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-paralle l ionization chambers and their use in calibrating therapy electron be ams. A hierarchy of methods is presented. The first is to calibrate th e plane-parallel chamber in a high energy electron beam against a cyli ndrical chamber which has an N-gas(cyl) value that has been obtained f rom a NIST traceable Co-60 beam calibration. The second method, which is recommended for implementation by the ADCLs is an in-air calibratio n 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 th e 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 com mercially available plane-parallel chambers: the Capintec PS-033, the Exradin P-11, the Holt, the NACP and the PTW-Markus. The calculation o f these N-gas ratios introduces a K-comp factor which is also calculat ed for the five parallel plate chambers. The use of the plane-parallel chambers follows the 1983 AAPM protocol for absorbed dose calibration s of electrons, except that new energy-dependent P-repl values are giv en 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 t he issue of the use of plane-parallel chambers in calibrating photon b eams.