Abstract PS1-07-05: 2.5-year follow-up results of a five-fraction external beam radiotherapy accelerated partial breast irradiation for early-stage breast cancer after breast-conserving surgery

Abstract Background: External beam accelerated partial breast irradiation (APBI) has emerged as a convenient alternative to the conventional 4-6 weeks of whole breast irradiation following breast-conserving surgery. This study presents 2.5-year follow-up data assessing the feasibility, safety, and efficacy of the 5-fraction APBI Florence trial regimen in select patients with early-stage breast cancer. Methods and Materials: A retrospective analysis was conducted on patients who received an ultra-hypofractionated external beam APBI regimen of 30 Gy delivered in 5-fractions. Eligible patients were women aged ≥40 years with early-stage breast cancer (pTis or pT1-2), no axillary lymph node involvement, tumor size ≤30 mm, and no distant metastases. All had undergone breast-conserving surgery, had histologically confirmed invasive carcinoma or pure ductal carcinoma in situ (DCIS), and had tumor bed clips placed. Clinical data collected included patient and tumor characteristics, treatment parameters, oncologic outcomes, and acute and late toxicities assessed using CTCAE v5.0. Adverse events occurring within 90 days post-APBI were categorized as acute; those occurring thereafter were considered late toxicities. Results: Between April 2021 and December 2022, 54 female patients most commonly with pTis tumors (29.6%) underwent external beam APBI. The median age was 68.0 years (range, 50.0-85.0) and 98.1% had an ECOG performance status of 0-1. A body mass index ≥30.0 mg/kg2 was reported in 40.7% of patients. Tumor laterality was nearly evenly distributed, with 29 (53.7%) patients having left-sided and 25 (46.3%) right-sided breast cancer. The majority of tumors were localized in the upper outer quadrant (42.6%). Median pathological tumor size was 6.5 mm (range, 1.0-29.0). Histological subtypes included invasive ductal carcinoma (55.6%), DCIS (33.3%), and invasive lobular carcinoma (11.1%). Pathological staging was available for 52 of 54 patients: 16 (29.6%) were grade 1, 33 (61.1%) grade 2, and 3 (5.6%) grade 3. Estrogen receptor positivity was observed in 98.1%, progesterone receptor in 77.8%, and Her2Neu positivity in 1.9% of tumors. During the median follow-up of 37.7 months from diagnosis (range, 5.4-49.6) and 34.7 months from APBI (range, 0.6-46.8), majority of patients (87.0%) received adjuvant hormonal therapy post-APBI. Acute grade 1 toxicities were observed in 24.1% of patients, including breast pain (11.1%), dermatitis radiation (7.4%), and fatigue (7.4%). Late grade 1 toxicities were reported in 7.4% of patients, with breast pain (3.7%), and fatigue (3.7%) being the most common. No toxicities of grade 2 or higher have been reported. Breast tissue fibrosis occurred in 1 patient, while 3 patients developed fat necrosis at a median 18.9 months post-APBI (range, 4.3-21.2); all of which were asymptomatic and required no intervention. At the most recent follow-up, all patients remained alive, with no cases of locoregional recurrence or distant metastasis observed. Conclusion: External beam APBI after breast conservation surgery is a feasible treatment approach resulting in excellent tumor control and with minimal toxicities. The outcomes of this study are consistent with those of the external beam APBI Florence trial, further supporting its use as a standard treatment option for appropriately selected patients with early-stage breast cancer. Citation Format: R. A. Herrera, N. Strusberg Fernandez, Y. Weiss, A. Botero, C. Lopez-Penalver, S. Mautner, J. Mendez, G. Giron, J. Panoff, M. M. Rodrigues. 2.5-year follow-up results of a five-fraction external beam radiotherapy accelerated partial breast irradiation for early-stage breast cancer after breast-conserving surgery [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 PS1-07-05.