2022 Thayer Investiture

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PhD Thesis Defense: Muhammad Ramish Ashraf



8:30am - 9:30am EST


For info on how to attend this videoconference, please email ramish.ashraf.TH@dartmouth.edu

"Radioluminescence Dosimetry and Beam Monitoring for Small Fields and FLASH Radiotherapy"


For accurate dosimetric characterization of modern radiotherapy delivery techniques, a dosimeter should ideally be energy independent, exhibit high spatio-temporal resolution and dose-rate independence. Both radioluminescence and Cherenkov emission can be effective tools for performing accurate dosimetry because they exhibit the aforementioned ideal characteristics. Therefore, the main theme of the work presented in this thesis is about fast radioluminescent and Cherenkov based detection methods for beam monitoring and quality assurance purposes in a radiotherapy clinic. This work uses these methods to probe dose distributions in 1D, 2D and 3D with high spatio-temporal resolution, especially in the limits of small fields and a new form of ultra-high dose rate delivery termed FLASH.

First, a time-gated camera was successfully employed to perform end-to-end patient specific quality assurance by imaging 2D projected dose distributions in real-time for complex, highly modulated radiotherapy plans. The optical technique was shown to be more sensitive to MLC and Gantry angle errors (~1mm and 1° accuracy) compared to a commercial cylindrical diode array. Next, 3D dose distributions were also constructed, using application specific approaches (static vs complex dynamic plans), where different 3D reconstruction algorithms were used. For static beams, a single optical camera and iterative tomography based methodology, was used to reconstruct beams as small as 4 mm. For imaging complex dynamic plans, two optical cameras were employed providing orthogonal views, and these views were successfully combined to image beams in 3D with complex and small apertures. For FLASH dosimetry, most commercially available dosimeters lack the ability to provide accurate dose linearity at high dose-rates and in real-time. Using optical imaging, 3D per pulse imaging was performed and central axis data were obtained for individual linac pulses delivered at a mean dose-rate of 60 Gy/s. This technique enabled rapid quality assurance of FLASH beams on a per pulse basis and revealed important beam parameters such as the energy stability and ramp-up of intensity in the first few pulses.

Finally, a fast field programmable gate array (FPGA) based control system was designed for the purpose of the pulse-resolved dose accumulation and feedback for the FLASH machine via a fiber coupled scintillating detector. The hardware present in this work can be used to probe the temporal structure of the beam and even provide feedback for control of the beam delivery. These tools are critical to accurate delivery of FLASH-based radiotherapy.

Thesis Committee

  • Petr Bruza (Chair)
  • Brian Pogue
  • Rongxiao Zhang
  • Lesley Jarvis
  • Brian Winey (External)


For more information, contact Daryl Laware at daryl.a.laware@dartmouth.edu.