M e s o t h e l i o m a

The energy (source of radiation) used in external radiation therapy may come from the following:
X-rays or gamma rays, which are both forms of electromagnetic radiation. Although they are produced in different ways, both use photons (packets of energy).
X-rays are created by machines called linear accelerators. Depending on the amount of energy the x-rays have, they can be used to destroy cancer cells on the surface of the body (lower energy) or deeper into tissues and organs (higher energy). Compared with other types of radiation, x-rays can deliver radiation to a relatively large area.
Gamma rays are produced when isotopes of certain elements (such as iridium and cobalt 60) release radiation energy as they break down. Each element breaks down at a specific rate and each gives off a different amount of energy, which affects how deeply it can penetrate into the body. (Gamma rays produced by the breakdown of cobalt 60 are used in the treatment called the “gamma knife,” which is discussed in Question 8).
Particle beams use fast-moving subatomic particles instead of photons. This type of radiation may be called particle beam radiation therapy or particulate radiation. Particle beams are created by linear accelerators, synchrotrons, and cyclotrons, which produce and accelerate the particles required for this type of radiation therapy. Particle beam therapy uses electrons, which are produced by an x-ray tube (this may be called electron-beam radiation); neutrons, which are produced by radioactive elements and special equipment; heavy ions (such as protons and helium); and pi-mesons (also called pions), which are small, negatively charged particles produced by an accelerator and a system of magnets. Unlike x-rays and gamma rays, some particle beams can penetrate only a short distance into tissue. Therefore, they are often used to treat cancers located on the surface of or just below the skin.
Proton beam therapy is a type of particle beam radiation therapy. Protons deposit their energy over a very small area, which is called the Bragg peak. The Bragg peak can be used to target high doses of proton beam therapy to a tumor while doing less damage to normal tissues in front of and behind the tumor. Proton beam therapy is available at only a few facilities in the United States. Its use is generally reserved for cancers that are difficult or dangerous to treat with surgery (such as a chondrosarcoma at the base of the skull), or it is combined with other types of radiation. Proton beam therapy is also being used in clinical trials for intraocular melanoma (melanoma that begins in the eye), retinoblastoma (an eye cancer that most often occurs in children under age 5), rhabdomyosarcoma (a tumor of the muscle tissue), some cancers of the head and neck, and cancers of the prostate, brain, and lung.