By Dr K. Dessai, Independent Consultant in Metals, Mining and Energy
Minor metals are used in the commonly-known applications of aerospace, automotive and energy generation, to name a few, and they have also been extensively utilised in medicine and dentistry. In biomedical applications for example, titanium is commonly used as an implant material in dentistry and orthopaedics. A less well-known branch of medicine—nuclear medicine—has also been using minor metals for a number of decades. In this application, it is the radioactive properties of minor metals are utilised, in the form of radioisotopes.
Many chemical elements have a number of isotopes, which have the same atomic number—representing the number of protons – but a different number of neutrons, which results in different masses. The number of electrons is equal to the number of protons for an element in a neutral state, and it is the number of electrons which determines the chemistry of the element or atom.
Image Source: Oak Ridge National Laboratory
Radioactive isotopes (or radioisotopes) are atoms which are generally unstable in nature and must therefore be produced artificially. The number of neutrons and protons in a radioisotope will differ from the same element in its naturally occurring state. For example, strontium has a naturally occurring mass number of 88, while it also has radioisotopes Sr-82 and Sr-90. Thus this difference in mass number leads to differing physical and chemical properties, which consequently each have differing applications.
A nucleus of a radioisotope is unstable, as it is composed of an undesirable number of neutrons and protons. For this reason, it will undergo radioactive decay through the emission of an alpha, beta and/or gamma radiation to become a more stable atom. In nuclear medicine, radioisotopes are used within the body, differing from the traditional uses of radiation in medicine, such as X-rays, which are generated externally and passed through the body to form an image.
Radioisotopes are used in two ways in medicine: in diagnostics and in treatment. While a greater proportion are used in diagnosing illness and disease, their growing uses in the treatment of cancers are wide-ranging, especially in the form of radiotherapy. This is because rapidly dividing cells are extremely sensitive to radiation. The ability to target the type of radiation (alpha, beta and/or gamma emissions) from a close proximity to specific cells, tissues, organs or regions of the body makes the technique a particularly advantageous treatment.
In nuclear medicine, the use of radioisotopes which emit ionizing radiation must be carefully measured and managed so as to diagnose or treat only the required areas for a specific period of time—enough to obtain the necessary information or undertake a particular treatment. Minimum radiation exposure helps to reduce any unwanted side effects or damage to other healthy organs and tissue. Any radiation exposure to treatment providers or the general public also needs to be considered and maintained within safe limits.
Radioisotopes can also be selected based on their decay properties, typically measured using ‘Half-life’. The ‘Half-life’ is the time required for half of the quantity of radioactive isotope to decay, and is measured in minutes (mins), days and years. This means that they survive for long enough to effect the treatment, but decay quickly enough to minimise negative effects from radiation.
Radioisotopes are generally produced in one of two ways: in nuclear reactors, as the products of the fission process through the addition of neutrons, or in cyclotrons, through neutron-depletion by the addition of protons. The largest proportion of the world’s supply of radioisotopes or radionuclides have been produced by two reactors; one in the Netherlands at the Petten Nuclear Reactor and one in Canada at Chalk River Laboratories.
Diagnostic radioisotopes are typically those which emit gamma radiation, which is detected externally by a gamma camera. In this application the radioisotope attached to a carrier is taken internally; intravenously or orally. An example of this is Technetium-99m, for which the parent element is the minor metal Molybdenum-99. Mo-99 (half-life 66 hours) is one of the essential radioisotopes required in the production of other species for use in diagnostics.
It is produced during the fission of Uranium-235 in the Chalk River reactor by neutron irradiation of the nuclear fuel.
Another minor metal, Thallium-201 (half-life 73 hours) is used in cardiology for diagnosing coronary heart conditions, as well as detecting the location of low-grade lymphomas. In diagnostic applications, the radioisotope must have a half-life short enough for it to decay away quickly after required imaging has been completed in order to minimise the radiation dose received by the patient.
In therapeutic applications, beta emitting radioisotopes are typically used which can also have some gamma emission.
These are used to treat a wide range of cancers through weakening or destroying tumour cells, for example, Yttrium-90 (half-life 64 hours) is used for non-Hodgkin’s lymphoma and liver cancer. Holmium-166 (half-life 26 hours) is being developed for the diagnosis and treatment of liver tumours. There are also other therapeutic applications of minor metal radioisotopes for example, in the use of palliative care and pain relief from cancers and arthritis. Rhenium-186 (half-life 3.8 days) provides pain relief for bone cancer, Samarium-153 (half-life 47 hours) for secondary cancers lodged in bone, and Strontium-89 (half-life 50 days) for prostate and bone cancer. Erbium-169 (half-life 9.4 days) is used to provide pain relief from arthritis of synovial joints.
Another application of minor metals in medicine is their use in sterilisation of equipment. Traditionally, heating has been used for this purpose, however, for equipment such as gloves, syringes, clothing and instruments which would be damaged by this process, Cobalt-60 which is an energetic gamma emitter and has a half-life of 5.27 years, is the main radioisotope used.
A newer area of radioisotope application is in targeted alpha-therapy to treat cancers where alpha radiation emitters such as Bismuth-212 (half-life 1 hour) and Bismuth-213 (half-life 46 mins) are utilised to provide a higher energy treatment at close range to the cancer cells. This type of therapy is used particularly for melanoma, breast and ovarian cancers. Table 1 below summarises some of the minor metals used in nuclear medicine.
The use of minor metals in nuclear medicine as key radiopharmaceuticals or as parent elements has been wide and varied, and new applications continue to be developed. With the versatility they offer, in terms of radioactive decay, energy quantity, half-life and distance, radioisotopes will have a healthy future in diagnostics and therapy. In particular, their use in the treatment of cancers will only increase, given the many and varied forms of the disease, and the need to treat specific areas without damaging healthy tissue until such time as cures are developed.
To contact the author, Dr K. Desai, email met.min.en@gmail.com
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