Date Approved

8-12-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Robert Hammond

Second Advisor

Kristen Vu

Academic Year

2025/2026

Abstract

Single-isocenter multi-target (SIMT) stereotactic radiosurgery (SRS) utilizing Varian HyperArc streamlines clinical workflows but introduces geometric sensitivity for distant outlier lesions when rotational and setup uncertainties are taken into account. The default isocenter location in the HyperArc workspace is the geometric centroid, which is inherently biased toward clusters of lesions. This study evaluates the dosimetric plan quality of an alternative minimax isocenter strategy, which minimizes the maximum lesion-to-isocenter distance, shortening the rotational lever arm to enhance mechanical robustness. Two fractionated SRS (fSRS) plans were generated for ten patients with 3-5 brain metastases using identical beam configurations: one using HyperArc’s default geometric centroid isocenter, and one using the minimax isocenter. Plans were evaluated with a comprehensive set of plan quality metrics, including prescription target coverage (V100%), Paddick Conformity Index (CI), Gradient Index (GI), healthy brain sparing (V10Gy, V12Gy, V20Gy), organs-at-risk (OAR) maximum doses, and total monitor units (MU). Minimax optimization shifted the isocenter by 1.32 ± 0.57 cm, successfully reducing the maximum distance to the most peripheral lesion by 1.08 ± 0.68 cm across all cases. Despite a slight increase in average distance for the remaining targets (3.97 cm vs 4.39 cm), average planning target volume (PTV) dosimetry and overall plan quality remained statistically equivalent for all measured metrics. Target coverage (P = 0.9379), Paddick CI (P = 0.7076), GI (P = 0.3262), and brain V12Gy (P = 0.5935) displayed no statistically significant variance. All OAR constraints were consistent between the two techniques, as well as total MU. For patients with fewer than 5 brain metastases, the minimax isocenter placement minimizes the lesion-to-isocenter distance for the most peripheral lesions while maintaining dosimetric equivalence and delivery efficiency compared to the standard HyperArc geometric centroid placement. This technique represents a practical alternative to enhance plan robustness.

Download

Share

COinS