Date Approved
8-6-2026
Graduate Degree Type
Project
Degree Name
Medical Dosimetry (M.S.)
Degree Program
School of Interdisciplinary Health
First Advisor
Kristen Vu
Second Advisor
Karen Snyder
Third Advisor
Hannah Lee
Academic Year
2025/2026
Abstract
Purpose: To evaluate the dosimetric impact of varying rectal gas composition on a 40 Gy in 5 fraction SBRT prostate and SV treatment course.
Methods: Ten prostate SBRT cases planned to 40 Gy in 5 fractions using VMAT were analyzed. Five incremental HU density overrides representing 0%-100% gas content were applied to rectal contours of small and large rectum cohorts. Plan doses were re-calculated for each density override per patient using the initial 0% gas plan fields. In order to simulate a realistic five fraction course of treatment, gas and feces were contoured separately on five CBCT images to re-calculate the plan dose to each fraction. The five fraction cumulative plan was then compared against the original 100% feces and 100% gas plan doses.
Results: As rectal gas exceeded 50%, significant target degradation occurred. The PTV coverage decreased up to 1.7% (p< 0.05) in the larger rectum geometries for the V40 Gy metric. Near minimum dose PTV D99% coverage decreased up to 0.74 Gy. High-dose rectal metrics V 38 Gy and D 1cc decreased with increased gas contribution, while low to intermediate rectal dose metrics V 20 Gy increased by 3% and 3.1% for small and large rectum cohorts respectively, emphasizing the need to assess the full DVH and range of dose metrics. For the five fraction plan sum with realistic mixed gas and feces content, the PTV and rectum DVHs trended between the extreme 100% feces and 100% gas plan DVHs, emphasizing the value in assessing both extremes for a potentially high-risk clinical case.
Conclusion: Rectal gas exceeding 50% degraded target coverage and increased intermediate rectal dose. The realistic plan sum representative example demonstrated the potential clinical value in calculating both extreme rectal content uniform density overrides as a DVH range predictor for a challenging rectal filling or high-risk patient.
ScholarWorks Citation
Head, Shomanike, "A Quantitative Evaluation of Inter-Fraction Variability of Rectal Gas in Stereotactic Body Radiation Therapy Treatment of Localized Prostate Cancer" (2026). Masters Projects. 728.
https://scholarworks.gvsu.edu/gradprojects/728
