between 0.1 andO. 15 mill/kWh of nuclear electricity disposal operation for calcined solids in salt deposits produced. This cost approximates only one to two are estimated at 0-01 mill/kWh(e). Based on past per cent of the cost of nuclear power in an 8 - 10 laboratory and engineering scale cold unit operation mill/kWh economy. The ORNL engineering and data, and on an expected successful field demonstra economic studies indicate that the total cost of in tion and testing programme with high activity wastes, terim liquid storage, pot calcination, and shipping it is firmly believed that waste management opera over a 3000 mile round trip can be as low as tions should not constitute a major obstacle to the 0-02 mill/kWh(e). Tentative costs for a large-scale development of economical nuclear power. "
MEDICAL USES OF RADIOACTIVE CALCIUM REVIEW OF AN IAEA PROGRAMME TO PROMOTE THE APPLICATIONS OF CALCIUM-47
Calcium plays a number of biologically essen absorbed in a layer of matter less than 1 mm thick. tial roles, which have long been under investigation The relatively long half-life of the isotope results, in by various techniques available to medical science. most human studies, in an undesirably prolonged One of the most important of these techniques is irradiation of the subject and the isotope is there radioactive tracer analysis, i. e. study of the func fore considered rather hazardous for internal ad tions of calcium within the body with the help of a ministration. Also, the low energy level of its radia radioactive isotope of the element. tion often makes measurements difficult. Calcium- 49, on the other hand, has a half-life of only nine The calcium-47 programme of the International minutes, which is too short for biological tracer Atomic Energy Agency is intended to promote these applications, although convenient for certain other investigations by facilitating the production and use purposes. Calcium-47 has a half-life of about five of this isotope. The importance attached to calcium- days, which is long enough for numerous biological 47 is due to the special properties of this isotope, investigations and short enough to ensure that the which make it the most valuable tool for many cal person subjected to the investigation is not under cium studies by the radioactive tracer method. prolonged irradiation. It emits gamma rays in Calcium as found in nature is a stable element addition to beta rays, and since the gamma rays are and has six isotopes. The most common of these capable of penetrating many centimeters of tissue, six stable isotopes is calcium-40, which constitutes determination of calcium-47 in the body is possible about 97 per cent of natural calcium. The other by external measurements. Furthermore, measure stable isotopes are rare, and calcium-46, a stable ment of the calcium-47 content even of isolated isotope which is of particular interest in the produc samples, such as a few milliliters of blood, is far tion of the radioactive isotope calcium-47, has an more convenient than that of calcium-45. Thus abundance of only 0. 003 per cent. calcium-47 has distinctive properties which allow its use in biological investigations where calcium-45 Advantages of Ca^' would be unsuitable, and some of these uses are of considerable medical significance. and Methods of Production Unfortunately, calcium-47 is difficult to pro Apart from the stable isotopes, at least six duce. The irradiation of ordinary calcium in a radioactive isotopes of calcium have been artificially nuclear reactor, of course, results in the capture produced. Three of these have properties which of some neutrons by the stable isotope calcium-46, make them of possible use as radioactive tracers. which thereby is converted to calcium-47. However, Calcium-45 has been the most prominent of these because calcium-46 is extremely rare, the amount three, and has been used extensively during the last of calcium-47 produced in this way is insignificant 15 years. Calcium-49 has also been employed. But compared with the amount of calcium-45, which is for many purposes the most suitable is calcium-47. produced from calcium-44, a more abundant stable Why this is so is explained by a comparison of the isotope. There are several other reactions by physical characteristics of these three isotopes. which calcium-47 can be produced, but they are all Calcium-45 has a half-life of about 160 days and very expensive. At present the most economic emits low energy beta radiation which is completely approach is a sequence of two steps. First, calcium-
15 46 is separated from other natural calcium isotopes retained in the skeleton and therefore irradiate the in isotope separation machines. Secondly, this skeleton more than any other organ. Notable among nearly pure calcium-46 is then irradiated in a reac these is strontium-90. In order to devise an effec tor to produce nearly pure calcium-47. It should, tive method of eliminating these isotopes from the however, be remembered that very few centres in skeleton, it is necessary to have a detailed under the world have machines for isotope separation. standing of the metabolism of the skeleton, and in particular of calcium. Importance of Calcium Studies It might be worth while to examine at this Start of IAEA Programme point the reasons for the special interest in the pro duction of radioactive tracers for calcium studies. IAEA's attention was focussed on calcium-47 The reasons lie in the physiological importance of during a scientific meeting in 1958 when it became calcium, which - as is well known - is a primary apparent that several doctors were interested in constituent of the skeleton; in fact, the skeleton con using this isotope, but that at the time only one or tains more than 99 per cent of the body's calcium. two in the whole world were able to do so on account What is not so well known is that the element has of its scarcity and exceedingly high price. Later other essential physiological functions. A continuous that year the Agency's own scientific staff and its and rapid exchange of calcium goes on between the consultants, after a study of the problem, deter blood and other soft tissues on the one hand, and the mined that the demand for this isotope far exceeded reserves in the skeleton on the other. its current supply, and that production possibilities existed which wouia De mucn more economical n Before the advent of radioactive tracers it was production in quantity could be undertaken. Much virtually impossible to investigate the metabolic path of the Agency's work thereafter has been directed of calcium in the body: its absorption through the towards the stimulation of cheaper production and walls of the intestine into the blood, its deposition the demonstration of the usefulness of the isotope if in the skeleton, its return to the blood and possible it could be more cheaply available. incorporation into complex molecules, and its excre In 1959 a second meeting of Agency consultants tion. All these steps are subject to interference by examined production techniques in detail and certain disease and by nutritional conditions, and defects in lines of action emerged from these discussions. One the body's handling of the isotope may in turn cause research contract was negotiated, with financial illness. The isolation of metabolic pathways and the assistance from the United States Atomic Energy identification of disorders in any of these are pos Commission, to develop cheaper methods of enriching sible only when the scientist is able to administer a calcium in the isotope calcium-46, i. e. increasing "tracer" under chemical or physiological conditions artificially the calcium-46 content of natural cal appropriate to the study. Radioactive isotopes of cium. The existing producers of calcium-47 were calcium are particularly suitable as such tracers. stimulated to improve their techniques; in parti A somewhat different calcium-47 application, cular, several additional machines for the enrich which cannot be done with any other calcium isotope, ment of calcium with the calcium-46 isotope were put is the detection and localization of bone tumours. into operation at the Oak Ridge National Laboratory These tumours, whether originating in the skeleton in the USA. Recommendations were also made con or transferred there from a soft tissue tumour, are cerning the radiochemical purity of calcium-47 for usually characterized by comparatively rapid growth human applications. and deposition of increased amounts of calcium. If detected in time, their progress can be greatly re The research contract on enrichment techniques tarded by suitable treatment. When calcium-47 is was partially successful in developing new methods, injected into a patient thought to be suffering from although they have not yet been utilized due to the or vulnerable to bone tumours, the existence and simultaneous improvement in the supply of enriched location oi tumours can De estaDlished irom the calcium from existing methods. Since 1958 the price abnormal concentration of the isotope at any place, of calcium-47 has been reduced from over $1400 per the concentration being detected by scanning of the millicurie to $200 per millicurie, and its degree of gamma rays emitted by the isotope. It appears that purity has been increased. it is sometimes possible to detect skeletal tumours By the end of 1960 eleven more research con by this technique earlier than by radiography. tracts had been negotiated by the Agency on the appli In addition to its interest in intrinsic calcium cation of calcium-47 in medical diagnosis and re metabolism and in the detection of skeletal tumours, search. Most of these contracts financed only the medical science has a special interest in another cost of the isotope itself, so as to remove an impor aspect of the behaviour of the skeleton, and this is tant barrier to its use and thus to encourage compe an aspect which has a bearing on IAEA's functions in tent and otherwise suitably equipped investigators to the field of radiation safety. Many long-lived radio explore its potential applications. The contracts isotopes, having once gained entry into the body, are have been placed in the following countries: Austria,
16 Belgium, Denmark, France, Poland, South Africa, be expected - those that affect the skeleton. It was Sweden and the United Kingdom. also shown that calcium absorption is to some extent Most of these studies have now been in progress influenced by the amount of calcium in the diet and by for nearly two years, and it is expected that they will the character of the diet. A number of investigators be continued for one more year. It is foreseen that had concentrated on the elucidation of the metabolic thereafter the cost of caicium-47 will not be exces pathways within the body. After injection of the sive in comparison with the other costs of radioactive isotope, the concentration in the blood and excretions tracer investigations of medical and biological prob was followed in quantitative detail, and attempts were lems, and that the existence and possibilities of this made to describe mathematically the size and turn isotope will be widely recognized by the medical pro over rates of the several metabolic pools of calcium. fession. The isotope would then, perhaps, be treated Correlations were attempted between these findings like any other useful radioisotope and not need any and disease states, or treatment routines in parti special subsidy. cular diseases. At several laboratories calcium-47 had been extensively tested in tumour localization studies, and specific examples were given in which Results of Research this isotope had permitted particularly early recog Throughout this programme, an important goal nition of tumours. has been to make the Agency serve as a clearing house for information concerning calcium-47. A One interesting finding was the difficulty ex study of the special problems in the assay of calcium- perienced in accurate measurement of stable calcium 47 and the most economic means of performing the by chemical techniques, which in turn made it diffi task was undertaken by the Agency's scientific staff, cult to interpret some of the results of the tracer and its conclusions were made available to those investigations. This illustrates the need for well- interested. In December 1961 the Agency convened established conventional measuring techniques to a panel of experts to review the progress of Agency- enable the effective utilization of radioactive tracers. sponsored research contracts; all Agency contrac The proceedings of this meeting have recently tors for calcium-47 research and a few other interes been published by the Agency as a booklet entitled ted scientists were invited to the meeting. Each "Medical Uses of Ca47" (Technical Reports Series scientist reported on his experience with calcium-47 No. 10). in comparison with other calcium isotopes and in the light of his medical problems. In summary, the Agency has for four years been engaged in a comprehensive effort to bring cal It was clear from the discussions that the dis cium-47 into routine medical use. To this end, it tinctive characteristics of calcium-47 make it the has surveyed the need for this isotope, stimulated isotope of choice in a variety of applications, and, by its cheaper production, encouraged the investigation joint use with calcium-45, extend the power of the of its medical possibilities, and arranged for the dis calcium tracer technique. One group of investiga semination of the information thus obtained. The fact tors had employed the isotope in studies of calcium that calcium-47 is no longer considered an exotic iso absorption through the intestinal wall. It was demon tope is at least partly due to the Agency's efforts, strated that calcium absorption is modified in the in co-operation with interested scientists throughout presence of several diseases, particularly - as could the world.
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