Abstract Purpose/Objectives: Response to neoadjuvant systemic therapy (NST) is often associated with favorable outcomes in breast cancer patients. We have demonstrated that patients with pathologic complete response (pCR) or minimal residual disease following breast conserving surgery (BCS) and treated with whole breast irradiation (WBI) have in-breast recurrence rates low enough that de-escalation of therapy with accelerated partial breast irradiation (APBI) may be safe. However, the role of APBI in this population remains poorly described. Herein, we retrospectively evaluated the technical feasibility of APBI in women who received NST and BCS. Materials/Methods: We have an established database of 321 breast cancer patients at a single comprehensive cancer center treated with NST, BCS, and WBI from 2017-2021. From this group, we identified a subset of patients that had pre-chemotherapy magnetic resonance imaging (MRI) of the breasts and had not undergone oncoplastic reduction mammoplasty at the time of BCS. On the radiation treatment planning CT scans, we contoured the surgical bed gross tumor volume (GTVsb). A 1. 0-1. 5 cm margin was added to create the surgical bed clinical target volume (CTVsb), and a final 0. 5 cm margin was added to create the surgical bed planning target volume (PTVsbEval), which was cropped 0. 4 cm from the skin and did not extend outside of breast tissue. We also contoured the ipsilateral breast (IB) to calculate the ratio of the PTVsbEval volume to the IB volume (PTV/IB-R). Technical feasibility of APBI was defined as GTVsb location within the same quadrant as the primary tumor on MRI and PTV/IB-R≤30%. APBI treatment plans using volumetric modulated arc radiotherapy (VMAT) were retrospectively created for patients with PTV/IB-R in the range of 25-30% using the dose/fractionation schedule of 3000 cGy in 5 fractions and the same planning constraints as the University of Florence APBI trial. Our primary endpoint was to establish the technical feasibility rate of APBI in this patient population with a rate of ≥70% defined as worthy of moving forward with development of a prospective trial. Results: From our cohort, we identified 40 women who received NST followed by BCS and WBI. Median age was 53 years (IQR 47-61 years), with most patients being Hispanic White (50%), non-Hispanic White (22. 5%), Black or African American (10%), or Asian (7. 5%). Median body mass index (BMI) was 30 kg/m2 (IQR 26. 9-35. 2 kg/m2), and 35% were pre-menopausal. When delineating the GTVsb, 92. 5% had visible metallic clips, 52. 5% had no associated seroma, and 10% underwent local tissue rearrangement during BCS. All GTVsb volumes were concordant with the same quadrant as the primary tumor visualized on pre-treatment MRI. The median margin from GTVsb to CTVsb was 1. 0 cm (range, 1. 0-1. 5 cm). Median PTVₑval volume was 138. 2 cc (IQR 98. 3-232. 5 cc), and median IB volume was 993. 6 cc (IQR 708. 3-1587. 7 cc). The median PTV-IB/R was 15. 3% (IQR 11. 6-19. 5%). Three patients had PTV/IB-R values above 25% (29. 2%, 25. 5%, and 25. 5%). All 3 corresponding VMAT plans met all dosimetric planning objectives and normal tissue constraints for acceptable APBI plans. Therefore, the technical feasibility rate of APBI in this patient cohort was 100%. Conclusion: Our current analysis indicates that APBI is technically feasible in patients that receive NST followed by BCS. Pre-chemotherapy MRI is an important prerequisite to ensure that the GTVsb is accurately defined. Coupled with recent evidence from our institution demonstrating that patients with early-stage (cT1-3 cN0-1) breast cancer achieving pCR or lymph node-negative with minimal residual disease in the breast (cN0/ypN0 and ≤ypT1c) have low rates of in-breast tumor recurrence after WBI, the current study supports prospective evaluation of APBI in this favorable patient population. Citation Format: A. A. Kassardjian, J. Nall, T. Reilly, C. Wong, S. Yoon, T. Watkins, J. Bazan. Feasibility of accelerated partial breast irradiation (APBI) following neoadjuvant systemic therapy and lumpectomy abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32 (4 Suppl): Abstract nr PS1-08-02.
Kassardjian et al. (Tue,) studied this question.