ESTRO 2022 - Abstract Book

S1461

Abstract book

ESTRO 2022

1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom; 2 The Royal Marsden NHS Foundation Trust, Radiotherapy, Sutton, United Kingdom; 3 The Royal Marsden NHS Foundation Trust and the Institute of Cancer Research, Radiotherapy, Sutton, United Kingdom Purpose or Objective The HERMES trial (NCT04595019) is investigating the feasibility and safety of delivering prostate SBRT in two fractions (2#) compared to five using the Unity MR-linac platform (MRL, Elekta AB, Stockholm). We report on the 2# treatment workflow and the dosimetric results for the first recruited patient, who was randomised to the 2# trial arm.

Materials and Methods For 2#, the target structures and dose levels (cGy) are: • GTVpb_2700: intraprostatic lesion •

PTVpsv_2400: 3mm isotropic expansion of prostate including 1cm of seminal vesicles (SVs) • PTVsv_2000: 3mm isotropic expansion of prostate including 1-2cm SVs, depending on patient risk Treatment planning is performed using the Monaco v5.40.01 (Elekta) treatment planning system and consists of 11-field IMRT. PTV dose coverage is systematically reduced around rectum and bladder to achieve the mandatory dose constraints. Plans are adapted online via the Adapt-to-Shape workflow (ATS), with any post-ATS planning prostate baseline shifts corrected using Adapt-to-Position (ATP). Due to the extended beam-on time for 2# and absence of clinically-available real- time adaptive planning tools, each fraction is split into two sub-fractions (sub-#) which are treated as two separate ATS workflows, delivered back-to-back on the same day. Patients will leave the treatment room between sub-# for bladder re- preparation. For the first patient, to estimate target and OAR doses at the time of treatment, the clinically delivered treatment plans for each sub-# were re-calculated offline on the MRL MRI scans (T2-weighted) acquired at the time point closest to beam- on. OAR doses were accumulated over the entire treatment course by averaging their respective calculated dose statistics across all sub-fractions. Results Individual sub-# results are given in Table 1. Treatment course mean OAR doses are given in Table 2. Whilst OAR dose constraints were exceeded on individual sub-#, the simple dose accumulation suggests that OAR doses were within their mandatory constraints for the overall treatment course, with the bladder D15cc being the only exception. However, this dose accumulation approach is conservative and assumes that the same OAR regions received the highest doses on all sub- #. Hence this is likely an overestimation of dose.

The total times to complete each # were 2.2hr and 1.6hr respectively. Whilst both were within the allocated 2.5hr treatment slot, #1 was longer than expected due to necessary management of rectal distension and a Monaco dose calculation failure. Conclusion We have implemented a workflow on the Unity MR-linac for HERMES trial two-fraction prostate SBRT and successfully treated the first patient whilst predicting to maintain OAR doses over the course of treatment. A sub-fraction workflow approach was followed to reduce prostate motion during treatment. It is expected that this sub-fraction approach will become redundant once real-time online adaptive planning using MLC tracking is clinically available.