ESTRO 2020 Abstract book

S69 ESTRO 2020

dosage occurring near the skin. Fig. 1 shows that target volumes closer to the skin generally exhibited a lower planDmin and a larger drop in local coverage during treatment compared to less superficial CTVs, independent of the applied PTV/PRV margin. Analysis of the local target coverages near the BS showed that for one patient, an increased BS PRV dose could be avoided using 3 mm margins. For cases with more proximal CTVs, a higher planDmin to the local PTV was achievable with 3 mm plans. Nevertheless, the drop in coverage during treatment for the local CTV was always less than 2%, with accDmin > 95% Dpresc (Fig 2). Similar results were obtained for CTV coverage near the SC.

Conclusion For CTVs proximal to BS and SC, 3 mm margin plans with improved PTV coverage and PRV sparing preserved CTV coverage during treatment. Consistent with results of Chen et al. [2] who reported that ~50% of marginal LR recurrences were observed in superficial areas, our study only showed a drop in CTV coverage below 95% Dpresc in skin regions. Careful monitoring of the delivered dose is therefore warranted both for 3 and 5 mm PTV/PRV margins. [1] Navran et al. 2019. doi.org/10.1016/j.radonc.2018.06.032 [2] Chen et al. 2014. doi.org/10.1002/hed.23532 [3] Franzese et al. 2019. doi.org/10.1002/lary.28048 PH-0124 Dosimetric effect of intrafraction motion for different correction strategies in prostate SBRT A. Vanhanen 1 , P. Poulsen 2 , M. Kapanen 3 1 Tampere University Hospital, Department of Oncology and Department of Medical Physics, Tampere, Finland ; 2 Aarhus University Hospital, Department of Oncology and Danish Center for Particle Therapy, Aarhus, Denmark ; 3 Tampere University Hospital, Department of Medical Physics, Tampere, Finland Purpose or Objective Intrafraction prostate motion affects the accuracy of radiotherapy treatment delivery. Motion correction is especially important in stereotactic body radiotherapy (SBRT) due to few fractions with high fraction doses. This study investigates the dosimetric benefit of continuous motion monitoring based correction strategy with the Calypso system (Varian Medical Systems). The Calypso correction strategy is compared to pre-treatment image- based strategies using only a setup CBCT scan (CBCT