Abstract PS1-07-14: Five Fraction Stereotactic Partial Breast Irradiation (S-PBI) with a Cobalt Stereotactic Unit for Early Stage Breast Cancer

Abstract Purpose/objective For early stage breast cancer, the GammaPod is a stereotactic radiotherapy device capable of delivering ablative doses with sharp gradients through precise breast localization. We hypothesize that 5-fraction stereotactic partial breast irradiation (S-PBI) will achieve high 3 year patient cosmesis satisfaction, minimal toxicities, and comparable treatment outcomes compared to external beam radiation therapy. We report combined results from patients treated with S-PBI at 30 Gy in 5 fractions or 40 Gy in 5 fractions as part of a multi-institutional phase II dose-escalation trial. Materials/methods Patient eligibility included DCIS or invasive epithelial histologies, AJCC clinical stage 0-II, tumor size 3 cm, and negative margins. For those patients on the phase II trial protocol, 40 Gy dose was delivered in 5 fractions to the clinical target volume (CTV) with a minimum 30 Gy dose to the planning target volume (PTV). Cosmesis was assessed at multiple follow-up time points using a physician-rated score (4 point scale excellent, good, fair, or poor). Treatment-related toxicity was graded with NCI CTCAE v5.0. Results From November 2018 to June 2024, 100 patients were treated. Histology included 74 DCIS and 26 invasive carcinomas. Of those with invasive carcinomas, 23/26 were ER+. 83/100 (83.0%) patients received endocrine therapy. 16 patients received 40 Gy in 5 fractions, 71 patients received 30 Gy in 5 fractions, and 13 patients received alternative fractionation regimens outside these two standard options. Median elapsed time between the first and fifth treatment was 13 days (4-22 days). Median follow-up was 39.3 mo (1.6-72.8 mo). There were 30 acute Grade (gr) 1 toxicities and 4 gr 2 toxicities, with no gr 3 or higher acute toxicities ( 90 days). Gr 2 toxicities included radiation dermatitis (2), breast pain (1), and fibrosis (1). In the late period, there were 37 acute gr 1 toxicities with no gr 2 or higher late toxicities. Physicians scored cosmesis was excellent or good 79/80 (98.8%), 59/61 (96.7%), 33/34 (97.1%), and 29/30 (96.7%) respectively at baseline, 12 mo, 24 mo, and 36 mo post S-PBI. There have been two local recurrences and two distant recurrences. Conclusion: S-PBI was a safe and effective treatment and offers another treatment modality for PBI. Due to its sharp dose gradient and favorable side effect profile future studies will focus on further shortening treatment time for patients. Citation Format: O. Cong, S. McAvoy, A. Rahimi, M. Guerrero, S. Becker, E. Nichols. Five Fraction Stereotactic Partial Breast Irradiation (S-PBI) with a Cobalt Stereotactic Unit for Early Stage Breast Cancer 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 PS1-07-14.