Technical Note: On the use of cylindrical ionization chambers for electron beam reference dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Electron beam dosimetry relies on accurate measurements, with ion chambers being standard tools.
• Cylindrical chambers are widely used, but their energy independence in electron beams is a concern due to perturbation effects.
• Plane-parallel chambers are often preferred for electron beams, but present their own challenges.

Purpose of the Study:

• To evaluate the energy independence of cylindrical chambers for electron beam dosimetry.
• To investigate the variability of relative ion chamber perturbation corrections for cylindrical chambers.
• To determine if cylindrical chambers can be used reliably for all electron beam energies.

Main Methods:

• Measurements were performed using reference-class cylindrical and plane-parallel chambers in electron beams (8 MeV and 18 MeV) at various depths in water.
• Relative perturbation corrections were determined by normalizing chamber readings of similar chambers in a high-energy beam.
• Data were analyzed as a function of mean electron energy at depth, extending to R80, and validated in a Virtual Water phantom for a 4 MeV beam.

Main Results:

• The variability in relative ion chamber perturbation corrections for Farmer-type cylindrical chambers was found to be less than 0.4%.
• This variability is comparable to or better than that observed for plane-parallel chambers with similar specifications.
• The results indicate consistent performance across different electron beam energies.

Conclusions:

• Reference-class cylindrical chambers demonstrate sufficient stability for electron beam dosimetry across various energies.
• Their use could simplify the overall reference dosimetry procedure in radiation therapy.
• Implementing cylindrical chambers for all electron beams has the potential to improve the accuracy of beam calibration.