F. H. Attix

A solid water phantom material for radiotherapy x‐ray and γ‐ray beam calibrations

The formulation, manufacture and testing of an epoxy resin‐based solid substitute for water is presented. This ‘‘solid water’’ has radiation characteristics very close volumetrically to those of water. When it is used as a dosimetry phantom for x‐ and γ‐ray beams in the radiotherapy range, phantom‐to‐water corrections and density corrections are eliminated. Relative transmission measurements have shown that the transmission through 10 cm of solid water is within 0.2% of that through an equal thickness of water for x and γ rays. The use of this material for calibration phantoms can help achieve the goal of radiotherapy beam calibrations within ±1.0% of the true dose rate, easier to achieve.

Calibration of 192Ir high-dose-rate afterloading systems

A method is described for calibration of 192Ir high-dose-rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma-ray spectrum of iridium-192, an interpolation procedure is employed, using calibrations above (137Cs, 662 keV) and below (250 kVcp, 146-keV x rays) the exposure-weighted average 192Ir energy of 397 keV. The same total wall + cap thickness must be used for both calibrations, and for the 192Ir measurements. A wall + cap thickness of 0.3 g/cm2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source-chamber distance and room scatter during the source calibration in inverse-square-law geometry. A new well-type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated 192Ir source, and offers a simple means for verifying the decay rate and for calibrating 192Ir replacement sources.

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 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.

A new re-entrant ionization chamber for the calibration of iridium-192 high dose rate sources

A re-entrant (well-type) ionization chamber has been designed and fabricated at the University of Wisconsin for use with iridium-192 high dose-rate (HDR) remote after-loading brachytherapy devices. The chamber was designed to provide an ionization current of about 10(-8) ampere with a nominal 10 curie iridium-192 source. A narrow opening is provided into the sensitive volume of the chamber to insert a Nucletron MicroSelectron catheter, or catheters with similar diameters from other HDR manufacturers. The chamber exhibits a flat response (+/- 0.1%) for any source position within +/-4 mm of the chamber center. A 300 volt chamber bias yields a 99.96% ion collection efficiency. The chamber is capable of being calibrated directly with an iridium-192 source which has in turn been calibrated with thimble-type ion chambers. Reproducibility for readings in the current mode for 10 consecutive insertions of the MicroSelectron iridium-192 HDR source is within 0.02% or less. Two thimble chambers calibrated by the U.S. National Institute of Standards and Technology provide calibration traceability of iridium-192 HDR sources and re-entrant chambers to a primary national standards laboratory. Results of activity measurements of 6 commercial iridium-192 HDR sources are reported.

Determination of Aion and Pion in the new AAPM radiotherapy dosimetry protocol

Simple equations are given by which accurate values of Aion and Pion can be obtained for use in applying the AAPM Task Group 21 dosimetry protocol.

Uncertainty of calibrations at the accredited dosimetry calibration laboratories

The American Association of Physicists in Medicine, through a subcommittee (formerly Task Group 3) of the Radiation Therapy Committee, has accredited five laboratories to perform calibrations of instruments used to calibrate therapeutic radiation beams. The role of the accredited dosimetry calibration laboratories (ADCLs) is to transfer a calibration factor from an instrument calibrated by the National Institute of Standards and Technology (NIST) to a customers instrument. It is of importance to the subcommittee, to physicists using the services of the ADCLs, and to the ADCLs themselves, to know the uncertainty of instrument calibrations. The calibration uncertainty has been analyzed by asking the laboratories to provide information about their calibration procedures. Estimates of uncertainty by two procedures were requested: Type A are uncertainties derived as the standard deviations of repeated measurements, while type B are estimates of uncertainties obtained by other methods, again expressed as standard deviations. Data have been received describing the uncertainty of each parameter involved in calibrations, including those associated with measurements of charge, exposure time, and air density, among others. These figures were combined with the uncertainty of NIST calibrations, to arrive at an overall uncertainty which is expressed at the two-standard deviation level. For cable-connected instruments in gamma-ray and x-ray beams of HVL > 1 mm Al, the figure has an upper bound of approximately 1.2%.

A proposal for the calibration of plane‐parallel ion chambers by accredited dosimetry calibration laboratories

A procedure is described by which the AAPM-accredited dosimetry calibration laboratories could offer calibrations of plane-parallel ionization chambers for radiotherapy dosimetry applications, by comparison with a cylindrical ion chamber in a phantom irradiated by 60Co gamma rays. Ngas can thus be determined for the plane-parallel chamber under uniform conditions of photon scatter, and without the need for an electron fluence correction.

A comparison of methods for calibrating parallel-plate chambers

All dosimetry protocols for calibrating the output of electron beams recommend the use of parallel-plate ionization chambers, but the method of determining their value of Ngas is a matter of concern. The AAPM Protocol (TG 21) recommends a direct comparison with a calibrated cylindrical chamber in phantom at dmax with the highest available electron energy beam. This must be done by the user. Since all calibration laboratories traditionally use 60Co for megavoltage chamber calibrations, two alternate procedures based on exposures in-air, or in-phantom, have been proposed. All methods use correction factors in the data reduction. To verify the consistency of the three methods, we have measured Ngas using each of these techniques for six of the most commonly used and commercially-available parallel-plate ionization chambers. The paired cylindrical and parallel-plate ionization chambers, and phantom materials/buildup caps were matched to the wall composition of the plane chambers, as recommended in TG 39. A 22 MeV electron beam was used for the electron irradiations. The ionization chambers were then taken to an Accredited Dosimetry Calibration Laboratory (ADCL), where 60Co calibrations were performed. The results demonstrate that, by using the appropriate correction factors for the chambers described in this work, all three methods yield values for Ngas that are within 1% of each other.

Calculational methods for estimating skin dose from electrons in Co-60 gamma-ray beams.

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co-60 gamma-ray beams. Either the Klein-Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source-surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.

Topics of general interest, session II

To the panel Dr. Sidney 0. Stephens. Will each of the discussants estimate as briefly as possible the unit cost of the main unit that you were talking about (if you have a number of them, kindly confine it to the one that you most want to talk about), and tube replacement cost, if there is such a thing in the unit you are talking about. Dr. Lawrence Cranberg. I estimate that, for an initial trial unit, with dual accelerators, the cost would be about