Ionizing radiation: particulate radiation

Ionizing radiation can also be in the form of particulate radiation, which includes small charged or neutral particles traveling with high energy. These particles may be alpha particles, electrons (beta particles), neutrons, or protons.

Alpha particles are charged particles made up of 2 protons and 2 neutrons with zero electrons—essentially the nucleus of a helium atom. These particles are emitted from radioactive nuclei of uranium and radium. Because of their large mass and positive charge, alpha particles are highly effective in transferring energy to tissue but are also easily blocked by a piece of paper or clothing. These particles are only a concern when alpha-emitting isotopes are ingested or inhaled.

A well-recognized source of alpha radiation involves the decay of radium into radon gas. Radium is an alkaline earth metal and a decay product of uranium and is found in uranium-bearing rocks or ores. Radium decays into radon gas, which can accumulate in poorly ventilated areas such as basements. Inhalation of radon on dust particles can lead to substantial doses of alpha radiation to the bronchi or lungs. The US Environmental Protection Agency attributes 10,000-20,000 cases of lung cancer per year to radon exposure.

Beta particles are another type of particulate radiation. These particles are high-energy electrons emitted from decaying isotopes such as strontium-90. These high-energy electrons are also easily produced in linear accelerators and are commonly used to generate x-rays and in cancer radiotherapy. As in alpha radiation, the main hazard with beta particles lies with internal exposures. With significant skin exposure, however, beta particles have sufficient energy to cause cutaneous burns, “beta burns.”

A neutron is an electrically neutral particle found within the nucleus of an atom. Neutrons are slightly greater in mass than protons. High-energy neutrons rarely occur naturally but can be produced in a particle accelerator or in nuclear reactor as part of the fission process. Neutron exposure is most consequential in a nuclear reactor criticality accident or during nuclear weapons detonation.

A proton is a positively charged particle that is more than 1800 times the size of an electron. Protons make up a major component of cosmic radiation originating from the sun. All but a small amount of the sun's proton radiation is deflected by the earth's magnetic field.

Irradiation, contamination, and incorporation

Irradiation occurs when a person is exposed to ionizing radiation. For example, patients who receive x-rays or CT scans become irradiated. Once the radiographic machinery is turned off, radiation is no longer produced. Because these individuals are only in the path of radiation energy as opposed to carrying a radioactive source on their bodies, they pose no radiation exposure risk to others.

Contamination refers to the presence of radioactive material where it is undesirable. If a person's skin or clothing is contaminated with radioactive material, irradiation continues to occur until the radioactive material is removed. Under some conditions of high contamination, these individuals may pose an exposure threat to others. Unless the contaminated patients are severely ill from the exposure, it is unlikely that they pose a significant risk to other patients or healthcare workers.

Incorporation of radioactive material occurs with cellular uptake of radioactive material via inhalation, via ingestion, or through open wounds. These radioactive atoms participate in the same physiologic pathways as nonradioactive atoms. Because removing radioactive material from the body is very difficult once incorporation occurs, the best treatment philosophy is to minimize exposure and decontaminate to prevent incorporation. As with contaminated individuals, only those who are severely ill truly pose a risk to others.

Ionizing radiation causes injury to cells via breakage of DNA strands (direct action) or from the production of hydroxyl or peroxide radicals that cause oxidative damage to DNA (indirect action). Both mechanisms ultimately lead to DNA strand breaks. Single-strand breaks can be mended rather easily utilizing the intact template on the remaining strand. However, double-strand DNA breaks have no intact template and repair is much more difficult. If the cell is unable to repair the damaged chromosome, it loses reproductive integrity and ultimately undergoes mitotic death.

Apoptosis, or programmed cell death, can also occur after exposure to ionizing radiation. Some human cells are particularly sensitive to low levels of radiation. Once exposed to radiation, these cells exhibit activation of a signaling cascade that leads to DNA fragmentation and rapid cell death. Human cells that undergo radiation-induced apoptosis include lymphocytes and acinar cells of the salivary glands.

Date: 2015-01-12; view: 865