Date Approved

8-14-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Kelly LaBant, M.S., CMD, RT(T)

Second Advisor

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

Academic Year

2025/2026

Abstract

Purpose:

This retrospective study compared dosimetric differences between prone left whole breast radiation (WBRT) treatment plans originally generated for Halcyon using 6X flattening- filter-free (FFF) beams and replanned for a conventional TrueBeam using 6X beams.

Methods:

Ten prone whole left breast treatment plans were paired and retrospectively analyzed. Clinically treated Halcyon 6X flattening filter free plans were replanned on TrueBeam using flattened 6X beams in Eclipse v16.1. Contours for all plans were generated using artificial intelligence-based autosegmentation, reviewed by a medical dosimetrist and radiation oncologist, and forward planned with a tangent technique in EZFluence to achieve identical planning objectives. Dose calculations were performed using the Acuros XB algorithm and evaluated for endpoints including Breast PTV Eval coverage, lumpectomy PTV coverage, dose homogeneity, cardiac dose, left anterior descending artery (LAD) dose, ipsilateral lung dose, and contralateral breast dose. Dose volume histogram (DVH) data were analyzed using the sign test, with significance defined as p < 0.05 and Effect size (ES).

Results:

Both Halcyon 6X-FFF and TrueBeam 6X plans produced clinically acceptable target coverage and met all institutional planning goals. Breast PTV Eval coverage was similar between linear accelerators (linacs) without statistically significant differences. Lumpectomy coverage was significantly higher in Halcyon plans (P= 0.002, ES = 0.5). Dose homogeneity showed no difference in V105% (P = 1.0, ES = 0.0) or maximum dose (P = 0.75, ES = 0.1) between the two linacs. Heart mean dose was lower in TrueBeam 6X plans (P = 0.02, ES = 0.4), while heart maximum dose was lower in Halcyon 6X-FFF plans (P = 0.02, ES = 0.0). LAD doses for the mean (P = 0.7539, ES = 0.0) and maximum (P = 0.3438, ES = 0.0) remained well under planning objectives for both machines. The ipsilateral lung Dmean dose was significantly higher with Halcyon 6X-FFF than TrueBeam 6X (P = 0.0215, ES = 0.4). The Dmax, V4 Gy, and V12 Gy did not show a statistically significant directional difference between linacs (P = 0.75, ES = 0.0), (P = 0.25, ES = 0.0), and (P = 0.25, ES = 0.0) respectively. In addition to metrics that did not show a statistical difference in direction was the contralateral breast (P = 0.63, ES = 0.0).

Conclusion:

Both Halcyon 6X-FFF and TrueBeam 6X generated clinically acceptable prone left whole breast plans with effective coverage and organ at risk (OAR) sparing. While 6X-FFF and 6X beams have differing behaviors in beam profile and dose fall off, these minimally affected either plan. Treatment machine and beam selection should therefore be individualized based on clinical priorities and OAR sparing objectives.

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