Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Radiotherapy Physics

Background:

• High-energy photon beams are crucial in radiotherapy.
• Accurate dosimetry is essential for effective cancer treatment.
• Plane-parallel ion chambers are used for dose measurements.

Purpose of the Study:

• To evaluate the performance of plane-parallel ion chambers in high-energy photon beams.
• To compare measurement data with Monte Carlo simulations.
• To determine the suitability of these chambers for reference dosimetry.

Main Methods:

• Measurements of absorbed dose-to-water calibration coefficients and k(Q) factors for ten plane-parallel ion chamber types.
• Characterization of chamber behavior including leakage, settling, polarity, ion recombination, and stability.
• Monte Carlo simulations to calculate absorbed dose to air and water, and k(Q) factors.
• Investigation of systematic uncertainties in Monte Carlo calculations.

Main Results:

• Plane-parallel chambers exhibited variable behavior, particularly in leakage and ion recombination, performing less ideally than cylindrical chambers.
• Significant variations (up to 1.5%) in calibration coefficients were noted over eight months, while k(Q) factors remained consistent within 0.17% on average.
• Chamber-to-chamber variations for k(Q) factors of the same type were around 0.2%.
• Systematic uncertainties in Monte Carlo k(Q) factors ranged from 0.34% to 0.50%.
• Average differences between measured and calculated k(Q) factors were excellent, within ±0.18% for 6, 10, and 25 MV beams.

Conclusions:

• High agreement (0.2% level) was found between measured and Monte Carlo calculated k(Q) factors.
• The performance of plane-parallel ion chambers is currently inadequate for high-energy photon beam reference dosimetry.
• An enhanced quality assurance program is necessary for reliable use of these chambers.