ESTRO 2025 - Abstract Book

S603

Clinical - Breast

ESTRO 2025

Cancer Death: Meta-Analysis of Individual Patient Data for 10,801Women in 17 Randomized Trials. Lancet 2011, 378, 1707–1716. 2. McGale et al. Effect of Radiotherapy after Mastectomy and Axillary Surgery on 10-Year Recurrence and 20-Year Breast Cancer Mortality: Meta-Analysis of Individual Patient Data for 8135 Women in 22 Randomized Trials. Lancet 2014,383, 2127–2135. 3. Radiation Therapy Oncology Group (RTOG). Breast cancer atlas for radiation therapy planning: consensus definitions. Available from: http://www.rtog.org/LinkClick.aspx?fileticket=vzJFhPaBipE%3d&tabid=236.

4008

Digital Poster Breath-Hold Proton Therapy for Breast Cancer Patients: Selecting the Ideal Candidates Gloria Vilches-Freixas, Femke Visser, Esther Kneepkens, Kim van der Klugt, Cissy Stultiens, Marije Velders, Fleur Vereijken, Stéphanie Peeters, Karolien Verhoeven, Liesbeth Boersma Department of Radiation Oncology (Maastro), GROW School for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, Netherlands Purpose/Objective: Left-sided breast cancer patients are usually treated with photon therapy in moderate deep-inspiration breath-hold (DIBH) to reduce the dose to the heart by increasing the distance between the heart and the target volume. Proton therapy for breast cancer is typically performed in free breathing (FB) because previous in-silico studies did not show significant dose reductions to justify the increased complexity of DIBH treatments. This study aims to determine if there are subgroups of patients that benefit from DIBH in proton therapy. Material/Methods: We conducted a retrospective in-silico study involving 15 left-sided breast cancer patients with intra-mammary node (IMN) indications. For each patient, planning CT scans in FB and DIBH were available, and proton therapy plans were created on both. Dose-volume histogram (DVH) for organs at risk (OAR) were extracted (Fig.1). To extend the analysis, we used the obtained DVH values to compute normal tissue complication probability (NTCP) values for acute coronary events (ACE), secondary breast cancer, and secondary lung cancer based on the national indication protocol (NIPP) [1]. This was done for a female patient population aged 40-70 years, with and without cardiac risk factors, in 10-year age intervals. We aimed for ≥1% absolute risk reduction by DIBH to consider it cost-effective compared to FB. Additionally, we examined geometrical relationships between DIBH and FB CT scans, such as differences in IMN, heart and lung volume, and distances between IMN and heart. Results: Patients ≤ 50 years old with cardiac risk factors benefit most from proton therapy in DIBH, showing a >1% absolute risk reduction compared to FB (Fig.2). DIBH plans resulted in a higher mean lung dose (MLD) for 4/15 patients. Smokers require individual assessment regarding MLD since in the NTCP model in the NIPP only smokers ≤ 50 years are considered to have an excess risk of secondary lung cancer. The distance between IMN and heart in the cranio caudal direction was identified as a positive indicator for DIBH plan improvement. For example, 4 cm difference in the DIBH CT scan correlated with 1Gy improvement in mean heart dose.