Date Approved

7-25-2025

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Health Professions

First Advisor

George Spencer Arnould, M.S., CMD*

Second Advisor

Kristen Vu, M.S., CMD, RT(T)*

Academic Year

2024/2025

Abstract

Prostate cancer remains one of the most prevalent malignancies among men, with radiotherapy, particularly VMAT—playing a central role in its management. VMAT allows for highly conformal dose delivery, but treatment planning must strike a careful balance between achieving steep dose gradients around the prostate and minimizing radiation exposure to nearby organs at risk (OARs), such as the bladder and rectum. This study aimed to evaluate how prostate volume influences dose fall-off characteristics and OAR dose exposure in VMAT treatment plans, providing insights into optimizing radiotherapy for patients with varying prostate sizes. A retrospective analysis was conducted on 50 prostate cancer patients treated with VMAT. Dose-volume histograms and relative dose fall-off metrics (R30, R50, R70, R90, R95) were used to quantify spatial dose gradients beyond the planning target volume (PTV). Statistical analyses, including linear regression and Pearson correlation tests, assessed the relationship between PTV size and both dose fall-off behavior and OAR exposure. Results showed that larger prostate volumes were associated with steeper dose gradients, particularly at intermediate dose levels, suggesting improved conformity. However, this benefit was offset by a concurrent increase in radiation exposure to the bladder and rectum, as demonstrated by significantly higher V30 and V50 values. These findings highlight a clinically relevant trade-off: as prostate volume increases, treatment plans may achieve better dose conformity but at the cost of greater OAR exposure. This underscores the importance of personalized planning approaches—such as the use of rectal spacers, adaptive therapy, or image-guided techniques—to mitigate toxicity risks in patients with large prostates.

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