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.

Abstract

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 Ncylgas value that has been obtained from a NIST traceable 60Co 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 Nppgas value for the plane-parallel chamber. This report also gives Nppgas/NxAion)pp and Nppgas/(NkAion)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 Ngas ratios introduces a Kcomp 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 Prepl 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, Prepl is unity for 20 MeV electrons. This report does not address the issue of the use of plane-parallel chambers in calibrating photon beams.