NPL secondary standard radionuclide calibrator. Syringe calibration factors for radionuclides used in nuclear medicine
Introduction
Radiopharmaceuticals are widely used in hospitals for diagnostic and therapeutic purposes. Before these radionuclides are administered to a patient, it is essential that their activity is accurately known. The most common instrument of choice, for these types of measurements, is a radionuclide calibrator which incorporates a well-type ionisation chamber. These calibrators have long term stability and their principal of operation is relatively simple. The chambers, however, are sensitive to geometry, density of sample, and container type and any variation of these factors will affect the calibration figures and hence, the measurements in these instruments. If there is any change in any of these parameters, new calibration figures need to be derived.
The NPL secondary standard radionuclide calibrators are supplied with calibration figures for a number of commonly used radionuclides but these are only valid for the specified containers and geometries. A special perspex holder is supplied with the chamber, so that the geometry can be accurately reproduced. Medical radionuclides are usually supplied in a standard glass vial and, before any activity is administered to the patient, the glass vial is measured in the calibrator to ascertain its activity. An aliquot of the radionuclide is then withdrawn from the vial into a plastic syringe which will be used to administer the radionuclide to the patient. In recent years, both as a confirmatory measurement and as good practice for quality assurance purposes, it has become standard procedure to re-measure the activity in the syringe before administration to the patient.
The calibration figures used by hospital users for these syringe measurements are frequently the same as for the glass vial. By not recalibrating for the difference in container and geometry, the reading that is obtained by the calibrator can be significantly different from the true activity. The magnitude of the differences between the calibration figures is dependent upon the radionuclide and the energies and emission probabilities of its emissions, both photons and beta particles.
There are several syringe manufacturers each of which supply a variety of syringe sizes and all of which will have different calibration figures. The situation is further complicated, first, by there not being an agreed standard syringe type and, second, by the fact that very few nuclear medicine departments have any control over the type of syringes that they use. The purchasing departments normally obtain the syringes, and they are unaware of the importance of keeping the types of syringes the same.
Section snippets
Previous indications of problem
The NPL routinely organises radionuclide comparisons for UK hospitals (Woods (1981), Woods (1987); Woods et al (1996), Woods et al (1997a), Woods et al (1997b); Ciocanel et al, 1999; Baker and Woods, 2000). During comparisons of 111In and 123I especially, it became apparent that there were problems with measurements carried out in syringes. The results obtained showed significant discrepancies between the true and the reported activity of the sample. Some manufacturers recommend correction
Present work
Following discussions with the user community, based on the results of the various comparisons conducted by NPL, it was decided that NPL should derive calibration factors for various different types and sizes of syringes routinely used in hospitals and for the most commonly used radionuclides. This paper details the initial findings from those studies, highlighting the problems of calibration factors for two radionuclides: 125I and 131I. These two radionuclides were chosen because the energies
Results
Fig. 3, Fig. 4 show the results obtained for the 125I and the 131I respectively. The syringe activity values were calculated by dividing the ionisation chamber response by the calibration factor that would be used for the standard glass vial normally employed in activity determinations in hospitals. The values plotted in the figures are the ratios of the syringe activity values as determined above compared with the true activity values. The uncertainties on the ratios are typically much <±1%.
As
Discussion
The results obtained confirm that there are large differences in ionisation chamber response between glass containers and plastic syringes. The 125I results show responses of up to 80% higher than for the normally used P6 vial and clearly validate the conclusion indicated by the previous NPL comparison exercises that one of the manufacturers recommendations of a correction factor of 25% is insufficient.
The 131I results are not as sensitive to container differences, the responses being about 10%
Conclusions
There are serious implications for patient administrations of radionuclides if the calibration factors used for radionuclide calibrators are inaccurate. The results obtained to date indicate that there is a real problem with syringe measurements and this needs to be addressed. Further work is required to produce a comprehensive set of calibration factors for the NPL secondary standard radionuclide calibrator that encompasses the syringes and radionuclides most commonly used in nuclear medicine
Acknowledgments
This work has been supported by the National Measurement System Policy Unit of the Department of Trade and Industry.
References (8)
- Baker, M., Woods, M.J., 2000. Intercomparison of 123I solution sources in UK hospitals, 2000. NPL Report CIRM...
- Capintec, 1983. Radioisotope calibrator owner's manual for models CRC-7, CRC-12 and CRC-120. Pittsburgh, PA 15238,...
- Ciocanel, M., Keightley, J.D., Scott, C.J., Woods, M.J., 1999. Intercomparison of 131I solution and capsule sources in...
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