust DR, using the same robustness parameters as in planning, was performed in the two series of repeated 4D-CTs. Robustness tests were also run for initial, distorted (no adapted) and restored (adapted)