Calibration of photon and electron beams with an extrapolation chamber

摘要

A variable air-volume, parallel-plate, extrapolation chamber forming an integral part of a Solid-WaterTM phantom was built to determine the absorbed dose in Solid-WaterTM directly. The sensitive air-volume of the extrapolation chamber is controlled through the movement of the chamber piston by means of a micrometer mounted to the phantom body. The relative displacement of the piston is monitored by a mechanical distance travel indicator with a precision on the order of 0.002 mm. Irradiations were carried out with cobalt-60 gamma. rays, x-ray beams ranging from 4 to 18 MV, and electron beams between 6 and 22 MeV. The absorbed dose at a given depth in Solid-WaterTM is proportional to the ionization gradient measured in the Bragg-Gray cavity region with a Solid-WaterTM embedded extrapolation chamber. Measured charge is corrected for ion recombination and ion diffusion in the chamber air volume according to a comprehensive model for charge loss in an ionization chamber. The discrepancies between doses determined with our uncalibrated phantom-embedded extrapolation chamber and doses obtained with calibrated Farmer-type cylindrical ionization chambers following the AAPM-TG21 and AAPM-TG25 dosimetry protocols are less than 1% for photon and electron beams at all clinical megavoltage energies. Uncalibrated extrapolation chambers thus offer a simple and practical alternative to other techniques used in output measurements of megavoltage photon and electron beams.