2702 - Daily Online Adaptive Radiotherapy with Narrow Margins for Ultra-Hypofractionated Whole Breast Irradiation (A-FF-WBI): Dosimetry and Early Clinical Outcomes

X. D. Li¹, D. D. M. Parsons¹, J. Visak¹, M. Arbab¹, L. Chen¹, W. Lu², C. Y. Liao¹, C. Tye¹, S. Bahrami¹, N. Wandrey¹, P. G. Alluri¹, M. H. Lin¹, and A. S. Rahimi¹; ¹Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, ²Medical Artificial Intelligence and Automation (MAIA) Lab, Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, TX,, Dallas, TX

Purpose/Objective(s): Ultra-hypofractionated (Fast-Forward) whole breast irradiation (FF-WBI) has been increasingly utilized worldwide. We aim to improve the metrics of FF-WBI by daily online adaptive radiotherapy with narrow margins (A-FF-WBI). Materials/

Methods: We collected the clinical and radiotherapy data of all early-stage breast cancer patients who were treated by A-FF-WBI from an institutional prospective registry. The clinical target volume (CTV) was defined per Fast-Forward study. The adaptive planning target volume (PTV) was a 3 mm uniform expansion (in contrast to 10 mm used in Fast-Forward). Relevant plan metrics for coverage and major OARs (heart, ipsilateral lung and contralateral breast) were compared between the adaptive plans vs the scheduled plans. The range of CTV changes over the 5 treatments normalized to the baseline CTV (?CTV%) was calculated to reflect interfractional variation of breast volume. Correlation between breast volume change and host factors was analyzed by multivariate analysis.
Results: Thirty-four patients (33 female, 1 male) were treated by A-FF-WBI (26Gy over 5 daily fractions) between 02/2023-02/2024 with median age of 68 years (range: 33-88); median BMI of 28.9 kg/m² (range: 18.4-39.9); 26.5% diabetics; 11.8% former vs 88.2% non-smokers; 17 left vs 17 right sided. The adaptive plan was utilized in 98.8% of fractions compared to the scheduled plan. The adaptive approach resulted in significant improvements in major plan metrics when compared to the scheduled plan (Table 1) while no statistically significant differences were observed for hotspots or other OAR dose (unlisted). Heart V7Gy of adaptive plans were significantly lower than 3D plans reported elsewhere (1.5% vs 3-4%^a,b). Median ?CTV% was 7.0% (range: 1.5-14.4%) with 20.6% of patients had more than 10% of breast volume variations through 5 fractions. After multivariable analysis, larger BMI had a statistically significant positive correlation with larger change in breast volumes (p=0.046). Out of twenty-seven patients who have follow up data available, 17 patients had grade 1 acute toxicities (per CTCAE 5) with no grade 2 or higher toxicities reported. All patients are alive without early local failure.
Conclusion: Daily online adaptive radiotherapy with narrow margins for FF-WBI led to superior metrics in coverage and OARs sparing with excellent acute toxicity profile. Long term follow up is needed to confirm the clinical benefits and to identify the patient population that most likely to benefit from this innovative approach. Abstract 2689 – Table 1

Contour	Metric	Adaptive [%]	Scheduled [%]	Difference [%]	P value
CTV	V100%	94.7 ± 1.3	78.7 ± 11.4	16.0	<0.001
	V95%	99.5 ± 0.5	95.6 ± 4.0	3.9	<0.001
PTV	V100%	89.9 ± 0.3	75.9 ± 10.9	14.0	<0.001
	V95%	98.0 ± 1.0	94.8 ± 3.7	3.2	<0.001
Heart (left-sided only)	DMax(Gy)	15.6 ± 3.8	17.8 ± 4.5	2.2	<0.001
	V7 Gy	1.5 ± 1.2	3.3 ± 1.7	1.2	<0.001
Ipsilateral Lung	V8 Gy	13.2 ± 2.0	15.4 ± 2.8	2.2	<0.001