ESTRO 38 Abstract book

S279 ESTRO 38

meaningful reduction in risk of intrafraction motion than previous technique. OC-0530 Improving OAR volumes during prostate RT using daily patient feedback and standardized protocols I. Gagne 1 , K. Earnshaw 2 , S. Cowan 2 , J. Goulart 3 , A. Alexander 3 1 BC Cancer - Victoria, Medical Physics, Victoria, Canada ; 2 BC Cancer - Victoria, Radiation Therapy, Victoria, Canada; 3 BC Cancer - Victoria, Radiation Oncology, Victoria, Canada Purpose or Objective Prostate radiotherapy accuracy is impacted by inter- and intra-fraction volume variations associated with proximal organs at risk (OARs) such as bladder and rectum. Patients are therefore often advised to have a comfortably full bladder and empty rectum at the time of CTsim and treatment. The optimal hydration and bowel preparation is a topic of many studies and there exists to date no consensus. The goal of this study was to evaluate the impact of a standardized hydration and bowel preparation protocol (SHBPP) in conjunction with daily patient engagement on bladder and rectum volumes. Material and Methods In 2018, 32 patients with prostate cancer were sent a SHBPP advising them to administer a micro-enema 2-3 hours before CTsim and the first 10 treatments and to empty their bladder 1hour before CTsim and treatment and consume within 15 minutes, 750 ml of water. At the time of CTsim and treatment, radiation therapists (RTs) recorded the details of the preparation in the patient’s technical notes. Daily CBCT was performed and the consistency of bladder and rectal volumes in comparison to CTsim was assessed and recorded using a locally developed bladder and rectum scoring system. Scores and CTsim volumes obtained for this cohort were compared to 20 patients treated in 2013 who had CBCT on days 1,2,3 and weekly, and no SHBPP and daily feedback. In this non- SHBPP cohort, therapists were instructed to ensure bladder filling at time of CTsim placed the bladder dome at least 2 cm above the femoral head. Results To date, 19 patients on SHBPP have completed treatment, leading to 19 CTsim volumes and 533 CBCT scores for analysis. Using the Mann-Whitney test, the median bladder (279cc vs 387cc) and rectal (57cc vs 79cc) volumes were found to be statistically different at the P=0.05 level, with CTsim bladder and rectal volumes being on average, 15% and 30% smaller, respectively with SHBPP. While rectal volumes were found to have less variability (SD: 20cc vs 37cc), bladder volumes had slightly larger variability (SD: 200cc vs 180cc). The smaller variability in the non-SHBPP cohort could be due to the bladder filling requirement at CTsim. On treatment, the fraction of treatments with rectal volumes within “rectum+5mm” where the same (75%) while the fraction of treatments with anterior rectal distensions >10mm decreased by 3% (8% vs 11%) for the SHBPP group. The fraction of treatments with bladder volumes <2/3 decreased by 28% (13% vs 41%). On average, the difference in bladder filling scored by measuring the dome location difference between CBCT and CTsim was -0.3cm±0.9cm for the SHBPP group vs -2.0±2.5cm for the non-SHBPP group. Conclusion The implementation of SHBPP resulted in significantly smaller and less variable rectal volumes and slightly smaller bladder volumes at CTsim. The use of daily feedback significantly reduced the variability of bladder volumes at treatment and also reduced the fraction of treatments with rectal distensions >10mm by 3%.

enter the room during treatment. Before the implementation of HyperArc, patients were immobilised with BrainLab mask system and planned using 10X FFF VMAT. Objective  Compare the positioning accuracy of Encompass and the BrainLab frameless mask system  Determine the potential treatment delivery advantages of HyperArc  Investigate the intrafraction motion for the Encompass device Material and Methods 50 patients with brain metastases where treated with SRS. Lesions were <3cm and treated with a dose between 18- 24Gy. 25 patients, set up with Brainlab, were planned using mono-isocentre 10XFFF VMAT with co-planar arcs. These were compared to 25 subsequent patients, set up with Encompass, and planned with Hyperarc using a mono - isocenter technique with multiple non-coplanar arcs . CBCT imaging was acquired immediately before treatment and registered to the reference CT using a 6DOF automatching procedure. All corrections were applied before delivery. Beam on time (BOT) (min:sec) is defined as the aggregate time for delivery of total MU for all arcs in a given plan. Time In Room (TIR) (min:sec) is measured from the first alignment image to last beam off, inclusive of all pre-treatment imaging and/or shifts to correct patient position. Post treatment CBCT scans were acquired for Encompass to assess intrafraction motion. Two sample t-tests (two-tailed with a significance level of < 0.05) were used to test for differences between the techniques. Results Brainlab patients were treated with 2 coplanar arcs. Encompass patients were treated with 4 arcs delivered at floor rotation angles of 0, 45, 315 and 270. Mean setup shifts for Encompass and Brainlab were 1mm or less (Table 1). There was no statistically significant difference in mean set-up shifts comparing Encompass with BrainLab patients, although the mean values for Brainlab patients were slightly smaller and sub 1 mm. Mean BOT for HyperArc was on average 0.6 minutes less than 10XFFF VMAT (p=0.0005) - a 20% reduction (Table 2). Mean TIR for HyperArc was on average 7.1 minutes shorter (p<0.0001) - a 41% reduction. Intrafraction motion for Encompass is shown in Table 3.

Conclusion To our knowledge this is the first study to evaluate the benefits of Varian Hyperarc for treatment delivery. The setup accuracy of Encompass is comparable to BrainLab, and treatment position with Encompass is maintained during the delivery of non-coplanar beams. HyperArc delivers linac based SRS efficiently, with a faster treatment time than 10XFFF VMAT and a statistically significant improvement in TIR. This significant reduction in delivery time (TIR) could result in a clinically