S308
Clinical - Breast
ESTRO 2026
Toronto, Canada. 7 Departments of Paediatrics, University of Toronto, Toronto, Canada
breast cancer-specific death, all-cause death, contralateral breast cancer, and mastectomy stratified by treatment approach. Results: The population cohort includes 234,476 women were treated with BCS+RT for invasive breast cancer or DCIS; 3,913 (1.7%) developed IBR. Of these, 769 (19.6%) were treated by BCS+rRT, 816 (20.9%) were treated by BCS alone and 2,328 (60%) underwent mastectomy + RT. Median age of the overall cohort was 63 years (range 53-71) with a median follow up of 4.3 years (range 0.01-18.2). At 10-years, the cumulative incidence of IBR2 was 7.44% (95% CI: 5.25%–10.12%) for individuals treated by BCS+rRT and 9.55% (95% CI: 6.86%–12.75%) for those treated by BCS alone (p=0.26), with no significant difference in the cumulative incidence of mastectomy. Among patients treated by BCS+rRT (N=769), 10.9% received <40 Gy, 33.3% received 40-44 Gy, and 38.8% received >45 Gy; dose was not reported for 17.0%. The median number of fractions was 16.0 (interquartile range 16.0- 25.0) and RT boost was delivered to 26.8%. On multivariable analysis, age at IBR, index year of IBR, or time from initial diagnosis to IBR (per 1-year increase, or categorically as 0-3 vs >3 years, and 0-3 years vs 3-5 years vs >5years) were not associated with the risk of IBR2. Conclusion: There is considerable heterogeneity in the management of IBR after initial treatment with BCS+RT. Overall, individuals who developed IBR and received repeat breast-conserving therapy achieved favourable local control, supporting its further evaluation of as a safe and effective option for appropriately selected patients. Keywords: breast cancer, reirradiation, local recurrence Oligoprogressive Breast Cancer: A Systematic Review and Meta-analysis of Prospective Clinical Trials Vivian S Tan 1 , Gabriel Boldt 2 , Devin Schellenberg 3 , David A Palma 1 , Aisling Barry 4 , Phillip S Blanchette 5 , Prakesh S Shah 6,7 , Andrew Warner 1 , Jacques Raphael 5 , Joelle Helou 1 1 Department of Radiation Oncology, Western University, London, Canada. 2 Clinical Research Library, Verspeeten Family Cancer Centre, London, Canada. 3 Department of Radiation Oncology, BC Cancer, Surrey, Canada. 4 Department of Radiation Oncology, Cork University, Cork, Ireland. 5 Department of Medical Oncology, Western University, London, Canada. 6 Departments of Paediatrics, Mount Sinai Hospital, Mini-Oral 3640 Stereotactic Ablative Radiotherapy for
Purpose/Objective: Data supporting stereotactic ablative radiotherapy (SABR) to oligoprogressive lesions in breast cancer is controversial. The objective of this study was to pool outcomes from prospective trials to assess SABR in oligoprogressive patients with metastatic breast cancer. Material/Methods: A literature search using PubMed (MEDLINE), Cochrane Central Register of Controlled Trials, Cochrane Database of Systematic Reviews, EMBASE, and Web of Science was conducted from January 2010 to July 2025. Prospective studies of adult patients with oligoprogressive breast disease treated with SABR were included. Descriptive statistics were used to summarize study characteristics. Change in therapeutic strategy was defined as change in systemic therapy or event-free survival as defined in the AVATAR trial. A binary random effects model was used to conduct pooled analyses for 1-year overall survival (OS1) and 1-year progression-free survival (PFS1). Results: Four prospective trials met eligibility criteria and were included in the review including CURB[1] (n=24), STOP[2] (n=12), RADIANT[3] (n=23) and AVATAR[4] (n=32) (Table 1).
CURB and STOP were randomized, while RADIANT and AVATAR were single arm. CURB and AVATAR included breast patients only, while STOP and RADIANT were basket trials including multiple histologies. The breast cancer population varied across trials: CURB included 34% triple negative patients, RADIANT included predominantly HR+/HER2- (87%) with 9% triple negative while AVATAR included only ER+/HER2-. All trials defined oligoprogressive disease ≤ 5 lesions. The median number of lines of systemic therapy was four in CURB and three in RADIANT. In contrast, AVATAR included only patients that were on first-line therapy on aromatase inhibitor (AI) and CDK 4/6 inhibitors. Reported median progression-free survival (PFS) ranged from 4.4 to 12.0 months (n=3) and median time to change in therapeutic strategy ranged from 3.9
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