ESTRO 2020 Abstract book

S928 ESTRO 2020

but when accounting for interplay, 6 pts had the lowest HI for IMPT RO . In mean HI increased from 0.04 and 0.06 (4D dose) to 0.096 and 0.104 (4D interplay dose) for IMPT RO and IMPT ROPTV on pCT. On sCT mean HI for IMPT PTVRO and IMPT RO increased from 0.06 and 0.12 (4D dose) to 0.12 and 0.13 (4D interplay dose). Conclusion IMPT ROPTV is more robust toward respiratory motion than IMPT RO yielding high CTV coverage in all phases. However, for a few pts dose coverage was compromised by respiration and it is important to evaluate the effect of the motion for all patients treated with IMPT. 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 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.

Deformable dose propagation was used for dose accumulation (MIM Software). Results Fig 1 (left panels) shows box plots of V95% CTV in each of the ten phases for all pts for IMPT RO and IMPT PTVRO plans, showing better coverage for the IMPT PTVRO . V95% CTV exceeded 99% in all phases in 18 pts (IMPT RO ) and 23 pts (IMPT ROPTV ). In two pts, V95% CTV <95% for IMPT RO in at least one phase. Due to inter-fractional anatomical changes, the CTV coverage decreased on sCT leading to only 9 and 17 pts maintaining V95% CTV >99% in all phases (Fig 1, right panels). The CTV DVHs for pCT and sCT for planned dose, 4D dose and 4D interplay dose for pt 3 in Fig 2 (upper panels) show a clear coverage decrease on sCT due to inter-fractional changes. Furthermore, interplay effects of a single F decreased V95% CTV . The 4D dose maintained V95% CTV >99% for all pts (IMPT ROPTV ) and for 24 pts (IMPT RO ). The 4D interplay dose maintained V95% CTV >99% for only 14 pts(IMPT ROPTV ) and 10 pts(IMPT RO ), but V95% CTV >95% for 24 and 21 pts. The homogeneity index (HI= (D98% CTV - D2% CTV )/Dp) increased when accounting for interplay (Fig 2, lower panels). On pCT, HI is lower for IMPT ROPTV than for IMPT RO for all pts when the 4D dose is considered,