Particle therapy is a form of
external beam radiotherapy using beams of energetic
protons,
neutrons, or positive
ions for cancer treatment. The most common type of particle therapy as of 2012 is
proton therapy. Although a
photon, used in
x-ray or
gamma ray therapy, can also be considered a particle, photon therapy is not considered here. Additionally,
electron therapy is generally put into its own category. Because of this, particle therapy is sometimes referred to, more correctly, as
hadron therapy (that is, therapy with particles that are made of
quarks).