ESTRO 2020 Abstract Book

S877 ESTRO 2020

On average, the reduction in PTV1 and PTV2 coverage is considerable (Fig 2). This reduction was correlated with sagittal prostate intrafractional motion. Statistically significant differences were found from sagittal displacements > 4mm (p <0,01). In this case, the concomitant boost (PTV2) margin (3 mm to the GTV in all directions except at prostate–rectum interface) would not be enough to compensate the dosimetric impact of prostate intrafractional motion, instead the margin applied to the PTV1 (10mm except 7 mm at the prostate– rectum interface) would be enough.

systematic range uncertainties (±3.5%) and incremental isocenter shifts (ranging from 1.0 mm to 12.0 mm): a total of 2023 dose distributions were analyzed. An a-posteriori analysis of patient images was performed to evaluate the quality of the technique with actual daily positioning data. Results The dose distributions of nominal plans gave good results in terms of target coverage (table): the only limitation to the target homogeneity was the maximum acceptable MLD. The median value of MLD was 15.77 GyRBE and the constraints on all OARs were always below tolerance. The interplay effect analysis gave always clinically acceptable hot (less than 105% of prescription dose) and cold spot in the target. The OARs maximum doses were always under the constraints also taking interplay effect into account. The robustness analysis showed that the ITV to PTV margin was always such that in at least 90% of the scenarios D 95 % of the ITV was at least as good as D 95% of the PTV in the nominal plan (figure 2). Patients data confirmed that the BH technique used was adequate for inter- and intra-BH liver position reproducibility.

Conclusion Intrafractional prostate motion has a considerable impact on the delivered dose to rectum, bladder and on to the coverage of the volumes of interest. It would be necessary to correct the intrafractional movement, for example by tracking techniques, to increase the precision in the hypofractionated prostate treatment. PO-1610 Robustness strategies towards respiratory motion for proton PBS treatments of oesophageal cancer PO-1611 Clinical implementation of liver cancer treatments with pencil beam scanning proton therapy F. Fracchiolla 1 , F. Dionisi 1 , R. Righetto 1 , L. Widesott 1 , I. Giacomelli 1 , G. Cartechini 2 , P. Farace 1 , M. Bertolini 1 , M. Amichetti 1 , M. Schwarz 1 1 Centro di Protonterapia, Protonterapia ospedale di Trento, Trento, Italy ; 2 Università degli Studi di Trento, Fisica, Trento, Italy Purpose or Objective To present our planning technique for the treatment of liver disease and to evaluate the impact of inter- breathhold (BH), setup and range uncertainties on plan quality for the first patients treated at our proton therapy center. Material and Methods The first consecutive 17 patients treated for primary liver disease were considered in this study. For each patient, 3 CT were acquired, using the Active Breathing Coordinator (ABC) to force the expiration breath-hold, to evaluate the inter breath-hold reproducibility. An Internal Target Volume (ITV) was generated based on the 3 datasets. The ITV to PTV margin was 7mm; the plans were generated using the range shifter (RS) optimization technique [1] (figure 1) (i.e. splitting each beam into two components, one with the RS for the superficial part of the tumor and one without) in order to minimize the deterioration of lateral penumbra due to the use of RS and reduce the dose to organs at risk (in particular the mean liver dose - MLD). Each plan was evaluated with a in-house developed tool for the evaluation of the interplay effects considering the reproducibility of liver position between BHs. The robustness of each plan was also tested by simulating Abstract withdrawn

Conclusion We designed and implemented a method for the treatment of liver disease with proton pencil beam scanning. The method showed good results in terms of plan quality and robustness. The effect of Inter-BH liver reproducibility, setup and range uncertainties was systematically tested. The data available so far based on the first treatments support the hypothesis that the planning technique, the ITV to PTV margins and the BH method reached a satisfactory balance between plan robustness and quality of the nominal dose distribution. [1] Fracchiolla F. et al. A pre-absorber optimization technique for pencil beam scanning proton therapy treatments. Physica Medica 57 (2019) 145–152. PO-1612 Bladder filling during MR-linac treatment sessions. M. Den Hartogh 1 , A.L.H.M.W. Van Lier 1 , T. Willigenburg 1 , A.M. Werensteijn-Honingh 1 , I.M. Jürgenliemk-Schulz 1 , J.R.N. Van der Voort van Zijp 1 , P.S. Kroon 1 1 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands