Dose conversion factors for use with ionisation dosemeters for 6 to 35 MeV electron beams

The use of high-energy electrons in cancer therapy has created a need for sensitive and accurate dosimetric techniques which are applicable to the measurement of electron beams. The most logical first step is to extend the use of ionisation chambers in order to measure high-energy photon and particulate beams. This article deals with a comparative study of chemical and ionisation systems. The method and results of an investigation to determine conversion factors for use with commercial ionisation instruments are reported. Some incidental findings in the use of ionisation chambers in electron dosimetry are also presented. Using a variable volume, parallel-electrode chamber, experimental evidence was obtained to substantiate the Bragg-Gray cavity nature of the thimble chambers used in routine clinical dosimetry. The conversion factors obtained for a Baldwin-Farmer and Victoreen instrument show good agreement with the theoretically calculated values. The electron dose conversion factors reported by other investigators are reviewed. The availability of these doseconversion factors puts the high-energy electron dose measurements on a par with the measurements of conventional X rays and teletherapy beams.