ESTRO 2023 - Abstract Book

S108

Saturday 13 May

ESTRO 2023

constraints for SBRT (Diez et al, 2022) was recently published and its impact on plan acceptability for oligometastatic disease is evaluated. Materials and Methods The 6 benchmark cases were inspected and those with structures overlapping the PTV evaluated for compliance with updated constraints. Specifically, duodenum was assessed for the liver case, small bowel for the nodal case and cauda equina for the spinal case. A total of 59 approved SBRT benchmark plans (7 liver, 31 node and 21 spine) were reviewed. Dose prescriptions for these cases were 45 Gy, 30 Gy and 27 Gy for liver, node and spine, respectively, all in 3 fractions. All OAR dose-volume constraints satisfied existing guidance (Hanna et al, 2018). Dose to the duodenum (D0.5cc <22.2 Gy), dose to small bowel (D0.5cc <25.2 Gy) and dose to cauda equina (D0.1cc <24.0 Gy) were reviewed. Changes introduced by the 2022 Consensus for these structures were in terms of near-maximum volumes. These were reduced from 0.5 cc and 0.1 cc to 0.1 cc and 0.035 cc for luminal structures and cauda equina, respectively. A Wilcoxon sign rank test was performed to evaluate differences between the 2 sets of constraints. Results 5/7 liver plans and 30/31 nodal plans failed to achieve the D0.1cc constraint for duodenum and small bowel, respectively. 13/21 spine plans did not achieve the D0.035cc constraint for cauda equina. Mean near-maximum doses for each structure, for each set of constraints is detailed in Table 1. Differences observed between the 2018 and 2022 dose-volume constraints were found to be statistically significant. Table 1: Mean near-maximum doses achieved for the 2018 and 2022 Consensus constraints

Conclusion The effect of applying updated dose-volume constraints to normal tissues from existing benchmark submissions was assessed. The new constraints should not significantly impact planning unless OARs are adjacent or overlapping the PTV. 71 % of liver plans, 97% of nodal plans and 62 % of spine plans would have required re-optimisation. An already very steep dose gradient between target and cauda equina in most spine plans is reflected in the lower failure rate. Steeper gradients may be necessary in planning other anatomical sites where OARs are adjacent to the target. Figure 1: Variation in near-maximum doses as defined in the 2018 (D0.5cc) and 2022 (D0.1cc) Consensus guidelines

MO-0146 Practice-based Training Strategy for Therapist-Driven Prostate MR-Linac Adaptive Radiotherapy W. Li 1 , P. Lindsay 1 , J. Padayachee 1 , I. Navarro 1 , J. Winter 1 , J. Dang 1 , S. Raman 1 , V. Kong 1 , A. Berlin 1 , C. Catton 1 , R. Glicksman 1 , V. Malkov 1 , K. Kataki 1 , P. Chung 1 1 Princess Margaret Cancer Centre, Radiation Oncology, Toronto, Canada Purpose or Objective Online adaptive MR-guided radiotherapy is resource intensive, requiring a team of radiation therapists (RTs), medical physicists (MPs), and radiation oncologists (ROs) for each fraction. Implementation of a new treatment paradigm offers the opportunity to evaluate roles for each member of the multidisciplinary team, and optimize the use of each skillset to maximize efficiency. Here we report a three phase practice-based training strategy developed to transition from RO-driven contouring to RT-driven contouring for whole gland prostate MR-Linac radiotherapy to maximize resource efficiency. Materials and Methods In Phase One, seven MR-Linac RTs independently contoured the target and organs-at-risk on T2-weighted MR images from 11 previously treated MR-Linac prostate patients. The case mix was chosen to ensure a broad representation of differing patient anatomy. The Dice similarity coefficient (DSC) was calculated against contours RO generated during the online session. The RO also performed a qualitative contour review of the RT-generated contours using a 5 point Likert scale; a score of 4 or 5 was deemed a pass, and a 90% pass rate was required for Phase One. Phase Two involved RO supervised online contouring, plan generation and assessment during adaptive treatment fractions, with participants requiring a score of autonomy (no direction required by RO) on 10 cases (minimum 8 patients) to advance. The final phase of training involved independent RT-driven contouring and planning, supported by offline contour and plan review by RO prior to the next fraction. Results In Phase One, the mean DSC was 0.916 (range 0.847-0.966) for prostate, 0.895 (range 0.645-0.99) for bladder, and 0.981 (range 0.962-0.998) for rectum. Mean Likert scores for the prostate was 3.7 (range 3-4), the bladder was 4.1 (range 3.7- 4.6), and the rectum was 4.3 (range 3.6-4.7). Qualitative prostate contour differences included under-delineation at the base, and variation at the apex. Five RTs did not attain a pass rate of 90%, attended follow-up one-on-one review, and subsequently performed additional contours on a further training set of cases (n=5). Each participant completed a median of 12 (range 10 – 13) cases in Phase Two. Minor direction were required from the RO on 5 cases related to target contouring (contour shape, and contour variability at the rectum prostate interface, prostate base, and prostate apex). ROs reviewed