Rapidly dividing cells are particularly sensitive to damage by radiation. For this reason, some cancerous growths can be controlled or eliminated by irradiating the area containing the growth. External irradiation can be carried out using a gamma beam from a radioactive cobalt-60 source.
Many therapeutic procedures are palliative, usually to relieve pain. For instance, strontium-89 and (increasingly) samarium 153 are used for the relief of cancer-induced bone pain. Rhenium-186 is a newer product for this.
The radioisotope most widely used in medicine is technetium-99m, employed in some 80% of all nuclear medicine procedures - 40,000 every day. It is an isotope of the artificially-produced element technetium and it has almost ideal characteristics for a nuclear medicine scan. These are:
It has a half-life of six hours which is long enough to examine metabolic processes
yet short enough to minimise the radiation dose to the patient.
Technetium-99m decays by a process called "isomeric"; which emits gamma rays and low energy electrons. Since there is no high energy beta emission the radiation dose to the patient is low.
The low energy gamma rays it emits easily escape the human body and are accurately detected by a gamma camera. Once again the radiation dose to the patient is minimised.
The chemistry of technetium is so versatile it can form tracers by being incorporated into a range of biologically-active substances to ensure that it concentrates in the tissue or organ of interest.
Its logistics also favour its use. Technetium generators, a lead pot enclosing a glass tube containing the radioisotope, are supplied to hospitals from the nuclear reactor where the isotopes are made. They contain molybdenum-99, with a half-life of 66 hours, which progressively decays to technetium-99. The Tc-99 is washed out of the lead pot by saline solution when it is required. After two weeks or less the generator is returned for recharging.
A similar generator system is used to produce rubidium-82 for PET imaging from strontium-82 - which has a half-life of 25 days.
Myocardial Perfusion Imaging (MPI) uses thallium-201 chloride or technetium-99m and is important for detection and prognosis of coronary artery disease.
Although radiotherapy is less common than diagnostic use of radioactive material in medicine, it is nevertheless widespread, important and growing. An ideal therapeutic radioisotope is a strong beta emitter with just enough gamma to enable imaging, eg lutetium-177. This is prepared from ytterbium-176 which is irradiated to become Yb-177 which decays rapidly to Lu-177. Yttrium-90 is used for treatment of cancer, particularly non-Hodgkin's lymphoma, and its more widespread use is envisaged, including for arthritis treatment.
Iodine-131 and phosphorus-32 are also used for therapy. Iodine-131 is used to treat the thyroid for cancers and other abnormal conditions such as hyperthyroidism (over-active thyroid). In a disease called Polycythemia vera, an excess of red blood cells is produced in the bone marrow. Phosphorus-32 is used to control this excess.
A new and still experimental procedure uses boron-10 which concentrates in the tumor. The patient is then irradiated with neutrons which are strongly absorbed by the boron, to produce high-energy alpha particles which kill the cancer.
For targeted alpha therapy (TAT), actinium-225 is readily available now, from which the daughter Bi-213 can be obtained (via 3 alpha decays) to label targeting molecules.
ISOTOPES USED IN MEDICINE
Many radioisotopes are made in nuclear reactors, some in cyclotrons. Generally neutron-rich ones need to be made in reactors, neutron-depleted ones are made in cyclotrons.
Reactor Radioisotopes (half-life indicated)
Molybdenum-99 (66 h): Used as the 'parent' in a generator to produce technetium-99m.
Technetium-99m (6 h): Used in to image the skeleton and heart muscle in particular, but also for brain, thyroid, lungs (perfusion and ventilation), liver, spleen, kidney (structure and filtration rate), gall bladder, bone marrow, salivary and lacrimal glands, heart blood pool, infection and numerous specialised medical studies.
Bismuth-213 (46 min): Used for targeted alpha therapy (TAT), especially cancers.
Chromium-51 (28 d): Used to label red blood cells and quantify gastro-intestinal protein loss.
Cobalt-60 (10.5 mth): Formerly used for external beam radiotherapy.
Copper-64 (13 h): Used to study genetic diseases affecting copper metabolism, such as Wilson's and Menke's diseases.
Dysprosium-165 (2 h): Used as an aggregated hydroxide for synovectomy treatment of arthritis.
Erbium-169 (9.4 d): Use for relieving arthritis pain in synovial joints.
Holmium-166 (26 h): Being developed for diagnosis and treatment of liver tumours.
Iodine-125 (60 d): Used in cancer brachytherapy (prostate and brain), also diagnostically to evaluate the filtration rate of kidneys and to diagnose deep vein thrombosis in the leg. It is also widely used in radioimmuno-assays to show the presence of hormones in tiny quantities.
Iodine-131 (8 d): Widely used in treating thyroid cancer and in imaging the thyroid; also in diagnosis of abnormal liver function, renal (kidney) blood flow and urinary tract obstruction. A strong gamma emitter, but used for beta therapy.
Iridium-192 (74 d): Supplied in wire form for use as an internal radiotherapy source for cancer treatment (used then removed).
Iron-59 (46 d): Used in studies of iron metabolism in the spleen.
Lutetium-177 (6.7 d): Lu-177 is increasingly important as it emits just enough gamma for imaging while the beta radiation does the therapy on small (eg endocrine) tumours. Its half-life is long enough to allow sophisticated preparation for use.
Palladium-103 (17 d): Used to make brachytherapy permanent implant seeds for early stage prostate cancer.
Phosphorus-32 (14 d): Used in the treatment of polycythemia vera (excess red blood cells). Beta emitter.
Potassium-42 (12 h): Used for the determination of exchangeable potassium in coronary blood flow.
Rhenium-186 (3.8 d): Used for pain relief in bone cancer. Beta emitter with weak gamma for imaging.
Rhenium-188 (17 h): Used to beta irradiate coronary arteries from an angioplasty balloon.
Samarium-153 (47 h): Sm-153 is very effective in relieving the pain of secondary cancers lodged in the bone, sold as Quadramet. Also very effective for prostate and breast cancer. Beta emitter.
Selenium-75 (120 d): Used in the form of seleno-methionine to study the production of digestive enzymes.
Sodium-24 (15 h): For studies of electrolytes within the body.
Strontium-89 (50 d): Very effective in reducing the pain of prostate and bone cancer. Beta emitter.
Xenon-133 (5 d): Used for pulmonary (lung) ventilation studies.
Ytterbium-169 (32 d): Used for cerebrospinal fluid studies in the brain.
Ytterbium-177 (1.9 h): Progenitor of Lu-177.
Yttrium-90 (64 h): Used for cancer brachytherapy and as silicate colloid for the relieving the pain of arthritis in larger synovial joints. Pure beta emitter.
Radioisotopes of caesium, gold and ruthenium are also used in brachytherapy.
Carbon-11, Nitrogen-13, Oxygen-15, Fluorine-18:
These are positron emitters used in PET for studying brain physiology and pathology, in particular for localising epileptic focus, and in dementia, psychiatry and neuropharmacology studies. They also have a significant role in cardiology. F-18 in FDG has become very important in detection of cancers and the monitoring of progress in their treatment, using PET.
Cobalt-57 (272 d): Used as a marker to estimate organ size and for in-vitro diagnostic kits.
Gallium-67 (78 h): Used for tumour imaging and localisation of inflammatory lesions (infections).
Indium-111 (1.8 d): Used for specialist diagnostic studies, eg brain studies, infection and colon transit studies.
Iodine-123 (13 h): Increasingly used for diagnosis of thyroid function, it is a gamma emitter without the beta radiation of I-131.
Krypton-81m (13 sec) from Rubidium-81 (4.6 h): Kr-81m gas can yield functional images of pulmonary ventilation, e.g. in asthmatic patients, and for the early diagnosis of lung diseases and function.
Rubidium-82 (65 h): Convenient PET agent in myocardial perfusion imaging.
Strontium-92 (25 d): Used as the 'parent' in a generator to produce Rb-81.
Thallium-201 (73 h): Used for diagnosis of coronary artery disease other heart conditions such as heart muscle death and for location of low-grade lymphomas.
Gamma sources are normally more portable than x-ray equipment so have a clear advantage in certain applications, such as in remote areas. Also while x-ray sources emit a broad band of radiation, gamma sources emit at most a few discrete wavelengths. Gamma sources may also be much higher energy than all but the most expensive x-ray equipment, and hence have an advantage for much radiography. Where a weld has been made in an oil or gas pipeline, special film is taped over the weld around the outside of the pipe. A machine called a "pipe crawler" carries a shielded radioactive source down the inside of the pipe to the position of the weld. There, the radioactive source is remotely exposed and a radiographic image of the weld is produced on the film. This film is later developed and examined for signs of flaws in the weld
Non-destructive testing is an extension of gamma radiography, used on a variety of products and materials. For instance, ytterbium-169 tests steel up to 15 mm thick and light alloys to 45 mm, while iridium-192 is used on steel 12 to 60 mm thick and light alloys to 190 mm.
Naturally occurring radioisotopes:
Chlorine-36: Used to measure sources of chloride and the age of water (up to 2 million years)
Carbon-14: Used to measure the age of water (up to 50,000 years)
Tritium (H-3): Used to measure 'young' groundwater (up to 30 years)
Lead-210: Used to date layers of sand and soil up to 80 years
Artificially produced radioisotopes:
Used in backscatter gauges, smoke detectors, fill height detectors and in measuring ash content of coal.
Used for radiotracer technique for identification of sources of soil erosion and deposition, in density and fill height level switches.
Silver-110m, Cobalt-60, Lanthanum-140, Scandium-46, Gold-198:
Used together in blast furnaces to determine resident times and to quantify yields to measure the furnace performance
Used for gamma sterilisation, industrial radiography, density and fill height switches.
Gold-198 & Technetium-99m:
Used to study sewage and liquid waste movements, as well as tracing factory waste causing ocean pollution, and to trace sand movement in river beds and ocean floors
Strontium-90, Krypton-85, Thallium-204:
Used for industrial gauging.
Zinc-65 & Manganese-54:
Used to predict the behaviour of heavy metal components in effluents from mining waste water
Iridium-192, Gold-198 & Chromium-57:
Used to label sand to study coastal erosion
Ytterbium-169, Iridium-192 & Selenium-75:
Used in gamma radiography and non-destructive testing.
Used as a tracer to study sewage and liquid wastes