Date Approved

8-13-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Robert Hammond, M.S., CMD, RT(T)

Second Advisor

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

Academic Year

2025/2026

Abstract

Purpose: This study aimed to compare dosimetric characteristics and treatment delivery efficiency between 6-MV Flattening Filter (FF) and Flattening Filter-Free (FFF) photon beams for adrenal Stereotactic Body Radiation Therapy (SBRT). While FFF beams have demonstrated dosimetric and efficiency advantages at other SBRT sites, there is limited data about their use in adrenal SBRT.

Methods and Materials: A retrospective dosimetric comparison was performed using 10 patients previously treated with adrenal SBRT. For each patient, two Volumetric Modulated Arc Therapy (VMAT) plans were generated in Eclipse: one using 6-MV FF beams and one using 6-MV FFF beams, resulting in 20 total treatment plans. All plans were prescribed to 50 Gy in 5 fractions and normalized such that 95% of the planning target volume (PTV) received 100% of the prescription dose. Planning objectives, dose constraints, arc geometry, and optimization parameters were held constant between plans. Dosimetric evaluation included PTV coverage, Conformity Index (CI), Gradient Index (GI), V50%, V100%, Organs at Risk (OAR) dose metrics, Monitor Units (MU), and beam-on time. Statistical comparisons were performed using paired testing, with significance defined as P < 0.05.

Results: FFF plans demonstrated significantly shorter treatment times compared with FF plans (2.26 ± 0.21 vs. 4.71 ± 0.43 minutes, P < 0.001), despite requiring higher MU (3162.62 ± 292.85 vs. 2825.18 ± 259.26, P < 0.001). FFF plans also demonstrated improved plan-quality metrics, including lower CI100 (1.02 ± 0.01 vs. 1.04 ± 0.01, P < 0.001), lower GI (3.17 ± 0.13 vs. 3.36 ± 0.18, P < 0.001), and reduced V50% and V100% values. Most OAR dose metrics were comparable between techniques, although small increases in mean lung and heart dose were observed with FFF plans.

Conclusion: FFF beam delivery demonstrated meaningful improvements in treatment efficiency while maintaining comparable target coverage and OAR sparing relative to FF beam delivery. Modest improvements in conformity and dose-gradient characteristics were also observed. These findings support the clinical use of FFF beam delivery for adrenal SBRT when available.

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