Date Approved

8-13-2022

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Health Professions

First Advisor

Eric Niekamp

Academic Year

2021/2022

Abstract

Introduction

This study aims to investigate the comparisons in dynamic conformal arc, VMAT (no ASC), forced MU limitations (ASC), or a 50/50 hybrid arc (ASC) in how they can improve a lung SBRT case where the tumor is abutting a dose critical structure. The goal of this research is to investigate multiple treatment techniques in an attempt to increase the dose constraint pass rate. The variables in comparison are how well different plan iterations can successfully meet constraints as well as pass a 0.5 modulation complexity score (MCS).

Methods

10 previously treated patients were used in this study who exhibited tumors abutting the heart, great vessels, bronchus, brachial plexus, rib, and the spinal cord. All of these patients were planned in all 5 iterations. Each of the plan consisted of two partial arcs and the patient specific iterations were compared with the same gantry arcs.

Results

The dynamic conformal arc plan exhibited the highest MCS, however it also was unable to pass any of the constraints. The VMAT plan iteration failed the MCS parameter on every patient, however more plans were able to meet the dose constraints. With an introduction of a x1 MU forced limitation, x1.5MU forced limitation, and a 50/50 hybrid, MCS as well as dose constraint pass rate significantly improved. The 50/50 hybrid iteration exhibited the most plans that passed both of the constraint variables. This treatment method incorporates the dose conformality of a dynamic conformal arc with the dose painting abilities of a VMAT arc. The ASC that was applied to these beams prevents the beam from becoming overly modulated which would result in an increase in MU and an increase in low dose to the patient.

Conclusion

This study has been able to determine that a 50/50 hybrid technique or a VMAT (ASC) with a forced MU limitation can be considered a reliable and reproducible alternative for treating lung SBRT cases that abut a dose limiting organ.

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