S264
Clinical - Breast
ESTRO 2026
risk factors and predicted mean heart dose (MHD). We report PARABLE trial screening data together with results of a recruitment substudy designed to identify barriers to recruitment. Material/Methods: UK PARABLE centres (two PBT centres in London& Manchester; 13 non-PBT centres) completed screening logs aiming to capture all potentially eligible patients, recording reasons for ineligibility where possible. All participants approached for PARABLE were invited to complete an anonymous questionnaire capturing factors influencing decision to participate alongside demographics. Factors influencing decision to participate were rated on a 5-point Likert scale (Strongly Disagree=1, Disagree=2, Neutral=3, Agree=4, Strongly Agree=5). Factors with mean score >3.5 were considered important influencers in decision-making. Written comments were collected. Results: PARABLE screening data were received for 834 patients (from 15 centres) between 06/22 and 09/25. 101/834 (12%) declined prior to eligibility assessment, the majority (59/101) due to not wanting to travel away from home for PBT. 364/834 (44%) were found to be ineligible, 300/364 (82%) of whom had a predicted MHD below the eligibility threshold. 37 patients declined after being found eligible, 18/37 due to not wanting to travel. 170/834 (20%) consented to PARABLE. 72 patients participated in the recruitment substudy (to 30/09/25), including 48 PARABLE acceptors, 10 decliners and 13 ineligible due to MHD below threshold. Important factors in decision making included trust in the clinical team and travelling long distances (table 1). Acceptors particularly valued trust in the clinical team and clarity of information-giving. Decliners were most concerned around costs/ logistics of travel and being away from home. Comments highlighted concerns around potential delays to radiotherapy, additional food costs during PBT, caring responsibilities, costs and inconvenience of transportation. Conclusion: Most people screened for PARABLE were ineligible based on estimated MHD from photon planning being below the necessary threshold. Reluctance to travel to a PBT facility was the biggest barrier to participation in this trial (albeit PPI-facilitated charitable support for travel has since supported successful accrual). Ongoing PBT trials should offer practical support where possible, minimise pathway delays, and clearly communicate timelines to enable equitable access for eligible patients. References: Kirby AM, Haviland JS, Mackenzie M, Fleming H, Anandadas C, Wickers S, Miles E, Iles N, Bliss JM, Coles CE; PARABLE Trial Management Group. Proton Beam Therapy in Breast Cancer Patients: The UK PARABLE
who had a histologically confirmed pathological complete response (ypT0N0) to NST. According to the planned analysis, all patients were divided into three groups: a breast-conserving surgery (BCS) group, a mastectomy (ME) group, and a VAB group. The VAB group was represented by the patients from prospective “surgery deescalaltion” study and two other groups were created historical control by matching patients according to risk factors with VAB group. The primary endpoint was two-year ipsilateral breast tumor recurrence-free survival (IBTR-FS). Results: The analysis included 84 patients who met the inclusion criteria. The BCS group comprised 39 patients, the ME group 20 patients, and the VAB group 25 patients. The median follow-up was 24 months. The two-year ipsilateral breast tumor recurrence-free survival (IBTR-FS) rate was 100% in the BCS group vs. 95% in the ME group vs. 92% in the VAB group [p = 0.525]. Conclusion: Additional follow-up and large-scale randomized prospective trials are required to determine the safety of using VAB. The rate of complications did not differ statistically significantly between the different invasive interventions in this patient cohort. Keywords: surgery deescalation, breast cancer, radiotherapy Factors influencing eligibility and recruitment to breast cancer proton beam therapy trials: findings from the PARABLE trial and recruitment substudy Anna M Kirby 1 , Szeyi Ng 2 , Lucy Kilburn 2 , Hannah McLaughlin 2 , Stephanie Brown 2 , Natasha Iles 2 , Cecilia Brandt 2 , Fay H Cafferty 2 , Mairead MacKenzie 3 , Joanne S Haviland 4 , Judith M Bliss 2 , Charlotte E Coles 5 1 Department of Radiotherapy, Royal Marsden/ ICR, London, United Kingdom. 2 Clinical Trials and Statistics Unit, Institute of Cancer Research, London, United Kingdom. 3 Independent Cancer Patients Voice, ICPV, London, United Kingdom. 4 Wolfson Institute of Population Health, Queen Mary University of London, London, United Kingdom. 5 Department of Oncology, University of Cambridge, Cambridge, United Kingdom Purpose/Objective: Trials of advanced radiotherapy techniques available in limited centres face challenges in recruitment which, if affecting specific subpopulations, can impact on generalisability of results. The UK PARABLE proton beam therapy (PBT) versus photon trial (40Gy/15Fr/3weeks) aimed to recruit breast cancer patients with around 2% or higher lifetime risk of radiation-induced heart disease based on age, cardiac Proffered Paper 2023
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