S309
Clinical - Breast
ESTRO 2026
in 5 fractions is non-inferior to the moderatly hypofractionated (MH) regimen of 40.05 Gy in 15 fractions in terms of tumor control and acute skin toxicities. The present study aimed to compare the incidence and temporal pattern of acute radiation- induced dermatitis (ARD) between both regimens. Material/Methods: A retrospective-prospective cohort analysis was conducted on 179 breast cancer patients who received external RT between March 2021 and January 2024. The presence of integrated or sequential boost in the treatment was an exclusion criteria. Most patients had invasive ductal carcinoma (73.7%), predominantly luminal-like (92.7%). Adjuvant therapy was prescribed in 97.2% of cases. According to TNM staging, 61.5% of patients were pT1N0, 19% pT2N0, and 2.8% achieved a pathological complete response (pCR) after neoadjuvant chemotherapy. The detailed description of patient characteristics is provided in Table 1. Acute toxicity was defined as any grade of ARD occurring during treatment or within the first three months after RT. The onset of ARD was recorded at first clinical detection in the scheduled follow-up, during RT or after its completion, by nursing staff or the radiation oncologist. Toxicities were graded using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0; and the statistical analysis was performed with IBM SPSS version 25.0
to 14.5 months (n=4). Pooled OS1 was 93% (95% confidence interval [CI]: 85-100%, I2=59.2%, n=88) (Figure 1) and PFS1 was 21% (95% CI:3-39%, I2=62.4%, n=56) (Figure 2) for patients treated with SABR. The PFS1 for the standard of care arm of CURB and STOP was 9% (95% CI:0-20%, I2=0%, n=26). Pooled 6-month change in therapeutic strategy was 29% (95% CI:8-49%, I2=71.8%, n=64).
Conclusion: There were wide ranges of reported outcomes in currently published prospective trials in SABR for oligoprogressive breast cancer. Trials with predominantly hormone sensitive breast patients appear to have better PFS. Identifying optimal breast subgroups and timing in lines of systemic therapy for SABR treatment will be important next steps. References: [1] Tsai CJ et al.Consolidative Use of Radiotherapy to Block [CURB] oligoprogression:an open-label, randomised, controlled, phase 2 study.Lancet 2024;403(10422):171-182.[2] Schellenberg D et al.Stereotactic Ablative Radiation for Oligoprogressive Cancers:Results of the Randomized Phase 2 STOP Trial.Int J Radiat Oncol Biol Phys 2025;121:28–38.[3] Glicksman RM et al.The Role of Stereotactic Body Radiotherapy in Oligoprogressive Malignant Disease (RADIANT):Oncologic Outcomes From Phase 2 Nonrandomized Controlled Trial.Int J Radiat Oncol Biol Phys 2025;121:292–306.[4] David S et al.Stereotactic Ablative Body Radiotherapy for Oligoprogressive Estrogen Receptor–Positive Breast Cancer:A Phase II Prospective Multicenter Trial.JCO Oncol Adv 2025. Keywords: SABR, oligoprogression, breast Digital Poster 3653 Real-world data on ultra- vs. moderated hypofractionated breast radiotherapy: differences in the onset of acute skin toxicity. Sara Moreno-López 1 , Gabriela Oses 1,2 , Queralt Castells 1 , Maria Laplana 1 , Alvaro Martinez 1 , Carlos Clavell 1 , Meritxell Mollà 1,2 1 Radiation Oncology, Hospital Clínic of Barcelona, Barcelona, Spain. 2 Department of Medicine, University of Barcelona, Barcelona, Spain Purpose/Objective: Adjuvant radiotherapy (RT) remains a cornerstone in reducing locoregional recurrence among patients with early-stage breast cancer. Studies have demonstrated that the ultra-hypofractionated (UH) regimen of 26 Gy
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