For proton therapy, hydrogen electrons are drawn off, leaving only the positively charged protons. Called ions, these adulterated atoms are typically shown as H+. In the table of elements, you can see both the atomic number (how many protons/electrons) and the atomic weight (which includes neutrons, which have no charge). When you change the number of neutrons, the atomic number stays the same, and the atom is called an isotope. Look at uranium, with an atomic number of 92 and a weight of 238. If you read the section about proton therapy history (see HISTORY) you will recall that they sought to isolate the U-235 isotope for making an atomic bomb.

Note, only the first 94 elements exist in nature. The rest are artificial. Note also that the bottom line of the chart indicates the elements are radioactive. This happens because of imbalances in the nucleus, which lead to the emission of ionizing particles or rays (gamma, etc.). These are what a Geiger counter picks up when checking to see if something is radioactive.

Hydrogen ions (H+) have a positive charge. If you have ever held two magnets together, you may have noticed that the like poles repel each other. H+ ions would do the same thing, except that strong magnets force them into an organized into a beam.

First, however, the protons circle around and around in a cyclotron until they reach two-thirds the speed of light. How fast is that? They could circle the earth five times in one second. ONE second!

It is most practical to understand proton therapy as it compares to x-rays, so let’s talk about them for a moment. X-rays are high-energy photons (light waves). Gamma rays are also weightless packets of photons, but usually of higher energy. X-rays and gamma rays have the same basic properties but come from different parts of the atom. X-rays are emitted from processes outside the nucleus (see illustration) whereas gamma rays originate inside the nucleus.

In this chart, the downward slope represents x-rays, in which the dose starts at its strongest point and then diminishes. Protons start at a lower dose, reach a sudden peak and then stop. To cover the entire target, there are a series of overlapping peaks, known as the spread out Bragg’s Peak (SOBP). The cancer’s DNA is corrupted, stopping it from reproducing. Protons also physically damage the cancer’s DNA, breaking the helical strands. I portrayed this on the cover of one of my books (see BOOKS).

This is all very straightforward, yet remains controversial, mostly because to resistance by traditional x-ray radiologists. This resistance has led to many roadblocks for proton therapy, which is covered in the essay on issues. (See ISSUES.) 

A proton and an x-ray stop at a bar on the way home from the cyclotron. After they’ve had a couple of drinks, the bartender comes around and asks if they want another round. In slurred speech the x-ray says, “Sure, let’s keep going.” The proton demurs, saying, “No thanks. I know when to stop.”

The next day an atom and a proton stop at the same bar. More drinking ensues, causing the atom to fall off of its bar stool. “Are you okay?” asked the proton. “No,” said the atom, “I think I lost an electron.” “Oh,” said the proton, “that would be terrible. Are you sure?” The atom replied, “I’m positive.” (Get it?)

Consider a target shaped like a “U” wrapped around a vital organ. With protons, you can treat half of the “U” from one side (then stop) and half from the other side (then stop) without radiating the inside of the “U.” With x-rays, the beam goes right across the “U” and radiates the vital organ in the middle. Because they stop, protons have access to difficult areas where x-rays would be too damaging. For more details, see the essay describing which cancers can be treated with proton therapy (see CANCERS).

Now that we know what protons can do, let’s look more deeply into the technology itself. Essentially, proton therapy requires a source of protons, an accelerator to ramp up the energy, a selection system to adjust the energy level, a magnet-controlled vacuum line to transport the protons, a delivery system, and a positioning system, and a patient.

An electric charge zaps hydrogen gas to ionize it. Magnets draw off the positively charged protons and inject them into a cyclotron or synchrotron to accelerate them. Physicists measure proton energy in electron volts, eV, using MeV to represent a million electron volts. Cyclotrons can energize protons as much as 250 MeV or as low as 70 MeV.

There are various devices to adjust the energy level of the proton beam, which determines how deep it can go into the body. Sometimes this is done at the beginning of the beam line (using a degrader), and sometimes at the nozzle end (range shifter) or both. Once at the proper energy level, the protons travel down a vacuum beam line (to keep them from contamination from ambient air) constantly being shaped and controlled by magnets. The quality and consistency of the beam are of the utmost importance. Twenty-four hours a day engineers monitor, test and calibrate the beam.

To revolve the proton beam 360 degrees around the treatment table looks simple enough. The patient sees a moveable nozzle that rotates with the push of a button. However, behind the wall looms a huge gantry, three stories tall, weighing anywhere from fifty to one hundred tons, all of which turns along with the nozzle. Some smaller systems use a different technology that only rotates 180 degrees and calls for a smaller gantry. In other cases, there is no gantry, just a fixed beam, with a means for moving the patient into the right position.

At this point, the protons reach the cancer target. There are two basic ways to do this with protons, an older technology called double scanning and a more recent development called pencil beam scanning (PBS). The illustrations below show how they work.