Particle Beam

Radiation is generally known as a flow of high-energy particles (beam) with ionizing power. Light is one of the most well known types of radiation, and lights with especially short wavelengths (high energy) are called X-rays and γ-rays. Because these create ionizing power, they are used for radiation therapy.

A particle beam is a flow of high-energy particles, and this is also a type of radiation. The particles we are talking about here are the electrons and atomic nuclei that make up an atom. Flows of high-energy electrons are differentiated from other particle beams, and this is called an electron beam. A flow of high-energy nuclei is simply called a particle beam. There are as many particle beam types as there are atom types (see diagram). Hydrogen has the lightest nucleus, followed by helium, lithium, and so on. The atomic nuclei presently used for radiation therapy are hydrogen nuclei and carbon atom nuclei. There was a time in the past when helium and neon nuclei were used for therapy. Since a hydrogen nucleus is made up of a single proton, which is a constituent of an atomic nucleus, a flow of hydrogen nuclei are specifically called proton beam. Also, a flow of carbon nuclei is called a carbon-ion beam.

Particle beams with atomic nuclei that are heavier than the proton are sometimes called heavy particle beams or heavy ion beams. The reason why the proton beam is differentiated from other particle beams is due to the difference in biological effectiveness. Biological effectiveness refers to the difference in rate at which cells are killed with the same radiation dose. The biological effectiveness of proton beams is about the same as the X-ray and electron beam, but since the carbon-ion beam has 2-3 times the biological effectiveness of X-ray and electron beams, it is classified as a heavy particle beam. However, from a physical standpoint, the proton beam and carbon-ion beam are in the same category of particle beams.