S216
Clinical - Breast
ESTRO 2026
6 Pathology, European Institute of Oncology, Milan, Italy. 7 Oncology and Hemato-oncology, University of Milan, Milan, Italy. 8 Scientific Directorate, European Institute of Oncology, Milan, Italy Purpose/Objective: Currently, the main strategies for early-stage breast cancer (BC) treatment include breast-conserving surgery (BCS) and whole-breast radiotherapy. Given that most local recurrences occur near the tumor bed, accelerated partial breast irradiation (APBI), which delivers a higher dose per fraction over a shorter treatment course, is gaining interest as a viable option for selected BC patients. This study reports updated findings from CRYSTAL trial, which uses CyberKnife to deliver ablative preoperative radiotherapy (RT) in a single fraction. Material/Methods: The CRYSTAL trial (NCT04679454) is a monocentric phase I/II, single-arm, open-label study enrolling patients to receive an ablative dose of 18-21-24 Gy to the tumor with CyberKnife before surgery. The primary endpoint of phase I is to determine the maximum tolerated dose (MTD) meeting a predefined toxicity threshold (no grade 3–4 toxicity). For phase II, the primary endpoint is treatment efficacy, measured by pathological complete response rate. Results: Eighteen patients with median age of 62 years (IQR 62.5 - 68) were treated within the study between november 2021 and January 2025. Nine patients were enrolled in phase I (3 patients per dose step: 18-21-24 Gy), while 9 patients received 24 Gy in a single fraction within phase II. Stage at diagnosis was cT1N0 for 12 patients and cT2N0 for 6 patients (12 luminal A, 5 luminal B, 1 luminal B–HER2+). At pre-surgery MRI, 10 patients had stable disease, 6 partial response, and 1 complete response, with a median reduction in disease extent of 27.8%. Median time to surgery was 33 days; three underwent mastectomy and 15 BCS. All but 4 patients showed a reduction in Ki-67 index, with median change between biopsy and histologic exam 7% (range 2-27). Fourteen patients received postoperative RT, and 2 also chemotherapy. Only 1 grade >2 acute toxicity was observed, in terms of moist desquamation at the site of RT, and a single case of grade 3 chronic toxicity (breast pain) was reported.
varied. Clear organs-at-risk (OAR) dose guidance were most applied to the brachial plexus, heart, lungs, and skin, although specific thresholds varied. The top three knowledge gaps identified were: (1) long-term adverse events and oncologic reRT outcome data, (2) standardized and validated OAR planning reRT dose constraints, and (3) consensus on optimal dose– fractionation reRT schedules, compounded by the paucity of prospective clinical trials. Conclusion: This international survey demonstrates heterogeneity in reRT practice for breast cancer. The highlighted knowledge gaps underscore the need for harmonized guidelines and prospective studies to establish standards in patient selection, technical planning, and OAR dose guidance to improve outcomes for patients undergoing reRT for breast cancer. References: 1. Arthur DW, Winter KA, et al. Effectiveness of Breast- Conserving Surgery and 3-Dimensional Conformal Partial Breast Reirradiation for Recurrence of Breast Cancer in the Ipsilateral Breast: The NRG Oncology/RTOG 1014 Phase 2 Clinical Trial. JAMA Oncol. 2020;6(1):75-82. 2.N. Andratschke et al., "European Society for Radiotherapy and Oncology and European Organisation for Research and Treatment of Cancer consensus on re-irradiation: definition, reporting, and clinical decision making," Lancet Oncol, vol. 23, no. 10, pp. e469-e478, Oct 2022.3. C. S. Mayo et al., "Joining Forces to Advance Reirradiation: Establishing the Reirradiation COllaborative Group (ReCOG)," Int J Radiat Oncol Biol Phys, vol. 122, no. 2, pp. 234-240, Jun 1 2025. Keywords: Breast Cancer, Re-Irradiation, Survey Preoperative radiotherapy in a single fraction for early stage breast cancer: the CRYSTAL phase I/II study update (AIRC IG-23118) Maria Alessia Zerella 1 , Marco Lucarelli 1 , Samantha Dicuonzo 1 , Dmaris Patricia Rojas 1 , Anna Morra 1 , Marianna Alessandra Gerardi 1 , Giovanni Carlo Mazzola 1 , Cristiana Fodor 1 , Elena Rondi 2 , Silvia Penco 3 , Manuela Sargenti 4 , Paola Baratella 4 , Elisa Vicini 4 , Consuelo Morigi 4 , Sabrina Kahler-Ribeiro-Fontana 4 , Viviana Enrica Galimberti 4 , Sara Gandini 5 , Giulia Cursano 6 , Elisa De Camilli 6 , Giuseppe Renne 6 , Federica Cattani 2 , Paolo Veronesi 4,7 , Roberto Orecchia 8 , Barbara Alicja Jereczek-Fossa 1,7 , Maria Cristina Leonardi 1 1 Radiotherapy, European Institute of Oncology, Milan, Italy. 2 Medical Physics, European Institute of Oncology, Milan, Italy. 3 Breast Radiology, European Institute of Oncology, Milan, Italy. 4 Breast Surgery, European Institute of Oncology, Milan, Italy. 5 Experimental Oncology, European Institute of Oncology, Milan, Italy. Poster Discussion 728
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