ESTRO 2025 - Abstract Book

S574

Clinical - Breast

ESTRO 2025

4. Thomsen MS, et al. Breast induration and irradiated volume: DBCG PBI Trial. Radiother Oncol. 2022;177:231-5. DOI: 10.1016/j.radonc.2022.09.024. 5. Thomsen MS, et al. Age, smoking, and boost in the DBCG HYPO Trial. Radiother Oncol. 2024;201:110574. DOI: 10.1016/j.radonc.2024.110574

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Digital Poster Online adaptive radiotherapy in breast cancer patients – a prospective trial, BREAST-ART NL2021.0624 Angelique R.W. van Vlaenderen 1,2 , Judith G. Middelburg-van Rijn 1,2 , Koen J. Nelissen 1,2 , Karin N. Goudschaal 1,2,3 , Lars ter Beek 1,2 , Jessica van der Himst 1,2 , Nina Bijker 1,2 , Joost J.C. Verhoeff 1,2 , Ben J. Slotman 1,2 , Anna Dinkla 1,2 , Wilko F.A.R. Verbakel 1,2,4 , Desiree H.J.G. van den Bongard 1,2,5 1 Radiation Oncology, Amsterdam UMC location Vrije Universiteit Amsterdam, Amsterdam, Netherlands. 2 Cancer Treatment and Quality of Life, Cancer Center Amsterdam, Amsterdam, Netherlands. 3 Radiation Oncology, Amsterdam UMC location University of Amsterdam, Amsterdam, Netherlands. 4 Radiotherapy Solutions, Varian Medical Systems, Palo Alto, USA. 5 Cancer Biology and Immunology, Cancer Center Amsterdam, Amsterdam, Netherlands Purpose/Objective: In breast radiotherapy (RT), contour differences due to seroma, edema or different arm/body positioning can result in dosimetric deviations. Internal data revealed that in the majority of breast cancer radiotherapy additional CBCT imaging was needed for accurate positioning in at least one fraction during treatment, and that in 9%-15% of local (i.e. whole breast/chest wall) and locoregional RT (including axillary lymph node levels (L) I-II/IV), offline re-planning was needed. Online adaptive radiotherapy (oART) offers daily plan optimization which prevents the need for repositioning and offline replanning. The study aims to evaluate the feasibility of oART in breast cancer patients. Material/Methods: The BREAST-ART trial (NL2021.0624) is a single-arm prospective study for breast cancer patients referred for postoperative RT on Ethos (Varian, a Siemens Healthineers company, Palo Alto). The prescribed dose was 5x5.2Gy for local RT+/-axillary lymph nodes LI-II, and 15x2.6Gy for local RT+LI-IV. In left-sided breast cancer, voluntary deep inspiration breath hold was used. CTV and PTV (5mm margin) were cropped 5mm below the body contour. For treatment planning, 4-/5-beam tangential IMRT setup was used for local and locoregional RT, respectively. On-couch treatment was led by radiation therapists (RTTs) and supervised by a radiation oncologist (RO) and medical physics expert. Contours (CTV) were reviewed and adjusted as needed, treatment plans (TP) (scheduled (TP S ) and adapted (TP A )) were evaluated. Local treatment switched to RTT-only, where an RO checked the TP offline. Statistical comparisons of TP A en TP S dosimetry were conducted using the Wilcoxon signed rank test. Acute toxicity was assessed using the Common Toxicity Criteria Adverse Events v5.0, and patient satisfaction was evaluated by an in house developed questionnaire. Results: Between June 2022 and October 2024, 66 patients (380 fractions) were enrolled. Median times from CBCT1-CBCT3 ranged from 14.5-25.8 minutes (figure 1). RTT-only was applied in 95 right-sided and 25 left-sided local fractions. TP A were used for all fractions except for one. A significant dosimetric difference was observed between TP S and TP A for the CTV (figure 2). Patient satisfaction was good, 80% of all patients were satisfied with the treatment time. Acute treatment-associated toxicity was mainly grade 0-1.