Practical Real-time Permanent Magnet Energy Spectrometer using a Diode Array for Therapeutic Electron Beam Tuning and Quality Assurance

Author

Mason W. Heath, Louisiana State University and Agricultural and Mechanical CollegeFollow

Degree

Doctor of Philosophy (PhD)

Department

Physics

Document Type

Dissertation

Abstract

Purpose: Develop a practical, real-time permanent magnet electron energy spectrometer and evaluate it for beam tuning and quality assurance (QA) of therapeutic electron beams.

Methods: Aim 1: A 0.55 T permanent magnet was coupled to Sun Nuclear SRS MapCHECK® PC boards, providing two interlaced diode arrays. Detector response, R_meas(z_diode) versus diode position z_diode, was extracted from raw diode array readings, correcting for diode sensitivity and other factors. Using Monte Carlo (MC) computed, monoenergetic detector response functions, a curve-fitting algorithm extracted the energy spectrum, from R_meas(z_diode). The energy spectra for 7-20 MeV beams were measured for six matched Elekta accelerators (Linacs). Aim 2: ‘Synthetic’ percent dose versus depth (PDD) curves were produced by convolving the energy spectrum with monochromatic PDDs, precomputed in water using EGSnrc MC. Synthetic PDDs were compared to PDDs measured in water for 7-20 MeV beams. Aim 3: Spectrometer utility as a QA device was assessed by comparing it to daily and monthly clinical QA measurements. Aim 4: Spectrometer utility as a beam tuning tool was assessed using its energy spectrum, synthetic PDD, and metrics.

Results: Aim 1: A practical, real-time lightweight (11.4 kg), and compact electron energy spectrometer was developed. The energy spectrum for 7-20 MeV beams from six matched Linacs were measured and compared. The energy spectrum for cumulative (15 s) and real-time (0.8 s) modes agreed. Aim 2: Synthetic PDDs showed accuracy ≤1 mm and precision ≤0.5 mm. Spectrometer real-time operation (≥1 Hz) showed clinically insignificant differences from cumulative operation. Aim 3: The spectrometer performed similar (within 1%) to daily and monthly clinical QA measurements. Aim 4: Synthetic PDDs showed shifts equaling those of PDDs measured in water before and after beam tuning, and they showed the ability to return a PDD back into clinical tolerance through beam tuning. Using and synthetic PDDs, FWHM and R_80-20were reduced from clinical beam tune values, while maintaining clinical R₅₀ values for 7-20 MeV beams.

Conclusions: A practical, real-time permanent magnet electron energy spectrometer was developed that showed the ability to tune and perform QA measurements for therapeutic 7-20 MeV electron beams.

Date

11-3-2025

Recommended Citation

Heath, Mason W., "Practical Real-time Permanent Magnet Energy Spectrometer using a Diode Array for Therapeutic Electron Beam Tuning and Quality Assurance" (2025). LSU Doctoral Dissertations. 6938.
https://repository.lsu.edu/gradschool_dissertations/6938

Committee Chair

Matthews, Kenneth