Author Type

Graduate Student

Date of Award

Spring 4-22-2026

Document Type

Thesis

Publication Status

Version of Record

Submission Date

April 2026

Department

Physics

College Granting Degree

Charles E. Schmidt College of Science

Department Granting Degree

Physics

Degree Name

Master of Science (MS)

Thesis/Dissertation Advisor [Chair]

Wazir Muhammad

Abstract

In spot scanning proton therapy, the machine steers magnetically a narrow pencil beam across the target volume, delivering highly conformal dose distributions layer by layer. As beam size is very small, this introduces a dosimetry problem. The beam is dynamic and the dose gradients are steep — conditions that push conventional ionization chambers beyond their reliable operating range. Plastic scintillation detectors offer a potential solution. Their sub-millimeter active volumes avoid the volume-averaging effects over larger chambers, and their very high temporal resolution making real-time monitoring feasible. This thesis characterizes the Blue Physics plastic scintillation detector in spot scanning proton fields at 100 and 200 MeV on the Varian ProBeam system at the South Florida Proton Therapy Institute. Measurements were benchmarked against a PTW Bragg Peak Chamber and a TOPAS Monte Carlo model built on validated phase space files for the ProBeam nozzle. In depth-dose measurements, the raw detector signal tracked the Monte Carlo dose accurately through the entrance plateau but lost up to 44% of its signal at the Bragg peak due to ionization quenching — a known limitation of organic scintillators in high-LET fields. A Birks’ law correction, using dose-averaged LET maps scored from TOPAS, fully recovered the Peak-to-Plateau Ratio. The Bragg Peak Chamber confirmed the corrected values independently. Lateral spot profiles measured at Bragg peak depth agreed with simulation within clinical tolerance and captured the beam’s intrinsic X-Y asymmetry, a feature that large-volume chambers cannot resolve. Remaining discrepancies at 200 MeV point toward the need for energy-dependent geometric corrections, identified here as the primary limitation and focus of future work.

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