ESTRO 2023 - Abstract Book

S1695

Digital Posters

ESTRO 2023

Results The planned target coverage and kidney dose for each optimisation scenario is compared to the recalculated target coverage on the plan sum from the weekly CBCT manual bowel filling overrides (Fig 1).

For all three patient cases, CTV coverage was maintained for the CBCT plan sums at the D95% for all the three planning scenarios. For Case 1, the contralateral kidney dose constraint (V14 Gy < 10%) was maintained for all CBCT plan sums. For Case 2, the contralateral kidney dose constraint was only maintained for the plan optimised on the mean bowel filling. For Case 3, contralateral kidney dose was exceeded for all three planning scenarios on plan sums. Conclusion The clinical cases with weekly CBCTs investigated retrospectively in the present work do not demonstrate a conclusive benefit in optimising plans on an overridden bowel structure to simulate variation in bowel filling. This work demonstrates all three methods of optimisation maintain CTV coverage on plan sums with manual HU bowel overrides from CBCTs but that kidney dose is sometimes significantly impacted regardless of optimisation technique. The use of weekly CBCTs to estimate variation in bowel filling is a significant limitation of this study. As bowel gas present at planning simulation scan may not be representative of subsequent treatment anatomy, future work on an expanded cohort will continue investigating whether planning with a bowel override may result in a planning protocol consistently robust to bowel filling variation.

PO-1940 Online adaptive CBCT-guided VMAT radiotherapy using an Elekta Synergy CBCT-linac

M. Groot Koerkamp 1 , G. Bol 1 , P. Kroon 1 , A. Scheeren 1 , A. Zoetelief 1 , L. Krikke 1 , T. Harderwijk 1 , A. Plat 1 , S. van der Vegt 1 , J. Hes 1 , I. van Gasteren 1 , G. Schimmel-de Kogel 1 , E. Renders 1 , J. van Kaam 1 , R. Onnink-Vijfvinkel 1 , D. Winkel 1 , M. van Rijssel 1 , A. Wopereis 1 , J. Noteboom 1 , S. van de Pol 1 , W. Eppinga 1 , J. van der Voort van Zyp 1 , C. van Es 1 , J. Bossenga 1 , B. Raaymakers 1

1 University Medical Center Utrecht, Radiotherapy, Utrecht, The Netherlands

Purpose or Objective The aim was to develop and implement a first-in-person daily online adaptive radiotherapy workflow for an Elekta Synergy CBCT-linac. Requirements were to use clinically approved tools and to minimize user interaction. Materials and Methods In the proposed workflow, the daily CBCT is used as planning scan and is sent to the treatment planning system (TPS). In the TPS, the following steps take place: contour propagation and bulk-density propagation from planning-CT to CBCT, contour editing, plan optimization (VMAT) and plan verification. If approved, the plan is exported to the treatment delivery software to start treatment delivery. During workflow development, we evaluated the CBCT image quality, created a planning template, evaluated the workflow duration, performed a risk analysis and implemented measures to minimize risks. Multiple dry runs were performed to test the workflow. Results We successfully developed the workflow for treatment of palliative bladder patients (Fig. 1). The TPS part is scripted using Monaco scripting tools. Interaction with the TPS is only required for patient selection, start of CBCT import, contour checking and editing, and plan approval. A VMAT plan is optimized from scratch without user interaction. After export, the online plan is manually entered into the treatment delivery software to start treatment delivery. In-house developed tools are used to send the data between the different systems and to add safety measures to minimize the risks identified.