Permanent-magnet energy spectrometer for electron beams from radiotherapy accelerators

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Spectroscopy

Background:

• Accurate measurement of electron energy spectra is crucial for effective radiotherapy.
• Existing methods for measuring electron beam energy can be complex and expensive.
• A need exists for a more accessible and lightweight spectrometer for clinical applications.

Purpose of the Study:

• To adapt a lightweight, permanent magnet electron energy spectrometer for measuring therapeutic electron beam energy spectra.
• To develop and validate an irradiation geometry and measurement technique for this spectrometer.
• To demonstrate its utility in measuring spectra from a clinical linear accelerator.

Main Methods:

• Developed a measurement technique using a permanent dipole magnet spectrometer and computed radiography (CR) phosphor strips.
• Employed dual-pinhole electron collimators and a lead block to manage beam characteristics and reduce X-ray background.
• Utilized theoretical detector response functions (DRFs) and an iterative technique to derive energy spectra from CR data.
• Calibrated the spectrometer using percent depth-dose ( %DD) curves and recalibrated considering collimation effects.
• Transformed measured spectra to a standard 95-cm source to collimator distance (SCD).

Main Results:

• The spectrometer successfully measured electron energy spectra for seven therapeutic electron beams (7-20 MeV).
• Energy calibration confirmed the uniform magnetic field model (0.542 ± 0.027 T).
• Measured spectra and peak energies correlated well with %DD curve parameters (R90).
• Anomalous spectral broadening was observed for the 9-MeV beam, attributed to accelerator tuning issues.

Conclusions:

• An inexpensive, lightweight permanent magnet spectrometer is effective for measuring therapeutic electron beam energy distributions (6-20 MeV).
• The developed apparatus and analysis techniques provide a viable method for spectral characterization.
• Future work aims to develop a real-time spectrometer for applications like beam matching and Monte Carlo input.