THE CALIBRATION AND USE OF PLANE-PARALLEL IONIZATION CHAMBERS FOR DOSIMETRY OF ELECTRON-BEAMS - AN EXTENSION OF THE 1983 AAPM PROTOCOL REPORT OF AAPM RADIATION-THERAPY COMMITTEE TASK GROUP NO-39

This report is an extension of the 1983 AAPM protocol, popularly known as the TG-21 Protocol. It deals with the calibration of plane-parallel ionization chambers and their use in calibrating therapy electron beams. A hierarchy of methods is presented. The first is to calibrate the plane-parallel chamber in a high energy electron beam against a cylindrical chamber which has an N-gas(cyl) value that has been obtained from a NIST traceable Co-60 beam calibration. The second method, which is recommended for implementation by the ADCLs is an in-air calibration against a NIST-traceable calibrated cylindrical chamber in a Cobalt-60 beam to obtain a plane-parallel-chamber calibration factor in terms of exposure or air kerma. The third method places the two chambers in a phantom in a Cobalt-60 beam, and leads to an N-gas(pp) value for the plane-parallel chamber. This report also gives N-gas(pp)/(N(x)A(ion))(pp) and N-gas(pp)/(N(k)A(ion))(pp) values for five commonly used commercially available plane-parallel chambers: the Capintec PS-033, the Exradin P-11, the Holt, the NACP and the PTW-Markus. The calculation of these N-gas ratios introduces a K-comp factor which is also calculated for the five parallel plate chambers. The use of the plane-parallel chambers follows the 1983 AAPM protocol for absorbed dose calibrations of electrons, except that new energy-dependent P-repl values are given for the Capintec PS-033 and PTW-Markus chambers consistent with the consensus of reports in the literature. For all the chambers, however, P-repl is unity for 20 MeV electrons. This report does not address the issue of the use of plane-parallel chambers in calibrating photon beams.