By E. J. Browne

Throughout the world X-rays and radioactive materials are used very extensively for the diagnosis of many forms of disease, for the treatment of cancer and its related diseases, and for medical research of many kinds. While there is some general awareness of this fact, it is not widely known Just how extensively X-rays and radioactive substances are used in clinical practice. The present article describes some of the medical uses to which they are being put in New Zealand.

Diagnosis

The refinement of X-ray techniques for the visualisation of the chest and lungs has played a major part in conquering tuberculosis. The development of safe contrast media, which are visible on radiographs because they absorb either more or less Xradiation than the surrounding tissue, have resulted in improved and new techniques for the examination of the heart, veins and arteries, kidneys, and other internal systems and organs. Radiography of the breast, mammography, is a valuable addition to the techniques of cancer detection with each centre evolving the technique best suited to its needs and available equipment. This examination, using low kilovoltages, has proved useful in assisting the diagnosis of tumours where clinical diagnosis is uncertain and in detecting unsuspected tumours in a supposedly normal breast.

Improvements in equipment and techniques have led to positive identification of many other types of abnormalities and tumours. In addition, X-ray localisation has made possible a more exact definition of the tumour volume and its relation to adjacent structures and this has resulted in more effective radiation treatment should this be required.

Modern medicine is becoming increasingly dependent on diagnostic techniques using minute amounts of radioactive material, known as tracer doses, for the identification of diseases and for the study of the course of disease after treatment. Any radionuclide' may serve as a true tracer for the element with which it is isotopic since its behaviour can be studied by measurements of its emitted radiations. Similarly, if a radioactive atom is incorporated into a molecule a "labelled" compound results whose physical or chemical reactions, may be observed in the same manner. For this reason they have found wide applications in tracing metabolic processes —the physical and chemical processes by which the living body is maintained and energy made available for various forms of work—and for the measurement of blood flow, blood volume, excretion, and other important parameters or functions.

In the diagnostic use of radioisotopes it is essential that no harmful concentration of the radioactive material occurs. Isotopes of high radiation energy which are retained in the body for long periods of time and which concentrate in small organs give the highest doses. A judicious choice of radioisotopes can often result in a radiation dose smaller than that from a conventional X-ray examination.

The capacity of the thyroid gland to concentrate iodine rom the blood has long been recognised. The use of radioactive iodine for measuring this capacity has become a standard clinical procedure and is the commonest application of radioisotopes in the field of clinical medicine both in New Zealand and overseas. When radioiodine is administered to the patient, gamma rays capable of external detection can be readily counted and the degree of thyroid activity measured. A more recently developed test which represents a significant advance in the search for a simple and reliable measurement of thyroid function is carried out by adding a resin impregnated with radio-iodine to a sample of blood taken from the patient. This test, which causes no radiation exposure to the patient, is significant in the presence of unrelated non-thyroidal factors which are known to complicate the interpretations of a conventional test finding.

Evaluation of the kidney function by radioisotope procedures is performed using a compound labelled with radio-iodine. Distinctive excretion patterns in normal and abnormal kidney states are obtained by external counting of radioactivity over each kidney. This method provides a simple, fast, and accurate means of evaluating kidney function, As the compound is excreted rapidly and exclusively through the kidneys, radiation exposure to the patient is minimised. This test often provides more diagnostic data than any one of the conventional kidney function tests.

Radio - lodinated human serum albumen is an ideal tracer substance and has proved to be of value in a wide variety of clinical procedures. It is derived from natural plasma protein and mixes readily with normal blood constituents. It can also be detected and measured easily and is excreted promptly following metabolic breakdown of the albumen, It is used in the determination of blood and plasma volumes and enables the physician to know the quantity of blood which has to be transfused to restore the blood volume to normal after an incident causing blood loss. Labelled albumen has also been used for the determination of blood circulation and heart output.

Although radioisotopes are not routinely used during pregnancy, certain clinical conditions warrant their administration. Localisation of the placental site is often of such importance as to justify a low level exposure of the foetus. The use of labelled albumen is an alternative to X-ray procedures for this examination and it generally results in a lower radiation exposure both to mother and foetus.

Before any procedure involving radio-iodinated serum albumen, a special substance is administered to minimise thyroid uptake of radio-iodine.

The measurement of the total red cell volume of a patient can be readily performed using radioactive chromium by a method known as the isotope dilution technique. By this procedure the patient's own red cells or those of a compatible donor are labelled, the total amount of radioactivity injected is measured, and after complete mixing has taken place the radioactivity per millilltre of red cells is measured. The total red cell volume may then be calculated. Radioactive chromium is also used in the study of red cell survival and in the detection of bleeding in the stomach.

If it is desired to determine how well a patient's liver is functioning, a small amount of radio-iodinated dye is injected into a vein in the arm. The passage of radioactive dye through the body is followed with an external detecting, apparatus similar to that used for thyroid tests. The radioactive dye accumulates in the normal liver and is then excreted through the gall bladder and bile ducts into the intestines. The manner in which the dye passes through the body indicates the degree of liver function. It also helps the physician to determine the type of liver disorder present, if any, and whether there is any obstruction.

The use of vitamin B12 labelled with radioactive cobalt is a well-established practice for the diagnosis of pernicious anaemia. Pernicious anaemia results when the stomach fails to secrete, into the gastric fluids, a biochemical known as "intrinsic factor." The entire body of a normal adult contains only a few milligrams of vitamin B12 which must combine with the (intrinsic factor before it is absorbed. The percentage of labelled vitamin B12 excreted in the faeces or urine is measured after administering an oral dose to the patient and this indicates the extent of intestinal absorption of vitamin B12 and confirms the presence or absence of intrinsic factor in the stomach.

Radioactive iron is one of the most versatile and most interesting of the radioisotopes available for medical diagnostic and research procedures. Radioisotope tracer techniques employing radioactive iron provide a comprehensive picture of the mech anism of iron metabolism and of the influence of disease, processes upon this mechanism. By means of radio-iron, one can measure the rate of absorption of iron from the intestine, the demand for iron by the bone marrow, the efficiency of the marrow in synthesising new red blood corpuscles, and the survival of labelled cells. Radioactive iron is also used for the diagnosis of many types of anaemic conditions.

Radioisotope scanner

Improvements of radiation detecting equipment and devices such as the whole-body scanner are rapidly increasing the scope and effectiveness of medical radioisotope work in hospitals and medical laboratories in New Zealand. Radioisotope scanning procedures for clinical diagnosis have been successfully applied to the thyroid, brain, liver, kidney, lung and bone. When the radioisotope has reached its anticipated tissue concentration maximum the radiation detector mounted on the scanning device is mechanically moved across the involved region of the body. As the detector moves, the amount of radioactivity under it is measured and recorded pictorially by the instruments.

The primary requirement of the scanner is its capacity to detect significant differences in the relative concentrations of radioactivity as the detector moves from point to point. In order to determine the position of the radioactive tissues relative to other body tissues, an X-ray picture may be taken of the area of interest, and the scanning image superimposed on the radiograph.

Research

Considerable research has been carried out in New Zealand and overseas to develop methods of using radioisot opes to localise tumours of the brain. Radioactive technetium and compounds labelled with radio-iodine have been injected and the tumour localised using the specially sensitive scanning devices.

Perhaps the most interesting development in the application of radioisotopes in medical research is the quantitative estimation of previously undetectable but vital biological compounds. This requires a sound knowledge of chemistry, biochemistry and radiochemistry. One of the techniques under development aims at the estimation of growth hormone in blood. In the normal child this hormone has a concentration in blood of the order of a thousandth 'of a microgram per millilitre and can only be measured by radioactive methods. An estimation of the level of growth hormone in the blood can determine if the lack of growth observed in a child is due to deficiency in growth hormone. For this test a pure extract of growth hormone is labelled , with radio-iodine. By a combination of extremely sensitive tests it is possible to estimate the amount of growth hormone in the patient's blood from a small sample of blood taken from the patient.

Radiotherapy

In the therapeutic application of X-rays and radioactive materials advantage is taken of the fact that radiation can destroy undesirable tissue. Broadly speaking, cells are; more vulnerable to radiation; in the growing stage. Since very active cell growth is a characteristic of cancerous tissue it follows that in many cases a destructive dose of radiation may be applied to the tumour without permanently harming the surrounding nealthy tissue. There are in general three methods by which radiation is applied for treatment.

A. By directing a beam of radiation into the tumour; from an external source.

B. By inserting a sealed source of radioactive material into the tumour.

C. By introducing unsealed radioactive materials into the body, either orally or, by injection.

A: Therapy using an external source

In this method of treatment a sharply defined beam of penetrating radiation is directed at the tumour. The successful application of external, beams of radiation demands a detailed knowledge of radiation physics. Over the years ingenious techniques have been developed to increase the tumour dose and to reduce the! exposure to the surrounding healthy tissue.

A method commonly used is to rotate the beam so that it is always aimed at the tumour which is to be destroyed. For the treatment of tumours located deep in tissue highly penetrating gamma radiation from radioactive cobalt-60 or X-rays generated at high; kilovoltages are used. For the treatment of skin conditions and tumours close to the surface of the body, low or medium energy X-rays, or beta or gamma emitting radioisotopes in the form of surface applicators are used. Radioactive strontium-90 is the most common beta emitter for surface application and is useful in eye applicators in which the limited penetrating power of beta radiation is an important factor in avoiding damage to the underlying radiosensitive tissues.

B: Insertion of a sea'el radioactive source into the body.

The treatment of various types of cancer by radium needles and tubes has been carried out for many years. The radium is inserted into the tumour by surgical means according to a predetermined plan to give the desired radiation dosage distribution. On completion of the treatment the radium is withdrawn. Radioactive cobalt-60 has been used as an alternative to radium. Among other similar treatment techniques is the permanent implantation of small radioactive "seeds." For these techniques radioactive materials which lose their radioactivity rapidly must be used. Radioactive gold, radioactive yttrium and gold encapsulated radon are commonly used for permanent implantation.

C: Introduction of unsealed radioactive materials into the body.

There are only two main procedures for this purpose. The better-known method alms at radioisotope localisation through metabolism since it results in selective absorption of the radioactive atoms inta the cells of the tissue or malignant growth to be treated. In this procedure radioactive iodine which concentrates in the thyroid gland is used successfully for the treatment of thyroid cancer, while radioactive phosphorus is used to suppress excess red cell formation in the blood and for certain types of chronic leukaemia and bone tumours. The other procedure achieves localisation by means of suspensions of nonsoluble particles of the radioisotope injected within a selected part of the organ to be treated. The radioisotope is used merely as an emitter of radiation and almost no account is taken of its biochemical property. Colloidal radioactive gold and colloidal radioactive yttrium are used for this form of therapy to reduce the spread of malignant tumours, particularly those in the abdomen and lung.

The procedures discussed represent some of the more important applications of Xrays and radioactive materials In medicine. In this rapid'y expanding field opportunities abound for graduates in medicine, physics, biology, or biochemistry, for the specialised study of the application of X-rays and radioactive materials which will enable them to contribute to the betterment of man's health and well-being.

Acknowledgements: The writer acknowledges with thanks the information supplied by a number of New Zealand medical physicists. The photographs were supplied through the courtesy of the Auckland Hospital Radioisotope Laboratory.

This article is published with the authority of the Director, National Radiation Laboratory. Department of Health, Christchurch.