S319
Clinical - Breast
ESTRO 2026
Material/Methods: Twenty eligible patients with HR+, HER2-, clinical stage I, unicentric, non-lobular breast cancers with no LVSI, Oncotype ≤ 25 and age ≥ 50 were accrued to an IRB approved protocol. Patients were treated with 3mo of ET followed by restaging ultrasound and ART (37.5Gy/5fxn qod), using an MR linear accelerator (MRLinac) when feasible. Patients underwent percutaneous vacuum-assisted, image-guided core biopsy (VAIGCB) of the tumor 6-12mo following ART with a minimum of 12 9G cores. Those without a pCR underwent surgery. pCR rates were reported along with the 95% credible interval (CI). Progression free survival was estimated using the Kaplan-Meier method along with the 95% confidence interval. Results: Median age at enrollment was 71 years (64-81), median Oncotype was 11 (7-18), and median Ki-67 was 6% (5%-15%). 16 of 20 patients (80%) were treated on an MRLinac and the remainder were treated using a linac with CT on rails. 19 of 20 (95%) of patients underwent VAIGCB, with 1 declining. Of the 19 biopsies, 10 (53%, 95% CI 30%, 73%) demonstrated pCR, with a pCR rate of 45% (95% CI 19%-72%) and 63% (95% CI 27%-87%), respectively, for biopsies performed 6 and 12mo following ART. Baseline ultrasound-assessed volume of tumor did not correlate with VAIGCB pCR status (p=0.29), but the volume at 3mo after initiation of ET (just prior to ART) and the percent volume decrease from baseline to this same time point were both associated with VAIGCB pCR status (p=0.02 and p=0.04, respectively). At a median follow-up time of 37.3 months for patients omitting surgery, the tumor progression rate was 0%. The 3-year PFS rate was 92% (95% CI 54%, 99%), with one patient dying of non-breast cancer-related causes. Conclusion: This first, prospective study of definitive, ablative radiotherapy and ET with omission of surgery in exceptional responders resulted in no tumor progression. VAIGCB pCR to ART was associated with the volumetric size of the tumor at the time of ART as well as its 3mo percent volumetric decrease in response to ET, highlighting potential imaging and biomarker opportunities to select appropriate patients for this approach. ART warrants study in a larger, multi-institutional study for breast cancer patients interested in non-surgical treatment options. Keywords: ablative radiotherapy, omission of surgery
Digital Poster 4198 Anatomical Predictive Factors of Cardiac Dose in Left-Sided Breast Radiotherapy: Analysis of Correlations between Lung Volume, PTV, and DAP Variation MARCIO LEMBERG REISNER 1 , STELA PALTRINIERI NARDI 1 , DAYANE CESAR TREVISAN 1 , CAMILA MARI MIZOBUCHI KAWAMOTO 1 , ROSANA RODRIGUES ANDRADE 1 , ISABELLA PEIXOTO BARBOSA 1 , JULIANA DEPRÁ PANICHELLA 1 , PATRICIA IZETTI RIBEIRO 1 , ADALEIA JESSICA ARAUJO ALVES DA SILVA 1 , SABRINA REIS 1 , DEBORA AZEVEDO VICTORINO 1 , JULIANA POMPEU PECORARO 1 , THAMIREZ VIEIRA 1 , FERNANDO METON VIEIRA 1 , RACHELE GRAZZIOTIN 2 1 RADIATION ONCOLOGY, AMERICAS ONCOLOGIA, RIO DE JANEIRO, Brazil. 2 RADIATION ONCOLOGY, INSTITUTO NACIONAL DE CANCER, RIO DE JANEIRO, Brazil Purpose/Objective: To identify and quantify the anatomical predictive factors of dose to cardiac structures during left-sided breast radiotherapy, comparing free breathing (FB) with deep inspiration breath-hold (DIBH), to optimize patient selection for cardiac-sparing techniques. Material/Methods: We conducted a retrospective analysis of 94 patients who underwent adjuvant left-sided breast radiotherapy. This study was approved by local Research Ethics Committee, protocol number 83104324.0.0000.5533. Correlations were evaluated between anatomical parameters—lung volume, Planning Target Volume (PTV), and anteroposterior diameter (APD) variation—and the doses delivered to the heart, left ventricle (LV), and left anterior descending (LAD) artery. All doses were normalized to 2 Gy-equivalent fractions (EQD2). Linear regression and ANOVA models were used to quantify dose-response relationships and compare patient subgroups. Results: Inspiratory lung volume demonstrated the strongest correlation with cardiac dose, showing a significant negative correlation with the mean LAD dose (r = - 0.438; p < 0.00001). Regression analysis indicated a reduction of 181 cGy in mean LAD dose for every 100 cm ³ increase in left lung volume during inspiration. PTV volume was positively correlated with LAD dose (r = 0.263; p = 0.011), resulting in a 125 cGy increase for every 100 cm ³ of PTV. The APD variation between inspiration and free breathing, a surrogate for thoracic expansion capacity, showed a trend towards correlation with heart dose reduction (20.34 cGy/cm; p = 0.054). Patients in the lowest tertile of lung volume (<2200 cm ³ ) had a 30% higher mean LAD dose compared to those in the highest tertile (>2600 cm ³ )
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