239 - Clinical Outcomes of Patients with Early-Stage Breast Cancer Treated with Five-Fraction Partial Breast Irradiation vs. Intraoperative Radiation Therapy

A. Bommireddy¹, S. M. Parker², A. Halima¹, Z. Al-Hilli³, S. Cherian¹, R. D. Tendulkar¹, and C. S. Shah¹; ¹Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, ²Department of Radiation Oncology, Cleveland Clinic, Cleveland, OH, ³Breast Center, Integrated Surgical Institute, Cleveland Clinic, Cleveland, OH

Purpose/Objective(s): Partial breast irradiation (PBI) offers several advantages over whole breast irradiation including smaller treatment volumes and potentially shorter treatment duration. A preliminary institutional series showed worse locoregional control rates with intraoperative radiation therapy (IORT) when compared to 5-fraction PBI; however, one limitation was the lack of long-term follow-up of the PBI cohort. We report a longer-term update to our institutional experience with IORT and 5-fraction PBI for patients with early-stage breast cancer. Materials/

Methods: Patients with early-stage breast cancer (Stage 0-IIA) who received IORT or PBI between 2011 and 2021 were retrospectively reviewed from a single institutional IRB-approved database. PBI patients received 30 Gy in 5 fractions using intensity modulated radiation therapy delivered every other day or daily. IORT patients received 20 Gy in 1 fraction using a 50 kv device prescribed to the applicator surface delivered at the time of surgery. Patients with less than two years of follow-up were excluded from analysis. The primary endpoint was local control (LC). Survival analysis was performed using Kaplan-Meier method with log-rank test.
Results: 245 patients with a median age 67 years old (IQR: 61-72) received 5 fraction PBI, and 202 patients with a median age 70 years old (IQR: 67-74) received IORT. Median follow-up was 3.5 years for PBI and 6.5 years for IORT. While most patients in both cohorts had T1 disease (76.3% for PBI and 91.6% for IORT), more patients had Tis in the PBI group (18.8% vs 2.5%, p < 0.001). There was no difference in estrogen receptor or progesterone receptor status between the two cohorts, but slightly more patients that received PBI were HER2-positive (3.7% vs 1.0%, p = 0.043). Twelve (5.9%) patients that received IORT also received external beam radiation therapy after final surgical pathology revealed positive margins or a positive sentinel lymph node biopsy. Adjuvant endocrine therapy was administered in 75.9% of PBI patients and 78.7% of IORT patients (p = 0.484). More PBI patients received chemotherapy (6.5% vs 1.5%, p = 0.008). Recurrence at any time occurred in 3 patients (1.2%) after PBI and 19 patients (9.4%) after IORT. 5-year LC after PBI and IORT was 99.6% and 94.7%, respectively (p = 0.013). No difference was seen in 5-year regional control (98.8% vs 99%, p = 0.718), distant metastasis free survival (99.5% vs 97.4%, p = 0.119), or overall survival (95% vs 94.1%, p = 0.633) rates.
Conclusion: In this updated large series of patients with early-stage breast cancer, IORT remains inferior to 5-fraction PBI with regards to local control. As adjuvant PBI becomes increasingly utilized in breast conservation therapy, IORT has a limited role as monotherapy.