Date Approved
8-3-2026
Graduate Degree Type
Project
Degree Name
Medical Dosimetry (M.S.)
Degree Program
School of Interdisciplinary Health
First Advisor
Kirk Luca MS, CMD
Academic Year
2025/2026
Abstract
Purpose: To evaluate whether planning target volume (PTV) margins can be safely reduced for intermediate-risk prostate cancer patients treated on the Radixact platform while maintaining adequate target coverage in the presence of uncorrected rotational uncertainties.
Methods: A retrospective planning study was conducted on five intermediate-risk, intact prostate cancer patients. For each patient, three PTV expansions were generated from the clinical target volume (CTV) using isotropic margins of 6 mm, 5mm, and 4 mm, with reduced posterior margins of 5 mm, 4 mm, and 3 mm, respectively. Treatment plans were created in RayStation using standardized templates and beam parameters. All plans were normalized to ensure that 95% of the PTV received the prescription dose of 6000cGy. Robustness analysis was performed by applying systematic pitch and roll perturbations to simulate rotational setup errors not correctable by the Radixact system. Dosimetric endpoints included CTV D98, D95, V100, and V95, as well as 0.035cc max dose organ-at-risk (OAR) metrics for the bladder and rectum. Worst-case degradation and failure rates (CTV D98 < 95%) were evaluated. Nonparametric statistical tests were used for comparisons.
Results: All margin strategies achieved acceptable nominal target coverage. Reduced margins demonstrated increased sensitivity to rotational perturbations. The 4-mm/3mm posterior margin showed the greatest degradation in CTV coverage under worst-case conditions, with a mean CTV D98 of 5699.0 cGy + 103.7 cGy at the largest simulated setup uncertainty (5 mm). The 5-mm/4mm posterior margin maintained comparable target coverage to the 6-mm/5mm posterior margin in most scenarios while reducing OAR dose. Differences between the smallest and largest margins were not all statistically significant. Statistical significance was defined as (P < .05).
Conclusions: Moderate 5-mm/4-mm posterior PTV margin reduction appears feasible for prostate radiotherapy on Radixact; however, aggressive margin reduction may compromise robustness. A 5-mm/4-mm posterior margin demonstrated the best balance between preserving target coverage and maximizing normal tissue sparing.
ScholarWorks Citation
Ubert, Deanne, "Non-inferiority of reduced PTV margins for Intermediate-risk Prostate cancer on Radixact: A retrospective margin analysis using robustness evaluation in RayStation" (2026). Masters Projects. 753.
https://scholarworks.gvsu.edu/gradprojects/753
