TRIUMF: Cancer Therapy with Pions

Each year more that 500 Canadians develop glioblastoma brain tumours, and 35,000 Canadians receive radiation for all types of cancer. In situations where the cancer has not spread beyond the original tumour site, localized radiation treatment may be more effective that other treatments in achieving tumour control and possible cure, if higher radiation doses can be delivered without excessive damage to normal tissues. This is the challenge for pion therapy.

Pion therapy

Pion radiotherapy is a novel form of cancer treatment that has been extensively investigated for tumours of the brain and pelvic area. A drawback of conventional radiation therapy (with photons) is the unwanted radiation which it delivers to the healthy tissue surrounding the tumour as it penetrates to where the cancer cells are located. In contrast, pion therapy concentrates the cell-killing power of the radiation more selectively in the tumour, while reducing the effects on nearby normal tissue.

What is a pion?

Pions belong to a group of short-lived subatomic particles called mesons. They are the lightest of the mesons, having about one seventh the mass of protons or neutrons. Some are electrically neutral, while others carry a single positive or negative charge. (Only the last kind is used in pion therapy.) Pions are not normally found in the free state in nature: they exist inside the nuclei of atoms, where they constitute the "glue" that holds the neutrons and protons together. But in some types of reactions, e.g. when a nucleus is struck by a proton having a certain energy, pions are ejected from the nucleus. TRIUMF uses this method to generate vast numbers of pions in its meson hall. Beams of charged pions can be guided, bent or focused by magnetic fields, just as light beams are controlled by prisms or lenses.

Protons to Pions

Producing pions

The pions employed for this unique form of cancer therapy are produced using the TRIUMF cyclotron. This cyclotron, the world's largest, accelerates hydrogen ions (which are composed of one proton and two electrons each) to 75% of the speed of light. The ions are then passed through a thin piece of metal foil which strips off the electrons, leaving a beam of protons. These protons travel away from the cyclotron at 225,000 kilometres per second inside a metal pipe. Next the protons collide with a target of carbon or beryllium, and pions (or "pi-mesons") are knocked out of the target's atoms. Although they exist for only 26 billionths of a second, the pions travel extremely fast. There is enough time for them to be channelled up a second pipe (called the biomedical beam line) to reach the cancer treatment area.

The "depth-charge" effect

A pion's average lifetime is approximately 26 billionths of a second. While it is moving, the pion does little damage to the material through which it passes, but at the last instant of its "life", a pion can destroy living cells.

Pion Stars Medical physicists can make precise adjustments so that the pions enter the patient at a speed which allows them to penetrate down into the tumour - but no further. By the time a pion reaches the tumour, it has slowed down so much that it can be drawn into the nucleus of an atom within a cancer cell. The capture of this foreign object makes the nucleus unstable, and it breaks up violently into smaller fragments which fly apart, producing what is called a "pion star". Since the fragments will damage surrounding cells within a short distance, more than just the unstable nucleus is destroyed.

The pion's action can be likened to a depth charge. It sinks through matter (healthy tissue) until, at the end of its "life", it comes in contact with the target (cancer cells) and produces a tiny "atomic explosion" within the cancer. In this way, pions can be used to destroy cancer cells without causing much damage to healthy tissue surrounding the tumour. In addition experiments have shown that the pion star radiation is, dose-for-dose, more effective against certain slow growing and hypoxic (starved of oxygen) cancer cells than conventional radiation. The effective cancer-destroying power of pion radiation, therefore, is higher than the same dose of photon radiation.

Target

Focusing the pion beam

There is an exact point in space where pions will have slowed down enough to be absorbed by a nucleus within a cancer cell. The location of this point must be not only known, but precisely controllable. A system was designed and built for collecting pions and concentrating them into a beam. It acts something like a telescope. As a telescope uses lenses to gather light from a large area and focus it into a single spot, the beam transport system uses giant electromagnets to select pions in a specific range of speeds as they exit in all directions from the pion-producing target, and to focus them into a narrow, circular beam. This complex focusing system consists of nine large electromagnets, weighing up to 5000 pounds each, and surrounding a 25-foot-long beam line pipe that extends from the pion production target to the treatment room in the biomedical facility. The magnets are remotely controlled and monitored by a computer to ensure that all pions reach the same irradiation focus. A high vacuum is maintained in the beam pipe to ensure that the pions do not scatter out of the beam line.

PET Scan At the point where the pions exit the beam line pipe, a special device is used to adjust the pions' depth of penetration into the tumour. It consists of a series of plastic slabs of different thicknesses, arranged in a circle. The pions are slowed down in proportion to the total thickness of the slabs inserted in their path at a given moment.

Thus, a combination of theoretical calculation and mechanics allows the medical physicists and doctors to control accurately the point where the pions will stop and react with surrounding matter. All that remains is to position the patient so the tumour sits within this pion-absorbing zone.

A computerized treatment couch

TRIUMF designed and built the computerized treatment couch which is able to move along the x, y and z axes (left/right, up/down, forward/back). The movement is controlled by a computer which can position it precisely in front of the pion beam. The patient lies on the couch and the computer controls the couch's movement so that the tightly focused beam of pions will sequentially irradiate throughout the tumour.

In order to keep the patient in the same spot on the couch, a mold is made of the affected area - hip or head. From this, a close-fitting, rigid plastic "mask" is formed. During treatment the mask surrounds the corresponding part of the patient and is fastened to the couch, firmly holding the patient stationary. The cancerous tumour is now at a known point above the couch, which can then be moved across the pion beam so that the central part of the tumour will receive radiation. The computer can easily direct the pion beam to within half a millimetre of any spot within the body. Prior to the treatment of a brain tumour, CT scans - special X-ray images of the head - are obtained, to assist in planning the treatment. The CT scan images define the exact size, shape and location of the tumour. This information is used to program the computer that operates the treatment couch. (Similar procedures are used to plan the pion treatment of pelvic cancer.) Data from the CT scans may also be used in producing the plastic slabs that are placed between the pion beam and patient, to control the penetration of the pion beam.

Patient Using Plastic Mask

How does it feel?

The movement of pions through the body and their absorption into nuclei happen on an atomic scale. That is, the processes occur on such a small scale that they are neither seen nor felt. Just as one cannot feel an X-ray, one cannot feel a pion beam. Like all forms of radiation therapy there are side effects on the surrounding tissues that limit the dose of radiation that can be administered.
Pion therapy is not currently being used for patient treatments, pending the final results of randomized studies performed in the early 1990's.

More information on radiation therapy and cancer treatments can be obtained from the
British Columbia Cancer Agency, 600 West 10th Ave, Vancouver.
(604) 877 6000. Toll free (BC only ) 1-800 663 3333.

The National Cancer Institute (US) has an excellent web site with detailed cancer information for both patients and health care providers.

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This page maintained by the Scientific Services Group. Last changes: Jan 02, 1997.