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Calibration of photon and electron beams with an extrapolation chamber.

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 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 calibrated 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 Water is proportional to the ionization gradient measured in the Bragg-Gray cavity region with an extrapolation chamber embedded in the Solid Water phantom. The discrepancies between the doses determined in Solid Water with our uncalibrated extrapolation chamber and doses obtained with a calibrated standard thimble ionization chamber are at most 1% for photon and electron beams at all megavoltage clinical energies. Uncalibrated extrapolation chamber thus offer a simple and practical alternative to other techniques used in output measurements of megavoltage photon and electron machines.