Types and Modalities of Radiation Therapy

Particles:

First, there are four major types of particles that are currently used in radiation treatments. Going back to Biology 101, you will remember that the nucleus of a cell is comprised of (in order of size) neutrons, protons,and electrons. There is also a type of particle called a photon, which is essentially a particle of energy (electromagnetic, light, etc.), and is much smaller than even an electron. These types of radiation work by ionizing the cancer cells they come in contact with by causing them to lose electrons, and in a roundabout way, destroying their DNA so they cannot replicate. Photon and proton radiation are the two most widely used, and the two I will discuss most in depth.

The most common particle used in current radiation therapy is the photon. Photons are used for everything from standard X-Rays to high-dose radiosurgery, and are the smallest of the three types of particles, thereby the most difficult to control and the least precise. There will be a radiation entry and exit point for photon radiation, as it travels well beyond the physical tumor.

Proton radiation uses the larger proton to make up the radiation beam, and uses a proton accelerator to speed the proton up to a precisely calculated speed prior to entering the body. Protons are only detrimental to cells they come in contact with at certain speeds. The radiologist calculates the exact speed at which the proton needs to enter the brain in order to not slow down enough to cause damage until it reaches the focal point (the tumor), and ensures that it stops before leaving the focal point. For this reason, there is no exit point for proton radiation.

Neutron radiation exists, but is not overly relevant to brain tumor patients as it is not widely available or widely used. It is extremely damaging to cellular structure, essentially beating the cells to death due to a neutron's large size.

Electrons are used in very few institutions, and are very small particles that are able to be delivered more accurately than photon radiation. I have heard electron radiation referred to as "poor man's proton radiation".  Due to the small size of electrons, they are thought to cause less trauma to healthy cells than some other types of particle beams, but research concerning long term effects is limited.

Now let's get into the ways they can be used. Photon and Proton radiation are typically delivered via External beam radiotherapy, which simply means the radiation source comes from outside of the skull. Photon and Proton radiation can both be delivered locally (to the tumor or tumor bed only), to the whole brain, or to the entire craniospinal axis. Typically, whole brain and craniospinal radiation are only performed on children over the age of three, and local radiation is usually only performed on children over the age of one. This is because 80% of the brain's development occurs by the age of two, including the myelination of nerve cell axons that speeds up processing in the brain (a newborn's brain processes information sixteen times less efficiently than an adult). Craniospinal radiation offers a much better chance of surviving AT/RT, as it irradiates the entire brain and spine, in hopes of preventing cancer cells throughout the entire CNS from replicating, rather than only those in the primary tumor bed. There are some additional risks with crainospinal radiation that certainly should not be ignored, and I will try to post more information on the after-effects of radiation later in this thread.

Photons

External photon beam radiation is most often delivered in the form of X-Ray radiation therapy, known as conventional or 2-Dimensional (2DXRT) radiation, which is a single strength beam of photon radiation. The course of treatment can broken up into multiple sessions, known as fractionalized therapy, or given all in one session, known as stereotactic radiosurgery (i.e. Gamma Knife).

External photon beam radiation can also be delivered in varying strengths attempting to conform to the 3D nature of the tumor, which is known as Intensity Modulated Radiotherapy (IMRT) or 3-Dimensional Conformal Radiotherapy (3DCRT). IMRT and 3DCRT, in theory, are similar to the conformal nature of proton therapy, but with less accuracy.

Protons

Proton radiation is almost always delivered via fractionalized therapy, and proton therapy is nearly always conformal radiation, meaning that it is 3-Dimensional in nature. Protons are accelerated in such a way that they slow to the proper speed for treatment either at the front of the tumor or at the back of the tumor, canvassing the entire tumor, but sparing the healthy tissue in front of and behind the tumor.

External Beam Proton Radiation versus External Beam Photon Radiation

Here is a very good article that describes the different types of conformal external beam radiation, including 3DCRT, IMRT, and Stereotactic Radiosurgery delivered via photon radiation, and conformal proton radiation. This article is an excellent read, and I have included some of the images from the article below to show the difference in affected radiation field between photon and proton radiation.

http://theoncologist.alphamedpress.o...t/full/9/4/442

Here, the red rectangle represents the tumor location, the blue shaded area shows the area that receives the proton dose (though a portion of that area receives no damage due to the speed of the protons as they pass through the area), and everything under the black line is the area that receives the photon dose. As you can see, much more healthy tissue gets irradiated with the photon dose.

In this next picture, the small blue circles on either side of the head represent the cochleas of the ear. The photon radiation dose hits the cochleas, which can cause hearing loss. The proton therapy does not reach the cochlea for this brainstem tumor.

In this last picture, you can see the penetration of each type of particle during spinal radiation. The proton therapy is contained almost solely to the spinal region, whereas the photon radiation passes through the organs, increasing the risk of damage to the organs, and/or future secondary malignancies due to the radiation treatment.