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Content archived on 2024-04-19

Patient dose from radiopharmaceuticals


In recent years, legal requirements of national regulatory authorities governing the administration of radioactive substances to patients have emphasised the need for making comprehensive dose estimates. Dose estimates were formerly based on limited biokinetic data; an unsatisfactory state of affairs which prompted the International Commission on Radiological Protection (ICRP) to appeal for the collection of the particular information needed for dose calculations and that this should be made available in the open literature. In recent years there has been a marked response and an increasing number of dosimetry orientated studies are appearing in the scientific literature. These have shown a trend to greater sophistication and accuracy in techniques of biodistribution and data analysis. This is exemplified in work undertaken in this contract (Smith - MRC) by the description of a biodistribution study designed to provide dosimetry information on a relatively new substance of proven clinical value (indium-111-antimyosin antibody.
The influence of paediatric biokinetics on dose estimates needs to be addressed and, to this end, the present contract has involved a study of the dosimetry of 99mTc-DMSA in children over a wide age range (Evans - Univ. London). This substance is typical of many for which there was formerly no alternative to the use of adult biokinetic data to predict doses to children.
In addition, internal dosimetry h as to embrace the full spectrum of available radiopharmaceuticals, especially to include the short-lived positron-emitting tracers. Until recently, very little dosimetry had been performed on PET substances, mainly because of the practical difficulties of carrying out biodistribution studies using tracers with half-lives of only a few minutes. This deficiency has led to the development of modelling techniques, combining both measured data and known physiological parameters, as a substitute of conventional methods. The description of such a model for estimation of the dosimetry of 15(O)-water is included in this contract (Smith - MRC).

In the paediatric studies which constitute part of this contract, attempts have been made to compensate for the stepwise nature of the phantom database by performing interpolation between phantoms according to the weight of the measured child in relation to the phantom weights (Evans - Univ. London).

An important part of the work of this contract has been to examine CT phantoms for this very purpose (Petoussi - GSF). Specific absorbed fractions have been calculated for a wide range of organs and, using biodistribution data from ICRP 53 or those supplied by another contractor in this project, organ of effective doses have been estimated. These data have been compared with values based on the MIRD type phantoms. Of particular importance are the results for bone and bone-marrow. These tissues are modelled by the CT technique in a fundamentally different way to that employed using MIRD phantoms, providing for a more scientific basis of skeletal dosimetry. It must be emphasised that, at the present time, the CT phantoms are limited in number and differences would be expected to exist, reflecting differences in stature between the human bodies used for derivation of CT phantoms and representative mathematical phantoms based on average body dimensions. However, this work has revealed that, in some circumstances, larger than expected differences are apparent, and further research is needed to establish the reasons for these observations.
The aim of this proposal, which is a continuation of an ongoing CEC project is:
- to improve dose estimates for patients in nuclear medicine, especially children and new-borns by:
a) Continued collection of biokinetic data for selected radiopharmaceuticals by serial uptake and retention measurements on patients and adult healthy volunteers. Special attention will be given to new radiopharmaceuticals, recently taken into clinical use. For these studies gamma cameras as well as whole-body counters will be used together with measurements on samples of blood and urine. The patient studies will be supported by measurements on phantoms. Animal experiments will be done in order to evaluate the absorbed dose contributions to various parts of the gastro-intestinal tract.
b) Improving the physical basis for the dose calculations by using detailed so-called voxel phantoms based on CT-data for calculations of new S-values or related parameters using Monte Carlo technique.
c) Calculations of mean absorbed doses to various organs and tissues as well as effective doses. To compare absorbed dose values for the GI-tract calculated by the old ICRP 53 (ICRP 30) model and recently proposed mOre realistic models for the GI-tract. To make comparisons between the numerical values of effective dose and effective dose equivalent for various nuclear medicine investigations.

and also

- to intensify the studies of the relations between diagnostic image quality and amount of administered activity using measurements on phamtoms as well as information from clinical investigations.

The proposal will yield improved dose estimations and give a base for optimization of image quality and patient exposure. -

This is a jolnt proposal from laboratories in Germany, Sweden and the United Kingdom.


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MRC Clinical Research Centre
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Watford Road
HA1 3UJ Harrow
United Kingdom

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Participants (3)