2739 - Pre-Operative, Single Fraction Stereotactic Partial Breast Irradiation (S-PBI) for Early-Stage Breast Cancer: Radiology-Pathology Correlation of Tumor Response in a Phase 2 Clinical Trial

C. Reverberi¹, A. Prisco¹, G. Facondo¹, E. Moretti², P. S. Scalchi², M. Orsaria³, E. Pegolo³, C. Di Loreto³, C. Zuiani⁴, L. Seriau⁵, S. Bertozzi⁵, C. Cedolini⁶, and M. Trovo¹; ¹Department of Radiation Oncology, ASUFC "Santa Maria della Misericordia", Udine, Italy, ²Medical Physics Unit, ASUFC Santa Maria della Misericordia, Udine, Italy, ³Institute of Anatomic Pathology, ASUFC "Santa Maria della Misericordia", Udine, Italy, ⁴Department of Medicine, Institute of Radiology, University of Udine "Santa Maria della Misericordia", Udine, Italy, ⁵Breast Unit, Department of Surgery, DAME, University Hospital of "Santa Maria della Misericordia", Udine, Italy, ⁶Breast Surgery, Department of Medicine (DAME), University Hospital of Udine, Udine, Italy

Purpose/Objective(s): To assess the value of breast MRI and contrast-enhanced mammography (CEM) to identify pathologic major response in patients with early-stage breast cancer undergoing pre-operative S-PBI. Materials/

Methods: We conducted a phase II clinical trial enrolling women older than 50, with proven breast invasive non special type carcinoma, hormonal receptors positive/HER2 negative, any grade, tumor size < 3cm, unifocal, withoutnodal involvement, and candidates to conservative surgery. The Gross Tumor Volume (GTV) comprised the tumor. The Clinical Target Volume (CTV) was equal to GTV. The Planning Target Volume (PTV) was created by adding a 3 mm margin to CTV. The total dose was 30-36 Gy prescribed to the 95% of the PTV. Patients had a breast MRI or CEM 3 months after S-PBI and underwent surgery at 4 months. The radiological response was described as no evidence of disease (rNED) or partial response (rPR) if any residual tumor was detectable. Pathologic response was classified as complete (pCR, absence of residual tumor), near pCR (npCR, <10% invasive tumor left), partial (pPR, <50% tumor left), or stable disease (pSD, no response). Pathologic “major response” included patients with pCR and npCR. Radiology-pathology correlation was conducted and post- S-PBI positive (PPV) and negative predictive (NPV) value calculated.
Results: From 01/2022 to 01/2024, 52 women had S-PBI followed by conservative breast surgery. The analysis was conducting on 46 patients who underwent post-S-PBI breast MRI (n=41) or CEM (n=5). rNED on imaging was reported in 29/46 (63%) cases, while major pathologic response was documented in 15/46 (33%) cases. Among the 29 patients with rNED, there were 14 cases with pCR+npCR, for a NPV of 48%. Among the 17 cases with rPR on imaging, there were 16 cases with residual tumor at pathology (pPR+pSD) for a PPV of 94%.
Conclusion: Post S-PBI breast MRI and CEM have high PPV (94%) for detection of residual tumor, although limited NPV (48%) for the detection of major pathologic response. Our results do not support the use of negative pre-operative imaging to avoid breast surgery.