Protons 101 -- the Bragg Peak

I have learned alot about how this treatment works. In a series of posts, I will try to explain both the theory of proton treatment and how it is applied at Loma Linda for prostate cancer.




We start at the particle level. Atoms. Made up of proton(s), neutrons, and electrons. Protons are the positively charged ones and electrons the negatively charged ones. We used to think that these were the most elementary part of nature, i.e. these were the building blocks for all matter. Now, however, it is believed that quarks,leptons, and gauge bosons are the fundamental particles, i.e. elementary particles not composed of even smaller things.

Traditional radiation relies on photons. These are also elementary particles and photons are used in gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. Photons have no mass -- an important point in comparing them to protons, which have a heavy mass (remember, they are the part of the atom that has the +sign).



Photon rays "release their energy" once they hit the body and continue to the target (breast tumor, brain tumor, prostate tumor). And, after they hit the target, they continue to release their energy on through the body as they exit the other side of the target. Most of the energy is released when the photon first comes in contact with the skin -- before it may reach a target deeper in the body. That is why people get "radiation burn" on the exterior of their body -- like a bad sunburn. That's why people receiving photons get it from a variety of angles on their body -- so that it won't do too much damage on the exterior of the body but enough will get to the target to do its work.

























This is a graph of the normal radiation beam (photon beam) as it releases its energy (does its work). You can see that the great portion is released just when the beam hits the first part of the body -- within 1-5 cm of its hitting the body, that's 1/2 to 2 inches. Good for surface tumors but not as efficient for those deep in the body. And, even for the surface tumors, notice that the beam still has about 25% of its energy at 30 cm -- that's 12 inches. So, much of its energy is being released between 1/2 to 12 inches. Tumors not that deep (I image few are) are going to get a lot of radiation around the tumors in areas that don't need it.

Proton beams have a different characteristic. Here is a graph of the proton beam releasing its energy and where most of that energy gets released. This is called the Bragg Peak after William Bragg, an English physicist who discovered it and won the Nobel Prize for physics in 1915 and specialized in x-ray technology even back then. There are 3 things to notice:

1. The very steep ascent of the proton beam's release of its energy and the very narrow range where it releases most of its energy.
2. The depth at which the proton beam releases its energy -- much deeper in the body than photons.
3. The extremely steep descent of the proton beam after releasing its energy (which means that there is no damage beyond the point at which it releases its energy.

Think of the proton beam as one of the "bunker busting bombs" used in Iraq (that did not explode when they hit the ground but are designed to explode -- release their energy -- at a certain depth in the ground in order to destroy underground bunkers. The proton also goes deep into the body before "exploding" or releasing its energy.

What does this all mean? If, and that is a big IF, you can control where the proton releases its energy, and can shape it to the area you want to release it, there is a very effective tool for treating diseases deep in the body without as much collateral damage when the beam enters the body and with virtually no damage as it exits the body.

From the graph above, however, the proton beam is releasing most of its energy at about 24 cm. What if the tumor is located at some other distance? And of course it will be. If a bunker busting bomb explodes 300 feet underground but the bunker is located at 200 feet or 500 feet, the target will be missed (imagine a bomb that does damage only in a very narrow area, say 5', like protons). How do you adjust the depth at which the proton does its work? And, if you want that bomb to destroy something that is 100' long, how do you increase its 5' working depth to 100' ?

Here's another problem. At Loma Linda, the proton beam enters the treatment room at only about 1 mm in diameter. If your target is the prostate, how do you enlarge that beam to the size of the prostate? The prostate is about the size of a walnut, far more than the 1 mm beam. The goal is to zap the entire prostate so that beam needs to be enlarged.

And, here's the last problem I will address. The prostate is not square. It is irregularly shaped. Imagine a series of tunnels and rooms for that bunker busting bomb. They are irregular and they are at different depths. You don't want to destroy anything other than the rooms and tunnels because nearby are kept the British Crown Jewels -- a priceless heirloom. But, if you don't get all the rooms and tunnels, 100%, then those Jewels will ultimately be destroyed. So, precision is key.

I will address each of these issues in later posts with such names as Proton 102, Proton 103, etc.

And, if any of you reading this have better methods of describing these things, feel free to do that as well as to make corrections.