Both kinds of radiation therapy have the same goal: to damage the genetic code of cancer cells so that they cannot grow and spread.
The fundamental difference between proton therapy and traditional radiation therapy is the type of energy they use. Traditional radiation therapy uses photons (particles of visible light, also known as X-rays) to treat tumors. Doctors have been using traditional radiation therapy to treat tumors for more than 100 years. It delivers large doses of radiation to tumors, but also to healthy tissues or organs in its path.
Proton therapy uses protons (subatomic particles found in the nucleus of all atoms). It is less common, but it has been used for decades at a small number of institutions. Proton therapy can deliver a high dose of radiation to the tumor but a much lower dose to healthy tissues. As a result, patients typically experience fewer short- and long-term side effects.