ESTRO 2020 Abstract Book

S95 ESTRO 2020

= 8, 4 and 12 for bladder, bowel and rectum, respectively. Significance was tested with the paired samples t-test for normally distributed variables and the Wilcoxon signed- rank test for non-normal variables. Results Target coverage was similar between RP and CP (Figure 1a), with D98% > 95% of prescribed dose for all targets in all plans. Mean (95% CI) reduction in D mean with RP was 2.1 Gy (1.3-3.0) for bladder, 1.0 Gy (0.6-1.3) for bowel and 1.5 Gy (0.6-2.4) for rectum (p < 0.05). Mean reduction in gEUD with RP was 0.9 Gy (0.6-1.2) for bladder, 0.9 Gy (0.6-1.2) for bowel and 0.6 Gy (0.4-0.7) for rectum (p < 0.05). Figure 1b-c and Figure 2 show how the inter-patient variation in rectum and bladder DVH was reduced with RP, mainly by limiting the volume receiving 20-50 Gy. The number of auxiliary structures was reduced from 10 in the CPs to 3 with RP. Estimated planning time with RP in our clinic is 55 min, where 30 min is VMAT optimization without user interaction, and the remaining 25 include preparations for planning and evaluation of the treatment plan. Planning time for the CPs was not recorded, but manual planning usually involved 2-3 rounds of repeated optimization and evaluation.

Conclusion In a single optimization, RapidPlan produced treatment plans with similar target coverage and improved OAR sparing compared to manually optimized plans. The potential for OAR dose reduction was largest for low and medium doses, likely because the higher doses have been given more attention in the manual planning, and the OARs are close to or overlapping with targets that have higher priority. KBP can improve plan quality and spare resources in the treatment planning process for high-risk prostate cancer, and has been routinely used in our clinic since July 2019. PD-0192 One-year experience with total body irradiation (TBI) using forward IMRT and on-line image guidance R. Van Leeuwen 1 , D. Verwegen 1 , P.G. Kollenburg 1 , M. Swinkels 1 , R.W. Van der Maazen 1 1 Radboudumc, Radiotherapy, Nijmegen, The Netherlands Purpose or Objective Total body irradiation (TBI) is a treatment used in the conditioning of patients prior to hematopoietic stem cell transplantation. Dependent on patient characteristics and conditioning regimen (chemotherapy), low dose (e.g., 1x2Gy) or high dose TBI (e.g., 6x2Gy) can be given. We developed a forward IMRT technique for radiotherapy treatment planning using a CT scan and Pinnacle treatment planning software (TPS, Philips, Best, NL), enabling a homogeneous dose distribution and sparing of critical organs (lungs) to lower doses. Material and Methods For our high-dose TBI, patients were scanned and treated in side position with bent knees fixed in a vacuum mattress. Low-dose TBI patients were treated in supine position. Treatment was performed on an Elekta Agility linac (Stockholm, Sweden) at 350 cm (source to isocenter) enabling a maximal treatment length of 160 cm. For treatment planning, we adjusted the Pinnacle beam model to better resemble absolute dose and beam profile at long SSDs. Using the CT scan and digitally reconstructed radiographs from both the Anterior (A) and Posterior (P) directions, various beams were created: (1) a beam following the patient contour that was used to position the patient using the light field, (2) a beam slightly larger than the projection of the lungs (Fig. c) for positioning the patients with respect to the beam using a mobile megavoltage (MV) imager (Cablon, Leusden, NL), (3) an open beam with collimator angle 45 degrees (Fig. a), (4)