DOI
https://doi.org/10.25772/Q8ST-C928
Defense Date
2025
Document Type
Thesis
Degree Name
Master of Science
Department
Medical Physics
First Advisor
Siyong Kim
Second Advisor
Tianjun Ma
Third Advisor
Chris Bartee
Abstract
Problems: A rediscovered radiotherapy technique, FLASH, involves increasing the dose rate to >40 Gy/s. For a given dose, FLASH is shown to decrease the normal tissue complication probability without affecting the tumor control probability. More preclinical studies are necessary to confirm and enable these desirable properties on humans. Current preclinical investigations have diverse constraints, requiring FLASH apparatuses with flexible parameters and accurate control. However, there are limited FLASH platforms available, the existing ones are costly. Our institution is not in possession of any such infrastructure. Even if available, dosimetry and beam control in FLASH are extremely lacking. There is a major need for novel techniques. Procedure: To establish a FLASH infrastructure at VCU, a cost-effective method of modifying current technology was employed. A LINAC was altered to achieve electron FLASH by selecting photon mode, retracting the target, shifting the carousel to an open port, and tuning waveforms. Prior to experimentation with the system, an extensive literature review was conducted. A novel control mechanism was developed around a commercial plastic scintillation detector equipped with an ultra-fast acquisition unit, display, and customizable microcontroller. Circuitry was developed to connect the acquisition unit’s output to an interface board on the LINAC, which enabled the assertion of a beam off. The microcontroller software was altered to enable pulse counting and dose-based control of the LINAC. The system was tested against previously validated passive film detectors by developing custom phantoms. Additionally, the scintillation detector underwent several characterization tests. Beam characterization of PDDs and profiles was also evaluated with film dosimetry. To preliminarily test the control system in a preclinical setting, a cell irradiation pilot study was performed. Results: The LINAC underwent the conversion. Film measurements revealed a mean dose rate of 168 Gy/s slightly past the isocenter, confirming the alteration was successful. The literature review indicated the uncertainty biologically if FLASH exists, highlighting a strong need for further investigation. The survey also provided a list of FLASH capable detectors, most of which were passive, and showed that new control systems are needed since all proposed methods are imperfect. The scintillation detector system was more than temporally sufficient to resolve individual pulses in FLASH, calculate dose in real-time, and be utilized for control. The control system was implemented but could use improvements to its accuracy. The scintillation detector appeared to degrade in FLASH, which requires recalibration after every use. Additionally, the detection system sometimes double counted pulses, was found to be nonlinear with dose, and experienced an afterglow effect. The control system was accurate enough to perform a preclinical cell-irradiation study to obtain a survival curve of tumor cells. Films revealed a practical range of less than the expected 3 cm owing to a decrease in beam energy from the conversion process. Conclusions: Gaps in current FLASH research were found via a literature survey. FLASH has been cost-effectively achieved at VCU by modifying available technology. A control system was enabled to count pulses and measure dose for flexible beam delivery. FLASH control was accurate enough for preclinical investigations. Improvements are needed before the use of FLASH on humans. More characterizations on the scintillation detector of several different compositions are required.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
4-23-2025