2948 - Clinical Evaluation of Setup Accuracy in Breath-Hold Lung SBRT Using a Combined Stereoscopic X-Ray and SGRT System Compared with Conventional Infrared Block-Based Gating

Purpose/Objective(s):

Breath-hold (BH) gating has conventionally used an infrared reflector block (RPM) on the patient’s thorax as a surrogate for inspiration level. Setup accuracy for this technique is limited by reproducible block placement, imprecise patient feedback, and the potential for patients to use alternative breathing maneuvers (e.g., abdominal expansion) to meet the inspiration threshold. ExacTrac Dynamic (ETD) provides a new Surface-Guided Radiation Therapy (SGRT) workflow for BH gating, potentially improving setup accuracy and reproducibility through more robust surface-based positioning. The purpose of this study was to compare the setup accuracy and reproducibility of BH gated lung SBRT when using ETD compared to conventional infrared marker-based gating.

Materials/Methods: Twenty patients receiving BH lung SBRT (4-5 fractions) were treated using the ETD CBCT based BH workflow. The workflow first prepositions the patient at treatment isocenter using the free breathing reference body contour, after which a respiratory tracking point is defined and subsequently the target inspiration threshold is automatically set from the BH reference contour. A BH CBCT was acquired and shifts to align to target volume were applied and recorded. During treatment delivery, the achieved inspiration level compared to the target level was recorded. A separate previously treated cohort of twenty patients receiving BH lung SBRT using the conventional RPM workflow was retrospectively analyzed, with applied shifts and achieved vs target inspiration level recorded. Applied shifts and difference in achieved inspiration level for the two cohorts were compared using a Mann-Whitney U test for statistical significance.

Results: Average shifts applied in all translational directions were lower for the ETD cohort compared to the RPM cohort, as demonstrated in Table 1. While only the lateral shift reached statistical significance (p < 0.001), the standard deviation of applied shifts was consistently lower for the ETD cohort. In addition, the average total shift vector was lower for ETD compared to RPM (8.7 mm vs 11.4 mm), however, this did not reach statistical significance. The target inspiration level was significantly more consistently achieved for the ETD cohort compared to RPM (0.4 mm vs 0.5 mm).

Conclusion: Lung SBRT patients treated using the ETD SGRT breath-hold workflow demonstrated improved setup accuracy and more consistent inspiration depth compared to patients treated using conventional infrared block-based gating.

Table 1: Setup accuracy results for ETD and RPM cohorts

Average Shift ± SD (mm)	ETD	RPM	p-Value
Vertical	3.6 ± 3.4	4.2 ± 4.3	1.00
Longitudinal	6.4 ± 4.6	7.1 ± 7.6	1.00
Lateral	2.2 ± 1.9	5.3 ± 5.6	< 0.001
Overall Vector	8.7 ± 4.4	11.4 ± 8.5	0.36
Average Difference Achieved vs Target Inspiration Level (mm)	0.4 ± 0.3	0.5 ± 0.3	0.02