Date Approved

8-7-2024

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Allied Health Sciences

First Advisor

Tasha Potts M.S.,CMD,RT(T)

Academic Year

2023/2024

Abstract

Introduction

Hybrid volumetric arc therapy (H-VMAT) is an advanced radiation therapy technique that merges three-dimensional and VMAT approaches. This integration enhances dose distribution and effectively spares organs at risk (OAR). By leveraging these technologies, H-VMAT ensures precise target coverage while minimizing overall radiation exposure. Although combining modalities can improve planning outcomes, there are disadvantages to using this technique. This study examines whether a novel modified arc VMAT (M-VMAT) technique can achieve dosimetric outcomes comparable to those of a hybrid VMAT plan.

Methods

Two planning techniques were created for retrospective dosimetric analysis of 11 patients with cancers of the middle and lower one-third of the esophagus. The subjects had previously completed a course of radiation therapy with volumetric modulated arc therapy (VMAT). Varian's Eclipse treatment planning system generated both an H-VMAT plan and an M-VMAT plan, with the latter featuring a modified arc that incorporated predefined avoidance sectors to produce a 3D effect. Each planning technique was prescribed the same total dose, and both plans' treatment parameters were templated. In addition, each plan was optimized identically, providing continuity for all plans and subjects. Lung volumes V5 Gy and V20 Gy, heart volume V40 Gy and mean dose, spinal cord maximum dose, and integral dose (ID) to the lungs were calculated for each plan. The amount of monitor units (MU) per plan and a delivery time estimate were also calculated for each plan.

Results

Statistical analysis, employing paired and one-sample t tests, was used to compare the mean differences between the H-VMAT technique and the M-VMAT plan. A p-value d, the standardized mean difference expressed in standard deviation units, was used to quantify the effect size within the t tests. Cohen’s d classifies effect sizes as small (d = 0.2), medium (d = 0.5), and large (d ≥ 0.8). Results are given for OARs, MUs delivered, and estimated treatment delivery time.

Conclusion

Although the novel M-VMAT aimed to create a 3D effect, H-VMAT plans were more effective at reducing lung V5 Gy, V20 Gy, integral dose (ID), and delivered MU. The M-VMAT plan excelled in minimizing heart and spinal cord doses and reducing treatment time. Nevertheless, both techniques achieved the required dose constraints, so the optimal approach should be tailored to each patient’s specific needs.

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