PURPOSE/OBJECTIVE(S): Interest has rapidly grown in past years in reducing the duration and volume of adjuvant radiation therapy for women with early-stage breast cancer. One such technique that accomplishes both is breast SBRT. We undertook an IRB-approved prospective clinical trial of 30 Gy/5fx SBRT to study local recurrence outcomes and skin toxicity in women who were suitable for partial breast irradiation (PBI) according to ASTRO guidelines. Here we report the plan quality, organ-at-risk dosimetry, and treatment efficiency for the treated patients. MATERIALS/METHODS: We accrued 20 women to the trial, with 19 treatments available for analysis at time of submission. 16 were treated in prone, and 3 in the supine position. Women had bioabsorbable fiducial markers placed at time of surgery, which we used for cavity delineation, patient alignment, and intrafraction triggered imaging. We added a 1cm CTV and 0.3cm PTV expansion to the cavity. Plans were delivered in five consecutive days. In addition to trial outcomes, we catalogued plan quality according to conformity (RTOG), R50 (V50%/V100%), OAR exposure, as well as treatment delivery time. Plans were delivered with 7-13 co-planar IMRT fields (median MUs/fx = 2189) with 10MV FFF beams on an Edge linear accelerator. RESULTS: Median lumpectomy cavity size was 7.2 ± 3.6 cc. Median PTV size was 78.3 ± 25.7 cc. Conformity (RTOGmedian = 1.04 ± 0.03) and gradient (R50median = 3.1 ± 0.4) of all plans were excellent. Heart dose was low for left and right-sided plans (103.4 ± 52.9 & 59.14 ± 21.7) was cGY respectively. Mean ipsilateral breast dose was 750 ± 257 cGY, mean contralateral breast dose was 142 ± 74 cGY. Median beam & total treatment times 4.5 ± 2.4 and 12.7 ± 10.2 min per fraction. CONCLUSION: Breast SBRT is an exceptional and convenient means of adjuvantly treating lumpectomy cavities with partial breast irradiation that spares OARs including non-at-risk ipsilateral breast tissue and allows patients to finish their treatment courses within one week with little to no additional time on the treatment table. Bioabsorbable fiducial markers facilitated lumpectomy cavity delineability and were also useful for patient positioning and intrafraction motion monitoring. Future work will assess the viability of condensing treatment to a single fraction to further reduce treatment burden on patients.