Date Approved

7-25-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Kristen Vu

Second Advisor

Alyssa Capizzi

Academic Year

2025/2026

Abstract

Purpose

To compare the dosimetric performance of manually generated optimizer rings and the Eclipse Normal Tissue Objective (NTO) for single-isocenter volumetric arc therapy (VMAT) planning of patients with multiple brain metastases treated with stereotactic radiosurgery (SRS).

Methods

A retrospective treatment planning study was performed using ten patients previously treated with SRS for three or more brain metastases. For each patient, three VMAT plans were generated using identical beam geometry. The three plans consisted of manually generated optimizer rings, NTO alone, and a hybrid approach that incorporated both optimization strategies. Plans were evaluated using conformity index (CI), gradient measure (GM), minimum planning target volume (PTV) dose, Brain-GTV metrics, mean brain dose, and monitor units (MU) required.

Results

CI and PTVmin were comparable among all three optimization strategies. GM differed significantly (p= 0.0022), with both manual rings and hybrid plans demonstrating significantly steeper dose gradients than NTO alone (adj. p= 0.0053 and 0.006). Although no statistically significant differences were observed in Brain-GTV metrics, the manual rings and hybrid plans consistently produced lower V24.5 Gy, V20.3 Gy, and Dmean values than NTO plans. NTO plans required significantly fewer MUs than the other optimization strategies.

Conclusion

Manual optimizer rings, either alone or combined with NTO, produced superior dose gradients compared to NTO alone while maintaining target conformity and normal brain dose metrics. Although NTO required fewer MUs, this improvement in delivery efficiency was accompanied by greater intermediate dose bridging between adjacent metastases. These findings support the continued use of manually generated optimizer rings for conventional VMAT planning of multiple brain metastases.

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