This radioactive form of strontium chloride - known as strontium-89 - has been developed for cancer treatment. Doctors say that it is not a cure, however, and has not been found to shrink tumours. But once injected into the bloodstream, it is carried straight to secondary cancer deposits in the bones where it can reduce pain.
The only problem is the price. A single injection, which lasts about three months, costs about pounds 800. Dr Valerie Lewington, from the Department of Nuclear Medicine at Southampton General Hospital, says: 'Strontium is an ideal substance to use because, like calcium, it has a natural affinity for bone and is avidly 'taken up' at sites of increased bone activity - such as around malignant deposits.'
Iris Webb, 69, from Portsmouth, was diagnosed as having breast cancer in 1987. Last year she developed excruciating pain from secondary cancer in the bones of her legs and was confined to a wheelchair. In November, she was given an injection of strontium-89. Within one week she noticed a significant reduction in pain levels. Now she is free from pain, fully mobile, and has taken up her old hobbies of painting and drawing.
Prostate and breast cancer are among the most common forms of the disease: in Britain alone, more than 10,000 new cases of prostate cancer are diagnosed every year. In the later stages, both cancers usually spread to the bones.
A combination of morphine-based drugs and conventional radiotherapy is normally used to relieve pain. Both have unpleasant side-effects such as constipation, nausea and drowsiness. Conventional radiotherapy requires frequent trips to hospital, and usually only one or two areas of the body can be treated at a time.
So far there have been three trials of strontium-89 in the UK involving nine hospitals, including Southampton. The second of these, a double-blind study involving 26 patients published in the European Journal of Cancer, found that strontium-89 compared extremely favourably with a placebo, with many patients reporting complete pain relief.
Results of a third major trial, involving 284 patients, have not been published, but they indicate that strontium-89 not only relieves pain but may also delay the development of new sites of distress. Researchers believe it may do this by attacking micro-deposits of cancer cells in bone, which are too small to be detected by scans. A recent Canadian trial has come up with similar results.
The advantages of strontium treatment are numerous. It involves only one visit to the outpatient clinic for one injection, the effects of which begin within two or three weeks and last at least three months. Because it 'targets' cancer deposits, it has no effect on normal tissue and can therefore be given in higher, more effective doses than conventional radiotherapy. It travels in the blood and is capable of reaching several cancer 'hot spots' simultaneously. Unlike morphine, it has no general effect on the nervous system; and unlike conventional radiotherapy, there is in theory no limit to the number of treatments that can be given.
No one yet knows exactly how strontium-89 relieves pain. Like conventional radiotherapy, it is thought to relieve the pain of bone cancer by killing off the abnormal cells which cause swelling and fractures. Researchers also believe it may interfere with the cells on the edge of cancer deposits, called osteoclasts.
The major drawback to this 'wonder drug' is its prohibitive cost. Amersham International, which markets strontium-89 as Metastron in the UK, says that the price reflects the cost of obtaining a purified source of strontium and producing the radioactive isotope from this 'stable' form. Since there are no suitable nuclear reactors in Britain, the active drug has to be prepared in Sweden, Belgium or the Netherlands and reimported. And, unlike other manufacturing processes, the cost of the drug does not drop with greater production.
Because it is so costly, the use of strontium-89, either on its own or in combination with other treatments, remains restricted. In the current climate of limited resources, many doctors are cautious about adding a new and expensive therapy to those already tried and tested.
Dr David Cole, consultant radiotherapist at the Churchill Hospital in Oxford, says: 'The cost of conventional radiotherapy is virtually nil once the initial outlay has been made on the equipment.' He has yet to be convinced that strontium is superior to available forms of treatment.
But according to Dr Lewington: 'One shot of strontium can make awful pain tolerable. Quality of life can be so improved that it is disappointing that more centres are not able to use it on a regular basis.'
The writer is a doctor currently working in Africa.