Date Approved

8-10-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Spencer Arnold

Academic Year

2025/2026

Abstract

Purpose: To evaluate dosimetric differences between breast boost plans generated on an initial prone whole-breast computed tomography (CT) simulation and those generated on a near-boost supine re-simulation CT acquired closer to boost delivery.

Methods: Ten patients receiving hypo-fractionated whole-breast irradiation followed by a sequential boost were retrospectively analyzed. Boost plans were generated using a three-field photon technique on both the initial prone CT and a near-boost supine CT. Target volumes, target coverage metrics, conformity index (CI), and organ-at-risk (OAR) doses were compared using paired statistical analyses.

Results: Mean clinical target volume and planning target volume were significantly smaller on the near-boost supine CT compared with the initial prone CT (both p = 0.0215). Target coverage remained clinically comparable between planning approaches, with no significant differences observed for V95%, Dmax, D2%, D98%, or D50%. The conformity index improved from 2.87 ± 0.89 to 1.61 ± 0.44 (p = 0.0011), demonstrating the largest observed effect size. Several OAR metrics, including mean doses to the heart, ipsilateral lung, contralateral breast, contralateral lung, and liver, demonstrated statistically significant differences between planning approaches.  Ipsilateral lung mean dose demonstrated the largest OAR effect size, whereas the remaining OAR endpoints demonstrated limited measurable effect. No significant differences were observed for low-dose lung volume metrics.

Conclusion: Near-boost supine re-simulation resulted in smaller target volumes and improved dose conformity compared with boost planning performed on the initial prone CT simulation.

Despite anatomical changes during treatment, target coverage remained comparable between 2  planning approaches. These findings suggest that near-boost re-simulation may improve conformity without compromising target coverage.

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