Evaluation of factors to convert absorbed dose calibrations from graphite to water for the NPL high-energy photon calibration service.
ABSTRACT 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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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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ABSTRACT: The ARPANSA calibration service for (60)Co gamma rays is based on a primary standard graphite calorimeter that measures absorbed dose to graphite. Measurements with the calorimeter are converted to the absorbed dose to water using the calculation of the ratio of the absorbed dose in the calorimeter to the absorbed dose in a water phantom. ARPANSA has recently changed the basis of this calculation from a photon fluence scaling method to a direct Monte Carlo (MC) calculation. The MC conversion uses an EGSnrc model of the cobalt source that has been validated against water tank and graphite phantom measurements, a step that is required to quantify uncertainties in the underlying interaction coefficients in the MC code. A comparison with the Bureau International des Poids et Mesures (BIPM) as part of the key comparison BIPM.RI(I)-K4 showed an agreement of 0.9973 (53).Radiation Protection Dosimetry 11/2012; · 0.91 Impact Factor
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ABSTRACT: An Elekta Synergy clinical linac facility is now in routine use at the National Physical Laboratory (NPL). For the purpose of therapy-level dosimetry, this has replaced the NPL research linac, which is over 40 years old, and in which the NPL absorbed dose primary standard for high-energy photons was established. This standard has been disseminated to clinical beams by interpolation of the calibration factor as a function of tissue phantom ratio TPR20/10. In this work the absorbed dose standard has been commissioned in all the beams produced by the Elekta Synergy linac. Reference standard ionization chambers have been calibrated in terms of absorbed dose to graphite and this calibration has been converted to one in terms of absorbed dose to water. The results have been combined with the calibration in 60Co γ-rays to obtain measured values for the quality-dependent correction, kQ, for these reference standard chambers used in the Elekta beams. The resulting data are consistent with the interpolated kQ to within 0.4%, which is less than the combined standard uncertainty of kQ, 0.56%.Metrologia 09/2011; 48(5):365. · 1.65 Impact Factor