Balance Beams: The Suitability of Proton Beam Therapy
by Robin Schoenthaler, MD
Cancer treatment is usually a team effort as it is often treated one, two,or three ways: surgery, drugs (hormones or chemotherapy), and radiation.
Radiation is usually given externally, using a broad beam of photons (x-rays) to hit the tumor and the normal tissue around it. The doses of “external beam radiation” try to hit “perfect pitch,” the dose needed to kill cancer cells with the minimum impact on the nearby normal cells. Different normal cells have different tolerances for radiation, and sometimes the dose to a tumor is limited by the dose that a nearby structure can take without getting damaged.
A perfect example is the spinal cord. Radiation oncologists try hard to limit the dose to the spinal cord because going to a higher dose runs the risk of injuring the spinal cord, which could result in paralysis. But what do you do if you have a tumor that lays right on top of the spinal cord and that tumor needs a higher dose than the cord can tolerate?
You need a special technique of radiation that hits the whole tumor but stops right at the tumor’s edge. Typical x-ray beams (photons) go deep enough to potentially cause tissue injury to precious organs like the spinal cord.
This is where protons come in. Protons are a very special, highly precise form of radiation that are much better at depositing their cancer-cell-killing energy right into the tumor — and then stopping right there, going no further, thus sparing the healthy tissue just underneath.
With protons, one of two things can happen — either the oncologist can give a higher dose of radiation to a tumor (possibly leading to better tumor control and cure) or avoid the side effects from radiating nearby tissue. Either way, it is a win/win.
Though, this only applies to the right kind of tumor. For people with tumors close to critical normal structures, protons are a godsend (and insurance-approved!). This includes tumors behind the eye (such as melanomas), along the spinal cord (such as chondrosarcomas), or by the base of the skull where it connects to the spine (such as chordomas). Anywhere you have a tumor that is super close to “critical structures,” that can’t take the same high dose as the tumor needs, there may be an advantage to using protons.
Likewise, anytime you have normal structures nearby that you don’t want to radiate much, protons may also provide substantial benefits. Nowhere is this truer than in children. Virtually all organs and tissues in children are actively growing and particularly susceptible to the cancer-inducing power of radiation, so you really want to minimize radiating anything you can. For instance, if you can minimize radiation to their bones, chances are higher they will have normal growth of that bone — and less chances of developing a radiation-caused bone tumor in the future.
So, why isn’t everyone being treated with protons instead of photons? Well, first and foremost, the machines that supply protons (cyclotrons) are highly sophisticated, incredibly complicated, and expensive to build. They are also expensive to run and maintain. In addition, giving radiation is always a team sport — you need physicists, dosimetrists, engineers, and technologists, along with nurses and doctors — and for protons, they all need special training and experience, all of which is costly.
Notice a theme? It’s an incredibly expensive type of treatment. It’s hard to get exact figures, but on average, it can cost almost three times as much as regular radiation. And it hasn’t been used that long or in substantial amounts of people in clinical trials, so some of its advantages haven’t been proven and are still theoretical.
What this means is that insurance companies can be reluctant to approve it across the board, mostly choosing to approve it only for limited circumstances, such as eye tumors, some spine tumors, base of skull tumors, and pediatric tumors. Some insurers now cover sinus tumors, some brain tumors, occasionally lung cancer, or cases where patients need re-irradiation.
A huge area of controversy is whether insurance should cover proton radiation to the prostate. Prostate cancer is a type of cancer that needs a high dose of radiation to get sterilized — a very high dose. And the prostate just happens to be located right between two organs that are easily damaged by radiation — the bladder and the rectum.
The radiation oncologist is caught between a rock and a hard place. We want to give the maximum radiation dose to the prostate and do our best to kill that cancer forever, but we also need to keep from damaging the vitally important bladder and rectum. We have figured out three ways to do this: brachytherapy seeds (internal radiation), IMRT (intensity modulated radiation therapy using photons), and protons. All are trying to provide a high prostate dose with much less dose reaching the rectum and bladder, resulting in, we hope, lower prostate cancer deaths and less long-term side effects.
Yet, so far, studies haven’t clearly demonstrated protons help that much — it’s still mostly theoretical. Some short-term studies seem to indicate fewer short-term side effects, but other studies have worryingly high rectal injury rates. And there aren’t any studies that show proton treatment leads to more cures. So, many insurance companies still question whether the small, early benefits we’ve seen in some trials are worth a huge increase in cost, and they won’t cover it unless the patient with prostate cancer is in a clinical trial.
So, the answer is we just don’t know yet if protons help a lot in prostate cancer, or many other cancers. Protons are like a lot of advances in medicine: step by step, we ascend the stairway of knowledge. Bit by bit, scientists are finding out the very best ways we can harness this incredible technology. Physicians are already using protons to save the lives of thousands of patients with rare tumors in eyes and spines, and hopefully in the next decades we will be curing millions more with common tumors as well.