ESTRO 2020 Abstract book

S929 ESTRO 2020

filling rate and bladder volume at baseline. Furthermore, filling rates were compared between fractions. Results 375 MRI scans were analyzed. The median time between MRI pre and MRI post was 30 minutes (range 21-57). Median time between MRI pre and MRI pv was 15 minutes (range 8-25) and between MRI pv and MRI post 15 minutes (range 10-33). Median filling rate for the total session duration (MRI pre – MRI post ) was 1.6 mL/minute (range 0.3-12.0). Figure 1A shows the total session filling rate per patient for each treatment session. Filling rate was not associated with bladder volume at the start of the treatment session (r 2 =0.13, p<0.05) (figure 1B). Filling rates between the first (MRI pre – MRI pv ) and the second (MRI pv – MRI post ) part of the treatment session differed significantly, with 1.3 mL/minute and 1.8 mL/minute, but were related to each other (r 2 =0.74, p<0.05) (Figure 2). Overall, there was a weak relation in bladder filling rates between fractions (r 2 =0.42). However, if the filling rate at the first fraction was below 2 mL/minute, the filling rate was strongly related to the filling rate at the following fractions (r 2 =0.90). Conclusion In healthy bladders, bladder filling during MR-linac treatment sessions varied greatly between patients and treatment sessions. In patients with a filling rate below 2 mL/minute, filling rates appeared more consistent. This dataset will be further analyzed to quantify bladder wall motion, which will allow for (individual) PTV margin calculations for potential future treatment of bladder cancer patients on an MR-linac. Alternatively, bladder filling data might be used for PRV calculation of the bladder for other treatment sites in the pelvis. This will enable comparison of treatment volumes between conventional linac and MR-linac treatments.

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 Purpose or Objective Treatment session times on an MR-linac are generally longer compared to session times on a conventional linac. Bladder filling during this prolonged session time could impact treatment volumes for potential future bladder cancer radiotherapy on the MR-linac. The purpose of this study was to quantify bladder filling during MR-linac treatment sessions and to investigate consistency in filling patterns between fractions. Material and Methods MRI scans acquired during MR-linac treatment sessions of 25 patients were retrospectively analyzed. Patients were treated in 5 fractions for non-bladder cancer related malignancies, i.e. stereotactic body radiotherapy for pelvic lymph node metastases. No drinking or voiding protocols were applied. During each MR-linac treatment session, MRI scans were made at the start of the session for treatment planning (MRI pre ), after dose calculation for position verification (MRI pv ) and after irradiation for post- treatment validation (MRI post ). The bladder was contoured on each MRI scan. Bladder filling rate was calculated as [Δ volume in milliliters / Δ time in minutes]. Regression analysis was used to investigate the relationship between