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.

Monte Carlo calculations of output factors for clinically shaped electron fields

“…This finding allows for elimination of the $P_{\text{ion}}$ in the formula for ICF (see below). $P_{\text{repl}}$ is unity for well‐guarded plane‐parallel chambers (17) . However, for poorly guarded plane‐parallel chambers, $P_{\text{repl}}$ is a function of depth as is the case for the chamber used in this study.…”

TG‐51: Experience from 150 institutions, common errors, and helpful hints

“…Physicists must compare with a cylindrical chamber in a high‐energy electron beam or apply an ADCL calibration in conjunction with either $k_{\text{ecal}}$ from TG‐51 1 or $N_{\text{gas}}false/N_X$ from TG‐39 9 or the literature 10 . There are two separate issues: (i) Using an ADCL calibration with $k_{\text{ecal}}$ values from the TG‐51 protocol can result, for some plane‐parallel chambers, in a dose error of up to 2%.…”

Differences in electron beam dosimetry using two commercial ionization chambers and the TG‐21 protocol: Another reason to switch to TG‐51

“…Others have made efforts to seek more appropriate values for $P_{\text{repl}}$ and $P_{\text{wall}}$ 19 for cylindrical chambers that in turn would render a different $N_{\text{gas}}$ value for a parallel‐plate chamber derived from that of a cylindrical chamber 17 . In addition to the uncertainty in the $P_{\text{repl}}$ and $P_{\text{wall}}$ values, another source of uncertainty that may assist in explaining the electron dose differences is the electron beam water to polystyrene fluence correction, ${\varphi }_{\text{poly}}^w$.…”

“…For electron beams the RPC applied a 2‐mm shift upstream from the center of the cylindrical chamber to the effective point of measurement for all measurements, even though TG‐21 does not recommend this correction at $d_{max}$ 16 . The MSK calculations using the Holt parallel‐plate ion chamber followed the TG‐21 and TG‐39 protocols 17 explicitly.…”