At present, cancer diagnosis using positron emission tomography (PET imaging) and treatment using irradiation by protons is available in just a handful of UK locations.

PET imaging uses very short-lived radioisotopes (with half-lives typically less than 1.5 hours) to produce the positrons needed, so the technique can only be carried out where there is a large (and expensive) cyclotron or Van de Graaff generator close at hand to make the radioisotopes.

Proton therapy needs very high energy protons produced in a cyclotron.

Dr. Ken Ledingham of Glasgow University detailed at the Institute of Physics Congress 2001 in Brighton on Wednesday how the breakthrough is a result of an amazing optical technique that was successfully applied to nuclear physics last year.

In 1988, it was predicted theoretically that if a sufficiently intense laser beam could be focused onto a small uranium target, light-induced fission would occur. At the time, laser technology was not advanced enough to achieve this.

But the sensational use last year of chirped pulse amplification -- in which a short laser pulse is stretched in time before being amplified and then compressed again -- means that fairly modest tabletop lasers can now be aimed at a uranium target and deliver huge amounts of power (a million million watts or even a thousand times this) in a million-millionth of a second or less, inducing fission in the uranium target.

In PET, a drug labelled with a positron emitter is injected into the patient. When it reaches a tumor, it is stopped by the cancerous tissue and emits two gamma-rays which move away from each other, back to back. A sphere of detectors around the patient detects the two gammas, making it possible to reconstruct exactly where they originated. The technique can pinpoint tumors in the brain with precision far better than any X-ray technique.

Protons have recently begun to be used for cancer treatment (proton oncology) and promise to be very effective. In radiotherapy, intense beams of X-rays produced by a linear accelerator are focused on a tumor. On reaching the tumor, the X-rays inevitably deposit some of their energy in -- and hence damage -- the surrounding healthy tissue, both when approaching and leaving the tumor site.

Protons, however, do much less harm to the tissue through which they pass, delivering the bulk of their energy when they stop in the tumor itself. By pinpointing the tumor with PET and then calculating the energy of protons that will just reach that tumor, treatment can be delivered extremely effectively.

It is clear that tabletop lasers able to put PET and proton oncology within the grasp of almost any hospital worldwide would be an enormous benefit. A meeting the researchers held last September with cancer specialists using the technique created great excitement and enthusiasm and Dr. Ledingham and his colleagues are now applying to various charities and funding bodies for money to develop a prototype laser.

"The cancer specialists were very keen that we start work directly on designing a big laser for proton oncology, but we think it would be better to develop a laser for PET and then upgrade it," says Dr. Ledingham. "Going straight for proton oncology is too big a step. We think we know how to do it, but we're certain we know how to design a laser for PET technology."

The Institute of Physics is a leading international professional body and learned society with over 30,000 members, which promotes the advancement and dissemination of a knowledge of and education in the science of physics, pure and applied.

The Institute works in collaboration with national physical societies, plays an important role in transnational societies such as the European Physical Society, and represents British and Irish physicists in international organizations.

In Great Britain and Ireland, the Institute is active in providing support for physicists in all professions and careers, encouraging physics research and its applications, providing support for physics in schools, colleges and universities, influencing government and informing public debate.

[Contact: Dr. Ken Ledingham, Dr. Alice Larkin]

22-Mar-2001