The absorption of energy from radiation in tissue often leads to ionization. Ionization involves actual ejection of one or more electrons from the atom.  Ionizing radiation is <b>electromagnetic</b> (photon, includes gamma rays and Xrays) or particulate radiation (like electrons or protons). X-rays are produced artificially and mechanically while gamma rays are produced by nuclear disintegration [decay of radioactive isotopes.]  [Harrison's Principles of Internal Medicine, 14th Edition, page 2559.]

<b>Xrays and gamma rays</b> can be thought of as beams of <b>photons</b> [packets of energy] traveling in straight lines. The photons have no weight or charge, and the amount of energy in each determines whether the radiation is ionizing or non-ionizing. Both Xray and gamma ray beams lose their strength [attenuate] continuously and steadily as they pass through tissue [from reacting with the tissue.]  Attenuation of the rays indicates use of the energy to create free radicals and other damage in the tissue.  

<b>Radiation particles</b> include electrons [beta particles], protons, neutrons, and helium nuclei [alpha particles].
 
<b>Electrons</b> are small, negatively charged, and can be accelerated. They penetrate tissue to a limited depth, and can be used to treat problems near to the surface. 

<b>Protons</b> are positively charged, and are about 2000 times heavier than electrons, and can be accelerated to increase their energy. Protons tend to stop abruptly when traversing tissue. In their sudden stops, most of their energy is abruptly given up, so there is a compact, enhanced region of ionization. This is called a Bragg Peak.
 
<b>Neutrons</b> are the same weight as protons, but are not charged. 

<b>Helium nuclei</b> consist of two protons and two neutrons. Their mass and charge are so heavy that unless accelerated to very high energies, they do not penetrate very far into tissue.

<b>Other RadioIsotopes</b> are also used to deliver ionizing radiation. Radioisotopes are unstable atoms, whose nuclei decompose, releasing gamma rays and/or electrons, protons, neutrons, or helium nuclei. What is released depends upon the particular radioisotope.  Radioisotopes can be placed in suitable containers and left in place either temporarily or permanently, or administered by mouth or intravenously to reach tissues that will preferentially take them up or injected straight into tumors, or placed in beads or resins and injected into liver arteries that feed tumors.

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