ESTRO 2025 - Abstract Book

S460

Clinical - Breast

ESTRO 2025

909

Digital Poster Evaluating the role of prior simultaneous integrated boost VMAT planning experience in protocol compliance for the PARABLE trial (ISRCTN14220944) Sabreena Ahmed 1 , Patricia Diez 1 , Zohal Nabi 1 , Luke Gilling 1 , Elizabeth Miles 1 , Lucy Gribble 2 , Fay Cafferty 2 , Anna Kirby 3 , Charlotte Coles 4 1 Radiotherapy Physics, National Radiotherapy Trials Quality Assurance Group, Mount Vernon Cancer Centre, Northwood, United Kingdom. 2 Clinical Trials & Statistics Unit, The Institute of Cancer Research, London, United Kingdom. 3 Department of Radiotherapy, Royal Marsden NHS Foundation Trust, The Institute of Cancer Research, Sutton, United Kingdom. 4 Department of Oncology, University of Cambridge, Cambridge, United Kingdom Purpose/Objective: PARABLE is a phase III multicentre randomised controlled UK trial evaluating VMAT versus Proton Beam Therapy in patients with breast cancer who are at higher risk of late cardiac complications. A comprehensive radiotherapy quality assurance (RTQA) programme is in place to support the implementation and use of optimal photon techniques, including simultaneous integrated boosts (SIBs). As part of pre-trial RTQA, participating centres completed a facility questionnaire and a planning benchmark case. This study aims to assess the impact of centres' previous experience with photon SIB VMAT benchmark planning on plan quality and protocol compliance. Material/Methods: Each centre's facility questionnaire and photon SIB benchmark plan were evaluated to assess planning technique and protocol compliance. Centres were classified into categories based on their prior SIB and VMAT experience: no experience, some experience and experienced. Plans were assessed for compliance with PARABLE guidelines through evaluation of both mandatory and optimal dose-volume constraints for PTVs and OARs, with feedback categorised as per protocol, acceptable variation or unacceptable variation from trial protocol. Unacceptable plans required resubmission. Additional support processes included pairing centres with more experienced centres using same planning system and similar technique plus drop-in support meetings. Results: Twenty-one centres completed the SIB VMAT benchmark case, of which 5 showed unacceptable variations on the first submission (Table 1). Only 2 of these were from inexperienced centres, one of which had no prior breast VMAT experience.

Both optimal and mandatory PTV constraints were achieved in most plans (Table 2), with no differences observed between less and more experienced centres. Compliance for OAR constraints was generally good.

Experience appeared to influence plan quality in achievement of mandatory dose constraints for the heart and the contralateral breast. Some optimal constraints were not achievable for this benchmark case, irrespective of the centre's experience.