Date Approved

8-10-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Sarah Johnson

Second Advisor

Kristen Vu

Academic Year

2025/2026

Abstract

Abstract

Purpose

Thoracic stereotactic body radiation therapy (SBRT) requires highly conformal dose distributions and steep dose gradients, making treatment delivery particularly sensitive to geometric uncertainty. This study evaluated the dosimetric effects of translational setup errors on thoracic SBRT plans generated using CyberKnife, Halcyon, and VitalBeam under standardized phantom conditions.

Methods

A stationary anthropomorphic thoracic phantom was used to eliminate respiratory motion and provide identical anatomy across treatment planning systems. A 1.5-cm spherical gross tumor volume with a 5-mm isotropic planning target volume expansion was planned to receive 50 Gy in 5 fractions. Translational setup errors of ±1, ±2, ±3, and ±5 mm were applied independently in the left-right, anterior-posterior, and superior-inferior directions, together with a compound multi-axis shift. Dose was recalculated without reoptimization or renormalization after each simulated setup error. Mixed-effects models evaluated changes in target coverage, intermediate-dose metrics, and organ-at-risk doses.

Results

Increasing translational setup error produced progressive reductions in target coverage across all treatment platforms. Estimated least-squares mean reductions in PTV D95 increased from 101.6 cGy (95% CI, 68.7–134.5) at 1-mm shifts to 1415.8 cGy (95% CI, 1382.9–1448.7) at 5-mm shifts. Shift magnitude (P < .0001) and direction (P = .0046) significantly affected PTV D95, whereas treatment platform was not a significant predictor (P = .0758). D2cm was significantly influenced by shift direction (P = .0053), but not by shift magnitude or treatment platform. Although CyberKnife demonstrated numerically smaller reductions in target coverage at larger translational shifts, increasing setup error magnitude had a substantially greater effect on dosimetric outcomes than treatment platform selection.

Conclusion

Translational setup error produced measurable degradation in thoracic SBRT plan quality across all evaluated treatment platforms. These findings emphasize that minimizing setup uncertainty through accurate patient positioning and image guidance is likely to have a greater impact on preserving target coverage than treatment platform selection alone.

 

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Available for download on Tuesday, August 10, 2027

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