2665 - Ultra-Hypofractionated Whole Breast Radiation Therapy Using a Novel Boost Regimen for Early-Stage Breast Cancer

M. A. Chakraborty¹, A. J. Khan², A. B. Tadros³, C. White⁴, M. Kim³, Z. Zhang⁴, L. Z. Braunstein², A. J. Xu², Q. LaPlant², D. A. Roth OBrien², J. J. Cuaron², S. N. Powell², and I. J. Choi^2,5; ¹Rutgers New Jersey Medical School, Newark, NJ, ²Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ³Breast Service, Department of Surgery, Memorial Sloan Kettering Cancer Center, New York, NY, ⁴Department of Epidemiology and Biostatistics, Memorial Sloan Kettering Cancer Center, New York, NY, ⁵New York Proton Center, New York, NY

Purpose/Objective(s): The 5-year results of the FAST-Forward trial demonstrate non-inferiority of 26 Gy delivered in 5 daily fractions (fx) compared to 40 Gy in 15 fx for breast cancer patients receiving adjuvant radiotherapy (RT). In this trial, patients with an indication for cavity boost received either 10 Gy in 5 daily fx or 16 Gy in 8 fx. We employed a novel boost regimen of 5.2 Gy in one fx for greater consistency with the overall treatment paradigm. We hypothesize that this boost regimen is feasible with high rates of disease control and limited toxicity. Materials/

Methods: Patients with non-metastatic invasive carcinoma or DCIS of the breast treated at our institution between 07/01/2019 and 6/1/2022 with adjuvant whole breast RT to 26 Gy in 5 daily fx with or without a single fx cavity boost were identified. Patient and tumor characteristics, treatment details, adverse events (AEs) and disease control outcomes were collected. Locoregional recurrence free survival (LR-RFS), regional recurrence free survival (RRFS), distant recurrence free survival (DRFS) and overall survival (OS) probabilities were estimated using the Kaplan-Meier method. Correlations between toxicities and patient, tumor, and treatment characteristics were assessed using logistic regression.
Results: A total of 284 patients were included in the analysis with a median age of 72 (range: 39-95) and a median follow-up of 22 months (range: 0-46). 167 patients (59%) received a boost. Acute grade (G) = 2 AEs, defined as those occurring =90 days after RT completion, developed in 25 patients (8.9%), with 8 patients (2.9%) experiencing an acute G3 AE. Late =G2 AEs developed in 15 patients (5.6%), with 7 patients (2.6%) experiencing a late G3 AE (G2: breast pain, n=1; breast lymphedema, n=4; arm lymphedema, n=1; fibrosis, n=3. G3: chest wall discomfort, n=1; fibrosis, n=6). Greater BMI (p=0.015), breast volume (p=0.002), cavity volume (p=0.041) and bolus use (p=0.048) were associated with an increased risk of =G1 acute AEs, and breast volume (p=0.017) and bolus (p=0.001) with =G2 acute AEs. Greater breast volume was associated with =G1 late AEs (p=0.029) and was borderline for late =G2 AEs (p=0.05). Receipt of a boost was not associated with higher toxicity risk. The estimated 24-month LR-RFS, RRFS, DRFS, and OS were 96%, 97%, 96%, and 97%, respectively.
Conclusion: A 5-fraction ultra-hypofractionated regimen for patients receiving adjuvant whole breast RT for early-stage breast cancer results in excellent early disease control and acceptable rates of acute and late toxicities. Receipt of an ultra-hypofractionated boost of 5.2 Gy in one fx did not appear to increase risk of toxicity. This regimen may be appropriate for carefully selected patients, such as those of advanced age or with logistical challenges, and continued follow-up will be of value to characterize long-term effects of this regimen. Additional data on patient-reported acute and long-term outcomes will be collected and analyzed by the time of presentation.