ESTRO 2025 - Abstract Book

S4320

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Additionally, reduced treatment times will enhance patient throughput. The technique favours planning automation, resulting in further efficiencies in the treatment planning process.

Keywords: Nodal breast, VMAT, robust

701

Digital Poster Evaluation of the dosimetric impact of bladder variability on a cervical cancer cohort Claire Nelder 1 , Amerah Alshamrani 2,3 , Ben Dobby 1 , James Tallon 1 , Robert Chuter 4,2 , Frank Brewster 4,2 , Peter Hoskin 5,2 , Cynthia Eccles 1,2 1 Research and Innovation, The Christie NHS Foundation Trust, Manchester, United Kingdom. 2 Radiotherapy related research, University of Manchester, Manchester, United Kingdom. 3 Research and Innovation, Princess Nourah Bint Abdulrahman University, Riyadh, Saudi Arabia. 4 Radiotherapy physics, The Christie NHS Foundation Trust, Manchester, United Kingdom. 5 Radiotherapy department, The Christie NHS Foundation Trust, Manchester, United Kingdom Purpose/Objective: Introduction of a bladder drinking protocol for the EMBRACE II study created challenges maintaining a consistent bladder volume (BV). This led to longer treatments, and multiple bladder adjustments for patients undergoing cervical radiotherapy. BV variability could compromise target dose or lead to overdose of OARs. This work investigated the dosimetric impact BV had on both target and OAR. Material/Methods: Seven patients treated between December 2020 and January 2021, following the introduction of a drinking protocol were included on a locally approved retrospective audit and planning study. Daily BVs were contoured on CBCTs by a single observer on Monaco treatment planning system, registered with the planning CT (pCT), and deformable registration was used to rigidly propagate OARs. The reference plan was saved as a template, and this template was used to generate new plans on each daily CBCT, following the EMBRACE II 45 Gy in 25# protocol. Patient inter fraction BV changes, time from CBCT to beam-on, and dosimetric variations were compared to the reference plan. Descriptive statistics were used to determine trends. Results: Nine pCTs (including 2 rescans), and 173 CBCTs were evaluated for 7 patients with a median age of 47 (32-82 years). Median time from CBCT to beam-on was 10 minutes (3-177 minutes). Six of 7 patients spent ≥45 minutes in the department on ≥1 day. Reasons for spending longer in the department included asking patients to empty or fill their bladder, to pass gas, be reset up, or to wait for a senior RTT to review images. Over all patients mean % difference from reference dose to CTV D98% was ≤4%, the greatest underdoses to CTV D98 were 11.72% on fraction 25 for patient 1, and 16.31% for patient 6 on fraction 11. ITV45 dose was in tolerance for all fractions except for patient 1 fraction 25, and patient 6 fraction 11, where it was underdose by 25.64% and 6.32% respectively. The underdose was due to bowel gas. Across all patients, dose was exceeded for one OAR for 95% of fractions (Figure 1). Bladder tolerance was exceeded most frequently on 47% of fractions.