How proton therapy works

There are effectively three ways to treat cancer: chemotherapy, surgery, and radiation therapy. Under the umbrella of radiation therapy, there are several different subtypes. Proton therapy is one of those subtypes. Due its many benefits, it is rapidly increasing in usage by oncologists and hospitals all over the world.   

Proton radiation was first suggested as a treatment for cancer in 1946 by physicist Robert R. Wilson, Ph.D. Early efforts to treat patients began in the 1950s, but it was limited to a few areas of the body. Since then, the use of proton therapy has grown, with 28 proton centers in operation in the United States alone. Beaumont is the first operational proton therapy center in Michigan

For patients facing many types of cancer, proton therapy can offer a safer, more precise alternative to traditional radiation. As the name suggests, the particles used during this therapy are “protons”, unlike the X-ray or gamma ray particles used in traditional radiation therapy. The use of these protons is what heightens the accuracy of the treatment. 

Understanding how proton therapy works and what its benefits are over other forms of traditional radiation therapy, involves understanding some basic concepts of physics, chemistry, and what happens when radiation reaches the human body. 

A quick background on “particles”

In both proton therapy and traditional radiation therapy, highly targeted particles are used to destroy cancerous cells such as those in a malignant tumor. But what exactly is a “particle”?

In physics, “particles” can best be described as the tiniest pieces of a whole. As a basic analogy, imagine dropping and breaking a drinking glass. The individual glass shards that make up that drinking glass could be viewed as its particles. An “X-ray”, for instance, is essentially just a wave of light found on the electromagnetic spectrum. While invisible to the human eye, this wave of light is also made up of microscopic pieces called “photons”. Each individual photon is a particle that makes up the entirety of an X-ray wave. Similarly, atoms are typically made up of three types of particles: protons, electrons, and in all but one of the elements on the periodic table, neutrons. This is why you may often hear protons referred to as “subatomic particles”. 

No matter what type of particle is used during radiation therapy, whether it be X-ray, gamma ray, or proton, the goal is the same. To send particles to “irradiate” cancer cells. This simply means to expose those cells to radiation which in turn will hopefully destroy them and prevent them from spreading. 

Turning protons into treatable particles

Protons aren’t inherently destructive particles. After all, they coexist happily within other particles in atoms; the basic building blocks for any ordinary matter. So how can a particle that helps make up ordinary matter be turned into a destructive force, capable of treating cancer? 

In order for it to do so, it must first become “ionized”, which means it needs to become electrically charged. For a proton to become electrically charged, it first needs to be separated from its electron partner in an atom, and then sped up. The faster that proton is moving, the more energy it will carry. The more energy it carries, the more destructive it will eventually become when making contact with a cancerous cell. 

Proton therapy uses an “accelerator” to accomplish this. The accelerator starts by taking a hydrogen atom, breaking it apart, separating its proton from its electron, and then speeding up the isolated proton so that it carries as much energy with it as is safely possible. The highly energetic, ionized proton is then ejected directly towards the tumor. 

There are a number of reasons an accelerator uses hydrogen atoms opposed to other elements on the periodic table during the ionization process. For one, hydrogen is the most abundant element in the entire universe, so it’s very easy to come by. For another, it’s structurally much simpler than other elements. A hydrogen atom only has one proton and one electron; it’s the only element on the periodic table without a neutron. Thus, isolating and ionizing a proton is much more easily accomplished with hydrogen than with any other element on the periodic table. 

Minimizing entrance doses, eliminating exit doses

Unfortunately, due to the destructive nature of any radiation particle, and because of how particles travel through the human body, damage isn’t completely isolated to the cancerous cells. For any particles to reach a tumor, it must first travel through some healthy tissue, most notably the skin. This in turn can cause unintended side effects during treatment. This is known as an “entrance dose”, and at least for now, it is unavoidable in both proton therapy and traditional radiation therapy. However, through the use of protons, entrance dosages are considerably reduced, compared to X-ray or gamma ray radiation. 

In traditional radiation therapy, it is possible for X-ray or gamma ray particles to cause further damage after they reach their intended destinations. This is what is known as an “exit dose”. Again, this can cause unintended harm to healthy cells. This can be particularly problematic when those healthy cells are located in or near vital organs of the body, as is the case when treating head and neck, gastrointestinal, or breast cancers

However, with Proton Therapy, the particles used to destroy the diseased cells cease to be harmful once they’ve reached their intended destination. Hence, exit doses are eliminated, protecting more healthy cells in the body that exist beyond the target. 

The Bragg peak of a proton

So how is it that protons can minimize entrance doses and eliminate exit doses entirely? What makes a proton particle so different from an X-ray or gamma ray particle? It all comes down to the unique physical properties of the proton itself and its “Bragg peak”.

For one, protons are subatomic. This means that it carries some mass with it. Hence, atoms can bond together to make things; water, human tissue, and literally any other form of ordinary matter that exists anywhere in the universe. In contrast, X-ray particles are photons. Photons do not have mass. The presence or absence of mass means that there will be some inherent differences in how a particle will behave when it comes into contact with something else like tissue in the human body. 

You might be familiar with Newton’s First Law that states that an object in motion will stay in motion at the same speed unless acted upon by another force. Think of a particle as the “object” here, and the “unbalanced force” as tissue within the human body. As a particle passes through the body, it slows down, depositing energy along the way, until it eventually comes to a halt. Whatever cells are in the way as this energy is deposited will be harmed, regardless of whether or not they are cancerous. In an ideal world, doctors would be able to deposit 100% of a particle’s energy at the precise moment when it reaches a cancerous cell. Unfortunately, this just isn’t physically possible, but with protons, doctors can get a lot closer to achieving this perfection than with X-ray or gamma ray particles.  

Ionized particles do not slow down at one consistent pace as they travel through the body, and consequently, the energy they carry will not be deposited evenly. In fact, they exist on what particle physicists call “The Bragg Curve”. It’s what makes using such particles so effective in treating cancer. When the ionized particle first makes contact with the body, it doesn’t slow down instantaneously, but rather ramps up the rate at which it slows down until it reaches its “peak”. Once the particle reaches the “peak” of that curve, it deposits the most amount of its destructive energy before quickly becoming harmless. In any cancer therapy, particles are targeted so that they reach their “Bragg Peak” at the precise moment when they reach the cancerous cells. 

The unique properties of a proton particle, notably the mass that it possesses, make its curve much more spiked than that of an X-ray or a gamma ray particle. It also makes the trajectory of a proton much easier to control. The Bragg curves of X-ray and gamma ray particles are comparably gradual, which means that more energy is deposited both before and after reaching the tumor. In contrast, the bigger spike associated with a proton’s Bragg curve ensures that far less energy is deposited on its way to the tumor, more energy is deposited once it reaches the tumor itself, and any lingering energy deposited after it passes the target will be too negligible to cause additional cell damage.  

Proton therapy advances at Beaumont

Beaumont’s ProteusOne single-room treatment system includes precision technologies. Intensity Modulated Proton Therapy (IMPT) with Pencil Beam Scanning technology and 3-D Cone Beam CT can target a tumor within less than a millimeter. 

Pencil Beam Scanning refers to the delivery of protons in a thin beam. Like a pencil, the beam uses back and forth motions to target the treatment area – the shape, size and depth. This technique uses a magnetic field to steer the position of the small proton beam and uses an energy layer selection system to choose the precise depth of the tumor, just like the 3D painting technique used in X-ray therapy. 

As leaders in the advancement of radiation oncology treatment, Beaumont radiation oncologists are continually looking for ways to improve patient outcomes and experiences, including developing new methods to deliver proton therapy, such as rotational arc for lung cancer treatment.

To learn more about proton therapy, or to find out if you are eligible for this treatment option, speak to a proton nurse navigator at Beaumont. Call 248-551-8402.