S225
Clinical - Breast
ESTRO 2026
recurrence free survival of breast cancer patients. Material/Methods: After approval of Scientific and ethical Local boards, registration numbers R-2023-1901-121, we initiated a retrospective cohort study, using the database of public health system hospitals in Mexico. We identified patients diagnosed between 2005 and 2024 treated in speciality and received treatment with radiotherapy. Retrospective cohort study was realized, including clinical registries of patients with histological confirmation of breast cancer, that received treatment with radiotherapy; they were assigned to one of two groups; intervention group, that received for at least two continuous months H1 or H2 antihistamines during or after radiotherapy (RT) treatment (AH); control group, without records of antihistamines use. Additionally, they were divided according schedule of RT received in Hypofractionated or conventional Schedule. Disease Free Survival (DFS) were analyzed with Kaplan-Meier curves and differences among groups with Log-Rang Test, using Jamovi v 2.3.28.0 statistical software. Results: A total of 1314 patients were analyzed and assigned into two groups, 619 cases at AH group and 695 at control group; mean age was 56 years and most cases corresponded to Stage III, Luminal molecular profile, treated with Radical surgery and chemotherapy; from these, 1251 patients have complete data to analyze radiotherapy schedule, 702 received conventional schedule and 549 hypofractionated radiotherapy. We observed a benefit in DFS of patients treated with Hypofractionated schedule (Fig 1, A) with HR of 1.65; this benefit was increased in patients that received AH, in both schedules, hypofractionated (Fig 1, B) and conventional (Fig 1, C), with HR of 0.66 and 2.118 respectively.
Conclusion: This finding is, to our knowledge, is the first report of a superior effect in breast cancer control with the use of hypofractionated regimens. Additionally, the use of antihistamines appears to enhance the effect provided by fractionation, suggesting a synergistic effect. References: 1. Abdel-Wahab M, Coleman CN, Eriksen JG, et al. Lancet Oncol. 2024;25(6):e270–80.2. Whelan TJ, Pignol J-P, Levine MN, et al. N Engl J Med. 2010;362(6):513– 20.3. Koulis TA, Nichol AM, Truong PT, et al. Int J Radiat Oncol Biol Phys. 2020;108(5):1150–8. 4. Haviland JS, Owen JR, Dewar JA, et al. Lancet Oncol. 2013;14(11):1086–94. Keywords: Hypofractionation, antihistamine, survival Digital Poster Highlight 921 Phase I study to evaluate the safety and tolerability of SABR treatment in inoperable breast cancer: final results Carolina De la Pinta 1,2 , Mercedes Martín 1 , Marina Alarza 1 , Ana Belén Capuz 3 , Rafael Moris 3 , Verónica Gamero 4 , Alfonso Muriel 5 , Ángel Montero 6 , María Teresa Muñóz 1 , Raúl Hernanz 1 , Sonsoles Sancho 1 1 Radiation Oncology, Ramon y Cajal Hospital, Madrid, Spain. 2 Biomarkers and therapeutic targets, IRYCIS, Madrid, Spain. 3 Medical Physics, Ramon y Cajal Hospital, Madrid, Spain. 4 Radiology, Ramon y Cajal Hospital, Madrid, Spain. 5 Biostatistics, Ramon y Cajal Hospital, Madrid, Spain. 6 Radiation Oncology, HM San Chinarro, Madrid, Spain Purpose/Objective: A significant percentage of elderly patients with early- stage breast cancer are not surgical candidates or refuse surgery, and exclusive hormone therapy offers poor local control. SABR, by delivering biologically ablative doses in a few fractions, could be a non- invasive and effective alternative. This phase I clinical trial's primary objective was to determine the safety,
In multivariate analysis, we found an incremented risk of recurrence for patients at the control group (non AH consumers), with an OR 0.72 (95% CI 0.55 - 0.95, p=0.020), additionally, the presence of late CS, Her2 positive subtype or treatment with palliative intention increase the risk of recurrence; but, interestingly, a significant improvement in risk of recurrence was found in patients treated with hypofractionated schedule of radiotherapy, with an OR 1.47 (95% CI 1.10 - 1.96, p=0.10), (Fig 2).
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