ESTRO 2025 - Abstract Book

S465

Clinical - Breast

ESTRO 2025

Conclusion: lateral decubitus positioning significantly reduces dose to the heart, to various cardiac substructures, to the homolateral lung and to the contralateral lung, compared with dorsal decubitus. This technique is easily implemented and can be widely recommended to reduce heart and lung doses to a minimum.

Keywords: Isocentric lateral decubitus, breast cancer, heart

1019

Digital Poster Impact of neoadjuvant systemic therapy on surgical and radiotherapy outcomes in patients with early-stage breast cancer: a cross-sectional study Yasmin A Civil 1,2 , Katya M. Duvivier 3 , Sanne A.L. Bartels 4 , Susanne van der Velde 4 , Willemien Menke - van der Houven van Oordt 5,2 , Ellis Barbé 6 , Berend J. Slotman 1,2 , Desirée van den Bongard 1,2 1 Radiation Oncology, Amsterdam UMC, Amsterdam, Netherlands. 2 Cancer Treatment and Quality of life, Cancer Center Amsterdam, Amsterdam, Netherlands. 3 Radiology, Amsterdam UMC, Amsterdam, Netherlands. 4 Surgery, Amsterdam UMC, Amsterdam, Netherlands. 5 Medical Oncology, Amsterdam UMC, Amsterdam, Netherlands. 6 Pathology, Amsterdam UMC, Amsterdam, Netherlands Purpose/Objective: Neoadjuvant systemic therapy (NST) can lead to non-concentric tumor shrinkage, complicating excision volume definition and potentially increasing radiotherapy boost volumes, which may affect cosmetic outcomes. This study aims to compare excision and boost volumes in patients undergoing breast-conserving surgery (BCS) after NST versus no-NST, and assessed the association with quality of life (QoL)and cosmetic outcomes cross-sectionally. Material/Methods: All women who underwent BCS for invasive breast cancer at Amsterdam UMC between January 2016 and March 2023 were included. Patient and tumor data were retrieved from records, and quality of life and cosmetic outcomes were assessed cross-sectionally using EORTC-QLQ-C30, -BR23, HADS and Breast-Q questionnaires. Treatment related toxicity was graded using the CTCAEv5.0. 1 A calculated resection ratio (CRR) of 1 indicated optimal tumor resection while preserving healthy breast tissue (Figure 1). 2 The boost volume was defined as the boost planning target volume on the radiotherapy planning-CT. Descriptive statistics were stratified by NST use, with sensitivity analyses for robustness.

Results: Among 403 patients, 136 (33.7%) received NST (Table 1). Median pathological (postoperative) tumor size was smaller in the NST group (7 mm vs. 14 mm, p<0.001). Total resection volumes (TRVs) were similar (262 cm 3 vs. 288 cm 3 , p=0.236), and CCR was higher after NST (8.9 vs. 4.5, p=0.056). A radiotherapy boost was administered to 30.1% in the NST group compared to 24.3% in the no-NST group (p<0.001), with lower boost volume in the NST group (55 cm 3 vs. 91 cm 3 , p=0.009). Within 3 months post-treatment, grade 1 breast induration occurred more frequently in