2960 - Fixed-Field IMRT with Ultra-Low Dose Constraints for Partial Breast Irradiation Improves Low Dose Bath Compared to VMAT

03:00pm - 04:00pm PT

Hall F

Screen: 21

POSTER

Presenter(s)

Elitza Koutleva, MD, MBA - University of North Carolina at Chapel Hill, Chapel Hill, NC

E. Koutleva¹, J. Williamson², B. M. Anderson³, J. L. Wright⁴, G. P. Gupta⁵, E. L. Jones³, L. B. Marks³, K. A. Pearlstein^2,6, and D. L. Casey⁷; ¹University of North Carolina at Chapel Hill, Chapel Hill, NC, United States, ²University of North Carolina, Chapel Hill, NC, ³Department of Radiation Oncology, University of North Carolina, Chapel Hill, NC, ⁴University of North Carolina at Chapel Hill, Chapel Hill, NC, ⁵UNC, Chapel Hill, NC, ⁶Department of Radiation Oncology, University of North Carolina at Chapel Hill, Chapel Hill, NC, ⁷Memorial Sloan Kettering Cancer Center, New York, NY

Purpose/Objective(s): Partial Breast Irradiation (PBI) is commonly delivered via volumetric modulated arc therapy (VMAT). However, VMAT can lead to unintended low-dose radiation exposure to the heart, lungs, and contralateral breast especially when ultra-low dose constraints are not employed. We herein compare dosimetric outcomes between fixed-field IMRT and VMAT-based PBI when ultra-low dose constraints for heart and lung are used in addition to standard planning constraints.

Materials/Methods: Planning images from ten patients (previously treated with PBI of 6 Gy x 5 = 30 Gy; 6 right-sided, 4 left-sided) were used to generate VMAT and fixed-field IMRT plans using dose constraints from the ASTRO PBI guideline. Additional ultra-low dose constraints were added including ipsilateral lung V3Gy<30%, contralateral lung V1.5Gy< 5%, and heart V3Gy< 5% for left-sided treatment. Dosimetric comparisons were created. On average, 2-3 partial arcs were used for VMAT plans and 3-5 non-coplanar beams for fixed-field plans. Metrics examined to evaluate low-dose spread included heart V1Gy and V3Gy, ipsilateral lung V1Gy and V3Gy, and contralateral breast maximum dose. In addition, ipsilateral breast V15Gy and V28.5Gy were compared as a measure of conformality. A paired two sample t-test was conducted for each metric with the two-tailed significance level set to alpha =0.05.

Results: When compared with traditional VMAT plans, fixed-field IMRT plans demonstrated a significant reduction in low dose exposure to the heart (V1Gy 2.98% vs. 7.28%, p=0.05), ipsilateral lung (V1Gy 40.25% vs. 57.15%, p=0.04), and contralateral breast (max dose 1.48Gy vs. 2.57Gy, p=0.004). By contrast, VMAT plans showed better reduction in ipsilateral breast V15Gy and V28.5Gy (see Table 1). Notably, both sets of plans met ASTRO PBI guidelines for all metrics, including ipsilateral breast doses. All plans additionally met our updated ultra-low dose constraints, excluding two VMAT plans and a single fixed-field plan in a medial tumor.

Conclusion: This analysis provides supportive preliminary data that fixed-field IMRT for PBI using novel ultra-low dose heart and lung constraints is feasible, meeting all ASTRO PBI guidelines and nearly all novel objectives. This technique decreases the low dose bath to heart, lung, and contralateral breast when compared with VMAT, with acceptable, but slightly decreased, conformality. Limitations include a small sample size. Further investigation is warranted to optimize planning technique based on tumor location and other anatomical factors.

Abstract 2960 - Table 1: Variations in dose with VMAT vs. fixed-field IMRT for PBI plans

Plan Type	VMAT	Fixed-Field IMRT	p-value
Heart V1Gy	7.28%	2.98%	0.05
Heart V3Gy	0.024%	0.079%	0.51
Ipsilateral Lung V1Gy	57.15%	40.25%	0.04
Ipsilateral Lung V3Gy	21.54%	18.63%	0.45
Ipsilateral Breast V15Gy	43.02%	49.20%	0.01
Ipsilateral Breast V28.5Gy	21.80%	23.74%	0.03
Contralateral Breast Maximum Dose	2.57 Gy	1.48 Gy	0.004