Calibration of megavoltage photon and electron beams with an extrapolation chamber

Abstract

A variable air-volume, parallel-plate, extrapolation chamber forming an integral part of a Solid–Water© phantom was built to determine the absorbed dose in Solid–Water©. The sensitive air-volume of the chamber is controlled through the movement of the chamber piston using a micrometer connecting the piston to the phantom body. A mechanical distance travel indicator monitors the relative displacement of the piston with a precision 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–Water© is proportional to the ionization gradient measured in the Bragg–Gray cavity region with a Solid–Water© embedded extrapolation chamber. Measured charge is corrected for ion recombination, ion diffusion, and charge multiplication 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 dosimetry protocol is 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.