Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams
Purpose: To investigate the behavior of plane-parallel ion chambers in high-energy photon beams through measurements and Monte Carlo simulations. Methods: Ten plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and k Q factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate k Q factors. Systematic uncertainties in Monte Carlo calculated k Q factors are investigated by varying material properties and chamber dimensions. Results: Chamber behavior was variable in MV photon beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5 were observed in calibration coefficients after a period of eight months although k Q factors were consistent on average within 0.17. Chamber-to-chamber variations in k Q factors for chambers of the same type were at the 0.2 level. Systematic uncertainties in Monte Carlo calculated k Q factors ranged between 0.34 and 0.50 depending on the chamber type. Average percent differences between measured and calculated k Q factors were - 0.02, 0.18, and - 0.16 for 6, 10, and 25 MV beams, respectively. Conclusions: Excellent agreement is observed on average at the 0.2 level between measured and Monte Carlo calculated k Q factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy photon beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers.