Measurement of the replacement correction factor for parallel-plate chambers in electron fields

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Radiotherapy Physics

Background:

• The AAPM dosimetry protocol suggests a P(repl),pp,E of 1.0 for parallel-plate chambers in electron fields.
• This recommendation is provisional, awaiting further experimental data.

Purpose of the Study:

• To experimentally determine the replacement correction factor (P(repl),pp,E) for parallel-plate chambers.
• To investigate the energy dependence of P(repl),pp,E and the gas-dependent component (Ngas,pp) across various radiation modalities.

Main Methods:

• Measured P(repl),pp,E for five parallel-plate chambers using a cylindrical chamber as a reference.
• Investigated energy dependence from 5.5 to 22 MeV for electrons.
• Assessed energy and modality dependence of Ngas,pp for Co-60, 4-, 6-, 24-MV photons, and 22-MeV electrons.

Main Results:

• Three chambers exhibited an energy-independent P(repl),pp,E, consistent with unity within 1-2 standard deviations.
• One chamber showed P(repl),pp,E consistent with unity above 10 MeV but decreasing at lower energies.
• A fifth chamber demonstrated a systematic decrease in P(repl),pp,E with decreasing electron energy.

Conclusions:

• The P(repl),pp,E is not universally constant and can be energy-dependent for some parallel-plate chambers.
• These findings highlight the need for chamber-specific measurements or updated protocol recommendations for accurate electron dosimetry.