Replacement correction factors for plane-parallel ion chambers in electron beams

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Charged Particle Beam Therapy

Background:

• Dosimetry protocols recommend plane-parallel chambers for electron beam measurements.
• The replacement correction factor (P(repl)) is often assumed to be unity for well-guarded chambers.
• Experimental data suggest non-unity P(repl) values for certain ion chambers, like the Markus chamber.

Purpose of the Study:

• Investigate the replacement correction factors (P(repl)) for plane-parallel chambers in electron beams using Monte Carlo simulations.
• Evaluate the influence of chamber design and beam characteristics on P(repl) values.

Main Methods:

• Monte Carlo simulations were employed to calculate P(repl) for various plane-parallel chambers in a water phantom.
• Simulations covered diverse electron beam qualities, phantom depths, and guard ring widths.
• Statistical precision was maintained at a high level.

Main Results:

• P(repl) values showed significant depth dependence in the dose fall-off region for low-energy beams, attributed to the gradient effect.
• The effective point of measurement often needs to be shifted from the cavity's front face.
• Calculated P(repl) values were near unity for the Roos chamber but varied for others, aligning well with experimental data for the Markus chamber.

Conclusions:

• Correctly defining the effective point of measurement is critical for accurate P(repl) in electron beams.
• With the correct effective point of measurement, P(repl) becomes largely depth-independent.
• Guard ring width and the diameter-to-thickness ratio influence P(repl); a 6 mm guard width is needed for a well-guarded NACP02-like chamber (diameter-to-thickness ratio of 5).