ESTRO 2025 - Abstract Book

S565

Clinical - Breast

ESTRO 2025

3200

Digital Poster Phase I/II study for accuracy and imaging response of MRI-directed preoperative accelerated partial breast irradiation in prone position (NCT02186470) Sasha J. Beyer 1 , Sachin R. Jhawar 1 , Erin Healy 2 , Yevgeniya Gokun 3 , Jose G. Bazan 4 , Michael Knopp 5 , Julia R. White 6 1 Radiation Oncology, The Ohio State University, Columbus, OH, USA. 2 Radiation Oncology, University of California Irvine, Irvine, USA. 3 Biomedical Informatics, The Ohio State University, Columbus, OH, USA. 4 Radiation Oncology, City of Hope, Duarte, CA, USA. 5 Radiology, University of Cincinnati, Cincinnati, USA. 6 Radiation Oncology, University of Kansas, Kansas City, Kansas, USA Purpose/Objective: The challenges of postoperative accelerated partial breast irradiation (APBI) include targeting inaccuracies and inability to measure tumor response. We hypothesized that the tumor could be reliably targeted with preoperative APBI (pAPBI) using magnetic resonance imaging (MRI) in prone position in combination with our established prone breast technique. We developed methodology for CT simulation and MRI in prone position, registration of MRI-CT for planning, and pAPBI delivery using the same immobilization platform. This study aimed to assess the accuracy and corresponding tumor response of pAPBI using our MRI/CT-guided prone technology. Material/Methods: This prospective study (NCT02186470) enrolled patients >50 yo with clinical (c)Stage IA ER+/PR+ breast cancer intending lumpectomy. Axillary ultrasound (US) and MRI imaging confirmed clinically negative nodes. Planning CT was followed by MRI using the same prone immobilization platform. pAPBI (3850 cGy in 10 fractions BID) was delivered with the same prone immobilization. Another MRI was obtained 4 weeks after APBI and the intensity, kinetics and volume were quantitatively measured to assess tumor response. Lumpectomy was performed 4-6 weeks after pAPBI. Simon 2 stage design required assessment after accrual of first 19 patients for assessment of feasibility. Results: Nineteen cStage IA ER+/PR+/HER2- breast cancer patients with median age of 65 (range 51-78) completed pAPBI followed by lumpectomy and endocrine therapy. Median follow-up was 73.8 months (IQR 58.6-82.5 months). Mean clinical tumor size was 1.1 cm ± 0.4 and mean path tumor size was 0.94 cm ± 0.6. There was complete pathologic response in 10.5% (2) patients and an additional 36.8% (7) were down-staged after pAPBI, resulting in a total response of 47.4% (9). Tumor response detected by MRI significantly correlated with tumor response on pathology (p=0.03). Cosmesis was rated as excellent/good in 89.5% (17) patients post-APBI. Cosmesis was fair in 10.5% (2) patients, one of which required adjuvant WBI after final pathology with triple negative breast cancer. Three patients had positive macrometastatic lymph nodes on pathology despite clinically negative nodes on imaging. One in breast recurrence outside the RT field was detected by MRI at 14 months and another patient was diagnosed with osseous metastases at 20 months. Conclusion: Using the same prone platform, patients successfully underwent CT simulation, MRI acquisition and pAPBI followed by lumpectomy. Nearly half of patients demonstrated tumor response to pAPBI based on imaging and pathology. While this methodology resulted in good cosmesis and tumor control, the challenge of excluding patients with subclinical lymph node positive disease remains.

Keywords: breast cancer, MRI, APBI