Date Approved

8-4-2025

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Kirk Luca

Academic Year

2024/2025

Abstract

Treatment of extensive prostate and pelvic lymph node volumes for prostate cancer patients provide unique challenges related to limitations in multileaf collimator (MLC) design in many modern linear accelerators. A strategy to overcome this challenge is by restricting the field size, improving modulation as well as using overlapping arcs to ensure complete coverage of the target. Using too few arcs can result in suboptimal target coverage, while too many arcs can lead to prolonged time on the table, increasing the risk of patient movement and potential side effects that may compromise treatment accuracy. This study aims to evaluate the optimal number of arcs required to ensure adequate coverage and sparing for organs-at-risk (OARs) while mitigating the potential drawbacks. Ten patients previously treated with definitive radiation therapy using volumetric-modulated arc therapy (VMAT) plans for prostate cancer were used to generate plans consisting of two, three, and four full coplanar arcs. Plan quality was evaluated by assessing conformity index (CI), gradient index (GI), and target coverage, while OARs were evaluated by examining doses to the bladder, colon, femoral heads, sigmoid colon, and small bowel. Plan quality showed minor but significant improvement when increasing from 2 arcs to 3 or 4 arcs. Differences between 3 and 4 arc plans were not found to be significant. Dosimetric endpoints to the bladder, colon, femoral heads, and small bowel showed no significant effect of differing numbers of arcs. For the rectum V60% and the sigmoid colon Dmax, the difference between 2 arcs versus 3 or 4 was significant, but improvement between the plans was minimal. These findings provide insights that can be used to help guide in choosing the optimal number of arcs for treating large prostate and pelvic nodal volumes.

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