In the last post in this series, I talked about how we can shape the beam of radiation in order to conform it to the shape of the tumor. If all we had to do is to shape the beam, point and shoot, this job would be easy. Unfortunately, it’s a lot more complicated than that. Getting the beam to the patient is, in some ways, the easy part.
When a beam of radiation interacts with a material, it undergoes attenuation. This means that the intensity of the beam decreases as it goes through the material. Therapeutic beams of radiation are made up of particles such as photons or electrons. A measure of the dose delivering ability of the beam is the number of these particles that pass through a certain area per second. This is called the fluence. The higher the fluence, the more dose the beam can deliver. As the beam enters a material, some of these particles will interact with the atoms in the material and give up their energy. These particles are removed from the beam, and the beam’s fluence decreases or attenuates. As the energy of the particles gets higher, they will usually travels farther than lower energy photons. Therefore higher energy beams will be less attenuated. If the material is uniform, the attenuation of the beam often decreases exponentially.
While the number of particles in the beam decreases at it penetrates farther, we are more concerned about how the dose to the patient decreases. Since more particles will deliver a higher dose, they are related, but the relationship is more complicated than it appears. For the most part, DNA is not damaged by the beam particles directly, but by the particles with which they interact. For example, a photon beam with an energy in the range of about 1 to 20 megavolts is most likely to interact with an atom by a process called Compton scattering. This process transfers energy from a photon to an electron which can then interact with cells in the body and kill them. These electrons are mostly scattered forward (along the beam). When a photon beam enters a patient, the dose is not delivered right at the skin, but further in the body where the Compton scattered electrons, either directly or by creating free radicals, damage the DNA. This results in what is called skin sparing, where a high energy beam of radiation delivers less dose to the skin than it does to points deeper within the body. Note that this is a greatly simplified version of the true physics of the situation (even more so than usual), but gives the general flavor of what is going on.
So what does the dose delivered to a patient actually look like? We can get an idea by measuring the dose delivered to a tank of water known as a phantom. Dose measuring instruments known as ion chambers move within the tank and show us the dose distribution. (Obviously a patient is not a tank of water, but we can correct for that in our calculations.) 
This figure shows a curve of dose versus depth within the phantom for a 6 megavolt and an 18 megavolt beam. Note that the point of maximum dose (called dmax) is deeper for the 18 megavolt beam. This makes sense as the higher energy photons will give more energy to the scattered electrons and drive them deeper into the water. Also note that the higher energy beam delivers a higher dose as a percentage of the maximum dose than the lower energy beam. These curves are called depth-dose curves and are the basis for radiation treatment planning.
In the next post I will show how these depth dose curves are used to plan treatments, and why we need full-time dosimetrists to do it.
