Abstract Purpose/objective The GammaPod is a breast-specific stereotactic radiotherapy device capable of delivering ablative doses with sharp gradients through precise breast localization. We delivered a 8 Gy tumor bed (TB) boost using the GammaPod system followed by a hypofractionated course (40 Gy in 15 fractions or 42.56 Gy in 16 fractions) of whole breast radiation in patients with early stage breast cancers. We hypothesized that 8 Gy to the TB compared to 10 Gy with low energy photons or electrons prior to whole breast irradiation will achieve high patient cosmesis satisfaction, minimal toxicities, and comparable treatment outcomes. Materials/methods Patients diagnosed with an invasive or non-invasive breast cancer treated surgically by a partial mastectomy were eligible if they had AJCC clinical stage 0-II, tumor size 4 cm, and negative margins. The clinical target volume (CTV), defined as the surgical cavity plus 5-mm, received 8 Gy and a 3-mm margin was added to create 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 March 2019 to November 2023, 63 patients were treated. Histology included 35 DCIS and 28 invasive carcinomas. Of those with invasive carcinomas, 22/28 were ER+. 41/63 (65.1%) patients received endocrine therapy. Typically completed within a week of the single fraction tumor bed boost, 32 patients received 40 Gy in 15 fractions, 29 patients received 42.56 Gy in 16 fractions, and 2 patients received alternative fractionation regimens outside these two standard options.Median follow-up was 52.9 mo (1.9-93.5 mo). There were 27 acute Grade (gr) 1 toxicities and 5 gr 2 toxicities, with no gr 3 or higher acute toxicities ( 90 days) following the entire course of RT (boost + whole breast). Gr 2 toxicities included radiation dermatitis (2), breast pain (1), and fatigue (2). In the late period, there were 37 acute gr 1 toxicities and 6 gr 2 toxicities, with no gr 3 or higher late toxicities. Gr 2 toxicities included radiation dermatitis (2), breast pain (2), and fatigue (2).Physician scored cosmesis was excellent or good in 37/38 (97.4%), 40/42 (95.2%), 27/28 (96.4%), and 21/22 (95.5%) respectively at baseline, 12 mo, 24 mo, and 36 mo post S-PBI. There have been two local recurrences and no distant recurrences. Conclusion: Single fraction boost prior to whole breast radiation was safe and effective and reduced overall treatment time. Citation Format: O. Cong, S. McAvoy, A. Rahimi, M. Guerrero, S. Becker, E. Nichols. Single Fraction Boost Prior to Whole Breast Radiation Using 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 PS1-07-13.
Cong et al. (Tue,) studied this question.