Indiana University       Research & Creative Activity       September 2000 • Volume XXIII Number 2

From Photon to Proton: Radiation as Remedy

by William Orem

To understand the Midwest Proton Radiation Institute at the IU Cyclotron Facility in Bloomington, you need to know a little about what radiation is, or more to the point, what it is not.

It is not what the atomic scare movies of the 1950s taught us: a universal evil turning insects, plants, and people into monsters. Quite the opposite. At the interface between modern physics and modern medicine, the bugbear of the Cold War generation has become at least partly a beacon of possibility for their children.

The term “radiation” does not designate a single thing, but a class of things. Sunlight is a type of radiation, as are the invisible waves picked up by your radio antenna and turned into an audio signal. Neither is radiation uncontrollable or unnatural. Radiation of various sorts is safely utilized by modern technology, such as the kind that cooks your dinner inside the microwave oven. A good quantity of the earth itself is radioactive, and another form of radiation from space—cosmic rays—is thought to play a critical part in the evolution of new species on our planet. In this sense, without radiation we would not exist at all.

But certain types of radiation most certainly are dangerous, especially because of their capacity to cause cancer. Sunlight is a good example, a fact to which anyone who has had a melanoma can attest.

John M. Cameron is professor of physics and director of the Indiana University Cyclotron Facility, a national research center in Bloomington, Ind. Photo Tyagan Miller.

As a therapeutic tool, then, radiation has been a classic double-edged sword: tantalizingly powerful, yet difficult to wield in the presence of the all-too-fragile human body. Which is exactly why John Cameron and an IU team at the MPRI are working to increase the amount of control doctors have.

“There is a good marriage between medicine and physics here at IU,” says Cameron, director of the IUCF. And that marriage is making possible a new technique whereby radiation is not enemy but ally, not the cause of cancer, but a novel part of the cure.

The new technique going on at the IUCF in Bloomington is called proton therapy. In a fundamental sense, though, it isn’t really new. Radiation in the form of high-energy electromagnetic waves—what we call light when it is in the lower frequency range to which our eyes react—has been used as a tool against cancer for several decades. What makes proton therapy different is its use of protons rather than photons, a distinction that may escape a nonspecialist but could make all the difference.

Despite the similar-sounding names, protons and photons are separate entities in the subatomic world. A photon is a particle of light, sometimes thought of as a tiny packet of energy. It moves, not surprisingly, at the speed of light and can be of differing frequencies, depending on how much energy is contained in the packet itself. Look around you for something red and then something blue. The photons hitting your eye in the latter instance are higher energy than in the former.

A proton, on the other hand, is one of the two major components of an atomic nucleus (neutrons are the other). Unlike photons, protons are massive and have a positive charge. They don’t naturally move at light speed, but they can be sped up close to the light-speed limit. The closer they come, the greater their energy. Such coaxing is done in an accelerator.

A 77-ton magnet was used to control the original cyclotron at Indiana University. The cyclotron, completed in 1941, was housed in IU Bloomington's Swain Hall. Photo IU Archives.

A cyclotron, such as the one at IUB, is a few gigantic and powerful magnets that permit ions (charged particles such as the proton) to be accelerated in an ever-widening spiral until they fly out of the cyclotron itself. The protons start in the form of a bottle of hydrogen gas and are moving at what Cameron calls “thermal velocity”: a paltry few meters per second. When they exit the machine, their velocity has been increased to 60 percent of the speed of light. To get a sense of how much of an increase in energy is imparted in this process, consider that light moves at 186,000 miles per second—fast enough, in an unimpeded journey, to leave your hand now and have reached the moon by now.

Now think of a cancerous growth. In essence, a tumor is the result of cell division run amok. In a localized region, cancerous cells begin multiplying without restraint. The natural self-regulating system can be shut down, and cells that are already reproducing inappropriately refuse to die off. The result is a growing mass in the body, dangerous in itself, and especially so when it holds out the possibility of passing pieces of disruptive tissue through the blood or lymphatic system, allowing them to circulate and take hold elsewhere (the process known as metastasizing).

Such lumps of growth are known as cancers because they reminded the physicians of ancient Greece of the hard shells of crabs. For much of the two-and-a-half millennia since those ancient times, nothing could be done about the appearance of these mysterious killers. It was not until the late twentieth century that advances in surgical technique, sterilization, and anesthetization made physical removal of the growths a possibility. But the danger remains high, and the emotional costs potentially even higher. The search for other less radical and less invasive methods of destroying tumors led to the advent of controlled radiation.

Traditionally, doctors have used this technology to try to bombard a tumor with a concentrated beam of high-energy photons known as X-rays. As an X-ray of a broken limb shows, this kind of radiation is able to pass into and through the body, being absorbed more or less by areas of greater or lesser density. The sustained collision of these photons with cancerous tissue can disrupt and kill the cells without having to surgically remove them from the body.

X-rays pack a punch, but they do not focus well. Much of the energy they deliver misses the mark and damages healthy tissue around the tumor. Imagine a line of soldiers in a brigade. One of the soldiers is a traitor. If left to his own devices, he will destroy the entire brigade. It is possible to kill the traitor by hurling rocks at him from a distance, but to remove the traitor in this way is to risk striking and injuring good soldiers on either side of the bad.

This danger has been one of the greatest drawbacks of radiation therapy. If the tumor is near a vital organ, the “spillage” of X-rays can be unacceptable. In pediatric cases especially, the damage caused to healthy growing cells can be heartbreaking. There comes a time when the danger posed by the treatment threatens to be as great as the illness it is intended to cure.

Proton therapy, Cameron explains, eschews the use of X-rays altogether. Instead, the “stones” being thrown at cancerous cells are accelerated protons. An accelerated proton has a much more focused energy distribution pattern than an X-ray. It slows down upon entering the body, so it still gives off energy in healthy tissue that should in principle be left alone, but the slowdown is part of an overall pattern of energy loss that gives its biggest kick at the end—just before the proton is halted altogether. This peak release of energy, called the Bragg peak, is something like the detonation of tiny explosives. The energy lost in travelling toward the Bragg peak is relatively small compared to the blast delivered at the peak itself.

This facet of proton behavior makes protons excellent tumor fighters. The amount of energy in the beam of protons can be controlled, which determines the depth in the tissue where the Bragg peak will occur. When that point is aligned with the tumor itself, the proton beam delivers a series of micro-explosions mainly inside the cancerous mass, with little in the surrounding tissue areas. The increase in the damage done to the target and the reduction of damage done elsewhere make protons as much as ten times more effective, by some measures, than traditional photons. With X-rays, scientists and doctors could at best launch a volley of stones in the general direction of the traitorous soldier, sacrificing some of those around him in the attempt. With a proton delivery beam, the IU team is more like David using a single rock to bring down Goliath.

Technicians work on the IU cyclotron shortly after it was installed on the Bloomington campus in 1975. Photo IU Archives.

The cyclotron at IUB has been up and running since 1975. In 1999, however, the pure physics work was moved to a newly completed synchrotron (a variant version of particle accelerator). The cyclotron has been rededicated as a tool used for proton therapy, and much of the site itself is being reconstructed as a clinic, with three treatment rooms in the works.

The cancer scenario is instructive, but the potential applications for proton therapy extend well beyond cancer treatment, as Cameron is the first to acknowledge. In cooperation with IU’s Schools of Optometry and Medicine, trials are also being run in areas ranging from treating macular degeneration in the eye to an NIH-funded program to help a damaged heart revascularize itself.

Many facilities dedicated to proton therapy are being developed throughout the country. A team from Massachusetts General Hospital, who pioneered proton therapy in the United States using a cyclotron at Harvard University, will soon move to a new facility at MGH. The Loma Linda University Medical Center in California, operational since 1991, treats about 800 patients a year. New accelerators being built around the world will have different capacities and may specialize in different treatments. Most likely, some will specialize in novel applications that have yet even to be imagined.

“I like to see projects going on like the one in cardiology,” Cameron says with enthusiasm. “There are lots of ways to use protons.”