Proton therapy is a kind of external beam radiotherapy where protons are directed to a tumor site.
Indications
Proton therapy's main interest is its ability to focus on very small areas, possibly rather deep in the body, without damaging the surrounding tissues. For this reason, it is favored for treating certain kinds of tumors where conventional X-ray radiotherapy would damage surrounding radio-sensitive tissues (optical nerves, spinal cord...), or where it is important not to leave any residual radioactivity which, previously, would be surgically removed from the surrounding tissues (which is particularly important in pediatrics).
In particular, proton therapy has had considerable successes in treating choroidal malignant melanomas, a type of eye cancer for which, until recently, the only known treatment was enucleation (removal of the eye). Today, proton therapy is one of the techniques that are capable of treating this tumor without mutilation. (see article in French)
How it works
Proton therapy, like all forms of radiotherapy, works by sending ionizing, energetic particles (in this case, protons) on the target tumor. These particles damage the DNA of cells and thus ultimately cause their death. Because of their high rate of division, and their reduced ability to repair damaged DNA, cancerous cells are particularly vulnerable to this attack on their DNA.
Proton therapy has a very small lateral dispersion; the ray beam stays focused on the tumor without much dispersion. Furthermore, most of the energy of the particles gets dissipated into the tissues after a specific depth in the body, called Bragg's peak . This depth depends on the energy to which the particles were accelerated by the protontherapy apparatus, which can be adjusted within some bounds. It is therefore possible to focus the activity of the proton beam on the very depth in the tissues where the tumor is situated; tissues situated before Bragg's peak receive some reduced dose, and tissues situated after the peak receive almost none.
Proton therapy, however, needs heavy equipment. For instance, the Orsay proton therapy center, in France, uses a synchrocyclotron weighing 900 tons in total. Such equipment was formerly only available within centers studying particle physics.
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