S207
Clinical - Breast
ESTRO 2026
treatment course can allow for timely treatment personalization. At the same time, it may reduce toxicity and costs while improving efficacy. This study aimed to assess the feasibility of using early changes in tumour size, as measured by ultrasound over 6 time points of NAC, to predict final pathological response in breast cancer patients. Material/Methods: This study included patients with biopsy-proven clinical stage l-lll breast cancer who received standard NAC and underwent ultrasound imaging at weeks 0, 1, 4, 8, 12, and 16 of NAC. A modified response grading system based on post-operative histopathological evaluation was used to classify patients as Complete Responders (CR; complete disappearance), Partial Responders (PR; both ≥ 30% reduction in tumour diameter and decreased cellularity to ≤ 5% in tumour bed), Weak Responders (WR; met one criteria of PR), or Non-Responders (NR; met none of PR criteria). MATLAB R2022b was used for tumour segmentation and extraction of dimensions from contours. Changes in tumour diameter, area, and volume were evaluated as early predictors of final pathologic response. Statistical analysis was performed to assess changes from baseline to each time point across pairs of groups. Results: One hundred and six patients were included in this study. The average age of patients was 50 (24 - 80). The median pre-treatment tumour size along the longest axis, i.e. diameter, was 4.0 cm (1.7 - 12.0). The most common molecular subtype was ER/PR+ HER-. The most common NAC regimen was dose-dense AC-T (±H) (63%), followed by FEC-D (±H) (26%) and KEYNOTE 522 (6%). Post-NAC response rates were as follows: 20% CR, 67% PR, 8% WR, and 5% NR. CRs displayed earlier and steeper relative reductions in mean diameter, area, and volume within the first week of NAC compared to PRs (21%, 31%, 43% versus 7%, 16%, 15%, respectively; see Figure). WRs demonstrated less pronounced reductions (9%, 5%, 14%), and NRs showed minimal reductions or even increases. The relative reductions in mean diameter (p=0.016) and tumour volume (p=0.018) between CRs and PRs from baseline to week 1 of NAC were significant. Conclusion: Early changes in tumour size may provide predictive value for final pathologic response to NAC but further validation is needed.
lung V17 exposure (18.3 vs. 21.6, p < 0.05), representing the first documented evidence of structured DIBH coaching impact on pulmonary dosimetry. This finding suggests that formal coaching protocols provide dual organ protection - both cardiac and pulmonary - challenging the traditional single- organ focus of DIBH implementation. Interestingly, while cardiac dose reductions were observed, they did not reach statistical significance, highlighting lung dose reduction as the primary measurable benefit of coaching interventions. Conclusion: DIBH coaching has consistently demonstrated benefits in reducing cardiac dose during left - sided breast cancer radiotherapy. Our findings highlight its additional potential in lowering pulmonary exposure, particularly relevant in LMICs where baseline lung function is often compromised. While lung dose reduction emerges as a promising advantage, its broader clinical impact warrants further exploration to strengthen dual - organ protection strategies. References: 1. Bergom C, Currey A, Desai N, et al. Deep inspiration breath hold: techniques and advantages for cardiac sparing during breast cancer irradiation. Front Oncol. 2018 Apr 4;8:87.2. Kim A, Kalet AM, Cao N, et al. Effects of preparatory coaching and home practice for deep inspiration breath hold on cardiac dose for left breast radiation therapy. Clin Oncol. 2018 Sep;30(9):571-7. Keywords: DIBH, Patient coaching, lung toxicity Early tumour size changes from neoadjuvant chemotherapy as a predictor of pathologic response in breast cancer Erika Z Chung 1 , David Alberico 1 , Lakshmanan Sannachi 1 , Archya Dasgupta 1,2 , Joyce Yip 1 , Maria Lourdes Anzola Pena 1 , Sonal Gandhi 3,4 , Belinda Curpen 5,6 , Gregory J Czarnota 1,2 1 Physical Sciences, Sunnybrook Research Institute, Toronto, Canada. 2 Radiation Oncology, Sunnybrook Health Sciences Centre, Toronto, Canada. 3 Medical Oncology, Sunnybrook Health Sciences Centre, Toronto, Canada. 4 Medicine, University of Toronto, Toronto, Canada. 5 Medical Imaging, Sunnybrook Health Sciences Centre, Toronto, Canada. 6 Medical Imaging, University of Toronto, Toronto, Canada Purpose/Objective: Neoadjuvant chemotherapy (NAC) is the standard of care for locally advanced, HER2-positive, and triple negative breast cancer. While complete pathologic response is associated with improved outcomes, response to NAC varies greatly and is only assessed after surgery. Identifying responses earlier in the Digital Poster 412
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