3025 - Comparative Dosimetric Analysis of Dynamic Collimator VMAT (DC-VMAT) Static Angle Modulated Port (STAMP) Optimization Strategies vs. VMAT for Breast Radiotherapy with Regional Nodal Irradiation

Purpose/Objective(s):

This study compares three Dynamic Collimator VMAT (DC-VMAT) Static Angle Modulated Port (STAMP) optimization strategies with varying control point (CP) counts against conventional VMAT, assessing effects on breast radiotherapy plan quality and delivery.

Materials/Methods:

Thirty breast cancer patient datasets receiving regional nodal irradiation were retrospectively selected. For each patient, one VMAT and three DC-VMAT plans with dynamic collimator were generated: arc-dominant (AD: 2 CP; 1 STAMP), balanced (BAL: 26 CP; 4 STAMP), and static-dominant (SD: 51 CP; 4 STAMP). All plans used two arcs and were optimized with identical target and OAR constraints. Prescription was 4005cGy in 15 fractions, normalized so 95% of breast PTV received 95% of the dose. Seven clinically relevant OAR metrics were evaluated to compare plan quality.

Results:

Hotelling’s T² tests showed significant differences between VMAT and all DC-VMAT strategies (p < 0.0001). All DC-VMAT plans maintained clinically acceptable nodal coverage and reduced all seven OAR metrics vs VMAT (p < 0.0001), with increased MU/dose reflecting a reduction in delivery efficiency (Table 1). BAL and SD were statistically equivalent across all OAR metrics (p > 0.1). Compared with these higher-CP strategies, AD yielded equivalent ipsilateral lung Dmean and contralateral lung V400 (p > 0.1), slightly lower ipsilateral lung V1600, and higher for the remaining four OAR metrics (p < 0.05).

Conclusion:

DC-VMAT techniques for breast radiotherapy significantly reduce heart, contralateral breast, and lung doses compared with VMAT while maintaining target coverage. These reductions are expected to decrease long-term cardiac risk and the likelihood of radiation-induced pulmonary toxicity, while lowering exposure to contralateral breast and lung, potentially minimizing secondary malignancy risks. Although dosimetric differences among DC-VMAT strategies were relatively small, higher CP approaches (BAL and SD) provided incremental improvements in specific metrics. The primary dose reduction is attributable to the rotating collimation DC-VMAT technique, while the additional STAMPs further enhance sparing, particularly for heart and contralateral breast. Despite slightly higher MU and longer delivery times, DC-VMAT offers a viable improvement in plan quality over VMAT, offering superior OAR sparing in breast radiotherapy.

Table 1. Average OAR metrics

	VMAT	AD	BAL	SD
Breast Contra Dmean (cGy)	380	213	206	208
Lung Ipsi Dmean (cGy)	1085	954	958	962
Lung Ipsi V1600 (%)	25.5	21.2	21.8	22.0
Lung Ipsi V400 (%)	67.0	59.0	57.2	57.1
Lung Contra Dmean (cGy)	278	195	188	187
Lung Contra V400 (%)	22.1	10.7	10.2	10.1
Heart Dmean (cGy)	270	201	196	194
MU/Dose Ratio	2.89	3.61	3.73	4.26
Delivery Time (min)	1.43	1.87	2.08	2.47