Evaluation of factors to convert absorbed dose calibrations from graphite to water for the NPL high-energy photon calibration service

National Physical Laboratory, Londinium, England, United Kingdom
Physics in Medicine and Biology (Impact Factor: 2.76). 03/2002; 47(3):441-54. DOI: 10.1088/0031-9155/47/3/306
Source: PubMed


The National Physical Laboratory (NPL) provides a high-energy photon calibration service using 4-19 MV x-rays and 60Co gamma-radiation for secondary standard dosemeters in terms of absorbed dose to water. The primary standard used for this service is a graphite calorimeter and so absorbed dose calibrations must be converted from graphite to water. The conversion factors currently in use were determined prior to the launch of this service in 1988. Since then, it has been found that the differences in inherent filtration between the NPL LINAC and typical clinical machines are large enough to affect absorbed dose calibrations and, since 1992, calibrations have been performed in heavily filtered qualities. The conversion factors for heavily filtered qualities were determined by interpolation and extrapolation of lightly filtered results as a function of tissue phantom ratio 20,10 (TPR20,10). This paper aims to evaluate these factors for all mega-voltage photon energies provided by the NPL LINAC for both lightly and heavily filtered qualities and for 60Co y-radiation in two ways. The first method involves the use of the photon fluence-scaling theorem. This states that if two blocks of different material are irradiated by the same photon beam, and if all dimensions are scaled in the inverse ratio of the electron densities of the two media, then, assuming that all photon interactions occur by Compton scatter the photon attenuation and scatter factors at corresponding scaled points of measurement in the phantom will be identical. The second method involves making in-phantom measurements of chamber response at a constant target-chamber distance. Monte Carlo techniques are then used to determine the corresponding dose to the medium in order to determine the chamber calibration factor directly. Values of the ratio of absorbed dose calibration factors in water and in graphite determined in these two ways agree with each other to within 0.2% (1sigma uncertainty). The best fit to both sets of results agrees with values determined in previous work to within 0.3% (1sigma uncertainty). It is found that the conversion factor is not sensitive to beam filtration.

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    • "The electron calorimeter is of a much simpler construction and is described by McEwen et al [22]. In both cases a set of transfer chambers are calibrated against the calorimeters firstly in terms of absorbed dose to graphite and then using the photon fluence scaling theorem, as described by Nutbrown et al [23] and McEwen et al [24], converted to absorbed dose to water. These transfer chambers are then used to calibrate the secondary standard chambers in various beams by the method of direct replacement. "
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    ABSTRACT: IAEA TRS 398 (1) recommends that proton dosimetry using ionisation chambers be based on absorbed dose calibrations. However, for protons, as no primary standard exists against which to calibrate chambers, TRS 398 provides factors to derive chamber calibrations in absorbed dose for use in protons from photon beams. Conversion factors from electron to proton beams can be derived as well as shown in this paper. This work aimed to compare measured absorbed dose in the proton beam derived from electron and photon calibrations of the same chamber. Measurements were conducted in the 60 MeV clinical proton beam at the Clatterbridge Centre for Oncology. Ionisation chambers used were the cylindrical NE2561/2611 and the Scanditronix NACP-02, PTW Roos and PTW Markus parallel plate chambers. All had been calibrated at NPL in terms of absorbed dose, the parallel plate chambers in 60Co and electron beams whilst the cylindrical chambers were calibrated only in 60Co. Measurements were made in modulated and umodulated beams. Starting with the same proton beam measurement the absorbed dose for each chamber was derived from the photon and/or electron beam calibration using TRS 398. A transmission monitor was used to limit the influence of beam variations. In a modulated beam the dose ratio, parallel/thimble, varied between 0.995 and 1.018 dependent on the parallel plate chamber type and choice of starting calibration. For the same beam and starting calibrations, the dose ratio parallel/NACP obtained using Markus and Roos chambers varied between 0.987 and 1.008. In an unmodulated beam (only parallel plate chambers were used) the ratio parallel/NACP varied from 0.986 to 1.019 for Markus and Roos chambers with photon beam and electron beam calibrations. We conclude that the agreement between the various ionisation chamber types is in general within about 2%, that there is no obviously better consistency with one of both calibration routes and that there is a discrepancy of about 1.5% between both routes which needs further investigation.
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