Air Kerma Rate Measurements Research Articles

Direct measurement of air kerma rate in air from CDCS J-type caesium-137 therapy sources using a Farmer ionization chamber.

A simple method for directly measuring the reference air kerma rate from J-type 137Cs sources using a Farmer 2571 chamber has been evaluated. The method is useful as an independent means of verifying manufacturers' test data.

In-water calibration of PDR 192Ir brachytherapy sources with an NE2571 ionization chamber

An ionometric calibration procedure for PDR brachytherapy sources in terms of dose rate to water is presented. The calibration of the source is performed directly in a water phantom at short distances (1.0, 2.5 and 5.0 cm) using an NE2571 Farmer type ion chamber. To convert the measured air-kerma rate in water to dose rate to water a conversion factor (CF) was calculated by adapting the medium-energy x-ray dosimetry protocol for a point source geometry (diverging beam). The obtained CF was verified using two different methods. Firstly, the CF was calculated by Monte Carlo simulations, where the source-ionization chamber geometry was modelled accurately. In a second method, a combination of Monte Carlo simulations and measurements of the air-kerma rate in water (at 1.0, 2.5 and 5.0 cm distance) and in air (1 m distance) was used to determine the CF. The obtained CFs were also compared with conversion factors calculated with the adapted dosimetry protocol for high-energy photons introduced by Tölli. All calculations were done for a Gammamed PDR source-NE2571 chamber geometry. The conversion factors obtained with the four different methods agree to within 1% at the three distances of interest. We obtained the following values (medium-energy x-ray protocol): CF(1 cm) = 1.458; CF(2.5 cm) = 1.162; CF(5.0 cm) = 1.112 (% for the three distances of interest). The obtained results were checked with TLD measurements. The values of the specific dose rate constant and the radial dose function calculated in this work are in accordance with the literature data.

Measurement of Ambient Dose Equivalent Using a NaI(Tl) Scintillation Detector

Ambient dose equivalent rates for terrestrial radiation fields were measured by a 3" diam. x 3" NaI(Tl) scintillation spectrometer. The factor for converting the measured pulse height spectrum of the NaI(Tl) scintillation detector to ambient dose equivalent was calculated using the response matrix of the detector. The relationship between air kerma rates and ambient dose equivalent rates for the measured terrestrial radiation fields at different locations was found to be H*(10) = 1.21 Ka + 1.26 where H*(10) is ambient dose equivalent rate in nSv.h-1 and Ka is air kerma rate in nGy.h-1. This linear relationship comes mainly from the facts that the ratio H*(10)/Ka for monoenergetic photons in the energy range from 300 keV to 3 MeV falls within 1.2 ± 10% and the effective energy of the terrestrial fields is usually within the above range. Ambient dose equivalent rate for the measured terrestrial field can be estimated using this formula within an error of ±7% from the measured air kerma rate in the range 80 ~ 280 nGy.h-1.

Second EUROMET comparison of air kerma rate and activity measurements of 192Ir brachytherapy wires

At the occasion of the EUROMET Workshop on Ionising Radiation and Radioactivity at the NPL, it was decided to carry out a second intercomparison on air kerma rate (AKR) and activity measurements on 192Ir brachytherapy sources in the frame of EUROMET project no. 219. The intercomparison has shown that both air kerma rate measurements and activity measurements of 192Ir brachytherapy wires are not trivial. One of the drawbacks is the appreciable inhomogeneity of the wires. Disregarding some outliers, the spread of the AKR measurements was below 1.5%, while the variation in the activity measurements was very large, due to reasons which are not completely understood.

Calibration of the NPL secondary standard radionuclide calibrator for 192Ir brachytherapy sources

Safe and effective treatment with brachytherapy sources requires an accurate knowledge of the local tissue absorbed dose rate derived from the source reference air kerma rate. It is desirable that these air kerma rate measurements be traceable to national standards. The NPL has embarked on a programme that will enable the user to assay brachytherapy sources to a convenient manner prior to treatment. Calibration figures have been derived for the NPL secondary standard radionuclide calibrator for 192Ir brachytherapy sources manufactured by Amersham International plc. The calibration figures enable the user to accurately estimate the reference air kerma rate and activity of such sources by measuring the ionization chamber response. Calibration figures for other brachytherapy sources are also being derived.

192Ir brachytherapy sources: Calibration of the NPL secondary standard radionuclide calibrator

The increasing use of brachytherapy sources has highlighted the need to confirm the traceability to national standards of air kerma rate measurements made by suppliers and to provide a simple means for the user to check the activity of such sources. A programme of work has been initiated under the auspices of EUROMET to confirm the traceability to NPL of air kerma rate measurements made by Amersham International plc for wire sources of 192Ir. Intercomparison exercises are also under way between a number of national laboratories. The wires used in this work have been used to calibrate the NPL secondary standard radionuclide calibrator both in terms of the air kerma rate and the activity, and thus provide the hospital user with the basis of a simple and accurate quality control system for these particular types of source.

A technique for the local measurement of air kerma rate from small Caesium-137 sources

A method is described in which a Farmer ionization chamber is used for the direct measurement of the air kerma rate in air from small spherical Caesium-137 sources at distances of 35 to 70 mm. The calibration factor and corrections for source and ion chamber geometry are examined. For quality assurance purposes, the results obtained are in sufficient agreement with other methods of calibration.

In-phantom calibration of Selectron-LDR sources

Source strength measurements were performed for cesium-137 spherical sources of nine Selectron-LDR remote afterloading systems in The Netherlands. The mean reference air kerma rate of a set of sources was obtained from air kerma rate measurements in a phantom at a distance of about 5.5 cm from a large number (24 to 30) of sources. The results were compared with the source strengths specified by the manufacturer. Discrepancies between measured and manufacturers data ranging from − 2.3 % to + 3.9 were observed. The frequency distribution of the strength of the sources within a set was measured by the use of a well-type ionisation chamber. The root mean square deviations of the nine source sets ranged from 1.0 to 3.2%. Application of the in-phantom calibration method would improve the precision of source strength measurements and therefore reduce the differences in dose delivery between institutes. The uncertainty of the in-phantom method, which could be estimated by statistical methods, was 0.3 (one standard deviation). That part of the uncertainty, which could not be evaluated by statistical methods, was estimated to be about 1.2% (one effective standard deviation). This latter part in the uncertainty needs further investigation in order to reduce the overall uncertainty.

Effect of Bedside Shielding on Air-kerma Rates Around Gynecologic Intracavitary Brachytherapy Patients Containing 226Ra Or 137Cs

An anthropomorphic phantom was implanted with 226Ra or 137Cs gynecologic intracavitary brachytherapy sources. Air-kerma rate measurements were taken at 10-cm increments along a horizontal plane from the side of the bed at 50 cm, 87 cm, and 136 cm heights above the floor. Five portable lead shields were placed at the head, at the foot and along one side of the bed and readings were taken again at the corresponding heights above, below and behind the shields. The readings were normalized to 100-mg Ra equivalence, and air-kerma rate curves were drawn allowing for the comparison of 226Ra and 137Cs with and without lead shields. The data demonstrated that the air-kerma rates for 137Cs were reduced more than those for 226Ra with the use of the portable lead shields. There was four times the transmission with 226Ra than with 137Cs. The optimal placement was with the lateral bedside shields proximal to the head and foot closest to the bed, with the middle shield overlapping in back. The shields at the head and foot should extend out and overlap the bedside shields. The level of the sources should be positioned near the bottom of the shields. This information will provide the medical health physicist with an estimate of air-kerma rates for both 226Ra and 137Cs with and without shielding for evaluating personnel exposures as well as the effectiveness of current shielding in relation to radiation protection requirements in adjacent rooms or hallways.