Replacement correction factors for cylindrical ion chambers in electron beams.

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Electron Beam Therapy

Background:

• The TG-21 and TG-51 dosimetry protocols use correction factors (Pgr, Pfl) for cylindrical chambers in electron beams.
• Pgr accounts for the effective point of measurement, while Pfl corrects for electron fluence spectrum changes.
• Accurate determination of these factors is crucial for precise dose delivery in radiation therapy.

Purpose of the Study:

• To investigate replacement correction factors (Prepl, Pfl, Pgr) for cylindrical chambers in electron beams using Monte Carlo simulations.
• To evaluate the accuracy of the TG-51 protocol's approach to these correction factors at the reference depth (dref).

Main Methods:

• Monte Carlo simulations were employed to calculate Prepl, Pfl, and Pgr for various cylindrical chamber dimensions in a water phantom.
• Simulations used established methods with high statistical precision (<0.1%).
• Electron beams of varying energies were simulated.

Main Results:

• Calculated Pfl values showed good agreement with experimental data when wall corrections were considered.
• An empirical formula for Pfl at dref was derived, dependent on chamber radius and R50.
• TG-51's use of Pfl at dmax for dref was validated.

Conclusions:

• The mean electron energy at depth is a reliable beam quality specifier for Pfl.
• TG-51's method of applying Pfl at dref using dmax data is accurate.
• The Pgr values in TG-51 may be inaccurate for Farmer-type chambers, particularly for low-energy electron beams (error > 1%).