2784 - Comparing Standard and Magnetic Resonance-Guided Linear Accelerators for Accelerated Partial Breast Irradiation: Early Toxicities and Dosimetric Outcomes
Presenter(s)
S. F. Lee1,2, F. Foroudi3, T. Leech3, S. L. Ng3,4, J. Chionh3, S. Jassal4, M. Law4, N. Zantuck4, C. Yong4, M. Cheng5, J. Dean3, E. Bevington3, W. M. Ooi3, S. W. Loh3, P. Newton3, E. S. Choong6, D. Lim Joon7, and M. Chao3,8; 1National University Cancer Institute, National University Hospital, Singapore, Singapore, 2Department of Medical Imaging and Radiation Sciences, School of Primary and Allied Health Care, Faculty of Medicine, Nursing and Health Sciences Monash University, Clayton, Australia, 3Olivia Newton John Cancer Wellness and Research Centre, Austin Hospital, Melbourne, VIC, Australia, 4Eastern Health, Melbourne, VIC, Australia, 5Eastern Health, Melbourne, Australia, 6Olivia Newton John Cancer Centre, Austin Health, Melbounre, VIC, Australia, 7Austin Health, Melbourne, VIC, Australia, 8Genesis Care, Ringwood, Melbourne, VIC, Australia
Purpose/Objective(s):
Comparative clinical data between magnetic resonance-guided linear accelerators (MRL) and conventional linac-based external beam radiotherapy (EBRT) for accelerated partial breast irradiation (APBI) in adjuvant settings remain limited despite the theoretical benefits of real-time imaging. We hypothesized that APBI delivered via MR-Linac yields non-inferior chronic toxicity profiles and superior dosimetric outcomes compared to standard EBRT.Materials/Methods:
A retrospective cohort study evaluated patients treated with APBI at a single institution, stratified into EBRT and MRL cohorts. Baseline clinical and dosimetric data were extracted, while CTCAE v5.0 toxicity data were recorded at 3, 6, 12, 24, and 36 months post-treatment. Categorical and continuous variables were compared using chi-square, Fisher's exact, independent t-tests, or Mann-Whitney U tests as appropriate.Results:
Of 115 evaluated patients, 39 received EBRT and 76 received MRL. Baseline characteristics were well-balanced, including mean age (65.7 vs 65.9 years; p=0.92), tumor size (11.6 vs 10.3 mm; p=0.14), and tumor grade (Grade 1: 59% vs 46%; p=0.24). All tumors across both cohorts were ER-positive, HER2-negative, and LVSI-negative. Most patients in both cohorts received 30 Gy in 5 fractions (113/115, 98.3%). MRL demonstrated significantly superior organ-at-risk sparing compared to EBRT, reducing median Heart V3Gy (1.42cc vs. 7.50cc; p<0.001) and median Ipsilateral Lung V10Gy (3.38cc vs. 8.67cc; p<0.001). Target coverage also differed slightly between platforms (mean PTV D95%: 28.39 Gy vs. 29.16 Gy; p<0.001). At 3 months, Grade 1 dermatitis occurred in 21% of EBRT and 16% of MRL patients, with 1% Grade 2 in MRL (p=0.74). Grade 1 breast pain was 15% (EBRT) and 8% (MRL) (p=0.33). By 6 months, dermatitis resolved completely in both groups; Grade 1 breast pain reduced to 5% (EBRT) and 3% (MRL) (p=0.60). While no Grade 2 or higher toxicities occurred at 12, 24, or 36 months, mild Grade 1 breast pain persisted at very low rates: 5% (EBRT) and 3% (MRL) at 12 months, and 6% (EBRT) and 0% (MRL) at 24 months.Conclusion:
MRL APBI is safe, offering comparable toxicity and distinct dosimetric advantages for normal tissue sparing compared to standard EBRT. Both modalities yielded exceptionally low acute and late toxicities. These findings support integrating MR-guided radiotherapy into early-stage breast cancer practice as it improves treatment efficiency and provides advantageous adaptive capabilities without compromising patient safety. Table 1: Selected Toxicity Outcomes at 3 and 6 Months Post-APBI| Toxicity Parameter | EBRT (n=39) | MRL (n=76) | p-value |
| Dermatitis (Grade 1, 3 months) | 8 (21%) | 12 (16%) | 0.74 |
| Breast Pain (Grade 1, 3 months) | 6 (15%) | 6 (8%) | 0.33 |
| Dermatitis (Grade 1/2, 6 months) | 0% | 0% | – |
| Breast Pain (Grade 1, 6 months) | 2 (5%) | 2 (3%) | 0.60 |