Date Approved

7-31-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Allied Health Sciences

First Advisor

Kristen Vu

Second Advisor

Jessica Johnson

Academic Year

2025/2026

Abstract

This retrospective study evaluated the effects of isocenter placement on dosimetric outcomes for sequential prostate boost planning using volumetric modulated arc therapy (VMAT). Ten patients previously treated with whole pelvis radiation therapy (WPRT) followed by a sequential boost were selected. For each patient, two VMAT boost plans were created using identical planning objectives and beam arrangements. One plan was optimized using the original WPRT isocenter, while the second plan had the isocenter placed at the center of the boost planning target volume (PTV). Target coverage, conformity, and organ-at-risk (OAR) dose metrics were compared between the two planning techniques. Both plans achieved similar target coverage and conformity, with no statistically significant differences in PTV Dmax, Dmean, V100%, or conformity index (CI). However, plans with the isocenter placed at the center of the boost PTV resulted in lower doses to organs at risk. The largest differences were observed in the rectum, where significant reductions were noted in Rectum Dmean, Rectum V17.44, and Rectum V20.93 (P = 0.0215). Bladder, femoral head, and penile bulb doses were also lower when the isocenter was placed at the center of the boost PTV, although these differences were not statistically significant. These results indicate that moving the isocenter to the center of the boost PTV may reduce dose to nearby normal tissues, particularly the rectum, and may provide a dosimetric benefit during VMAT prostate boost planning.

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