Abstract PS15-05: Hypofractionated volumetric modulated arc therapy for breast cancer: A propensity-score-weighted comparison of radiation-related toxicity according to fractionation and modality

Background: Recent trials support a shorter, hypofractionated regimen (HF-RT) for breast cancer. Based on START-B trial, 3-week schedules of hypofractionated-RT (HF, 40 Gy/15 fractions) has been adopted as the one of standard of cares. We studied the clinical benefit of combining volumetric modulated arc therapy (VMAT) and HF-RT in the incidence of radiation-related toxicities.Methods: We retrospectively reviewed 4209 patients treated with three-dimensional conventional fractionation (CF-3D, mostly 50·4 Gy/28 fractions) and 1540 patients treated with HF-RT (768 received HF-3D and 772 received HF-VMAT, mostly 40 Gy/15 fractions) between 2005 and 2017 at a tertiary academic center. A total of 2229 patients (38.8%) received regional node irradiation (RNI): 1642 (39.0%), 167 (21.7%), and 420 (54.4%) patients received RNI in the CF-3D, HF-3D, and HF-VMAT, respectively. VMAT was used to HF patients either to minimize cardiac dose in regional RT for unfavorable/challenging cardio-thoracic anatomy (n = 420) or to shorten the treatment time by using 3.2 Gy of simultaneous integrated boost (SIB) in 15 fractions for breast conservation (n = 352). Physician-reported events during/within 3 months after RT were defined as acute/subacute toxicity. Late toxicities included radiation pneumonitis, lymphedema, hypothyroidism, and cardiotoxicity. Propensity scores were calculated via logistic regression then inverse probability of treatment weighting analysis was performed for pairwise comparison.Result: The rate of grade 2+ acute/subacute toxicities were the highest in patients treated with CF-3D (15.0%, 2.6%, and 1.6% in patients treated with CF-3D, HF-3D, and HF-VMAT, respectively; p 0·05).Conclusion: HF was associated with decreased toxicities than CF in this real-world inverse probability of treatment weighting analysis cohort. HF-VMAT further decreased acute and late toxicity than HF-3D or CF-3D, especially in women underwent RNI, although prospective long-term follow-up is needed. The shortening of overall treatment time by SIB-VMAT in HF may be of value. Citation Format: Nalee Kim, Jee Suk Chang, Ki Chang Keum, Chang-Ok Suh, Hyejung Shin, Yong Bae Kim. Hypofractionated volumetric modulated arc therapy for breast cancer: A propensity-score-weighted comparison of radiation-related toxicity according to fractionation and modality [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS15-05.