Radiation Injuries Health Article

Definition

Radiation injuries is damage caused by ionizing radiation emitted by the sun, x-ray machines, and radioactive elements.

Description

Radio and television signals, radar, heat, infrared, ultraviolet, sunlight, starlight, cosmic rays, gamma rays, and x rays all belong to the electromagnetic spectrum and differ only in their relative energy, frequency, and wavelength. These waves all travel at the speed of light, and unlike sound they can all travel through empty space. The frequencies above visible light have enough energy to penetrate and cause damage to living tissue, damage that can be as minor as a sunburn caused by ultraviolet light or as extreme as the incineration of Hiroshima, Japan, during World War II. Lower frequencies do not penetrate, but can cause eye and skin damage, primarily due to the heat they transmit.

Atomic particles can also have enough energy to do damage. They come from radioactive isotopes as they decay to stable elements. Electrons are called beta particles when they radiate. Alpha particles are the nuclei of helium atoms—two protons and two neutrons—without the surrounding electrons. Alpha particles are too large to penetrate a piece of paper unless they are greatly accelerated in electric and magnetic fields. Other subatomic particles are much less common outside of nuclear reactors and particle accelerators.

The energy of electromagnetic radiation is a direct function of its frequency. The high-energy, high-frequency waves, which can penetrate solids to various depths, cause damage by separating molecules into electrically charged pieces, a process known as ionization. Atomic particles, cosmic rays, gamma rays, x rays, and some ultraviolet are called ionizing radiation. The pieces they generate are called free radicals. They act like acid, but they last only fractions of a second before they revert to harmless forms. Adjusting the energy of therapeutic radiation can select a depth at which it will do the most damage. Ionizing radiation also does damage to chromosomes by breaking strands of DNA. DNA is so good at repairing itself that both strands of the double helix must be broken to produce genetic damage.

Because radiation is energy, it can be measured. There are a number of units used to quantify radiation energy. Some refer to effects on air, others to effects on living tissue. The roentgen, named after Wilhelm Conrad Roentgen, who discovered x rays in 1895, measures ionizing energy in air. A rad expresses the energy transferred to tissue. The rem measures tissue response. A roentgen generates about a rad of effect and produces about a rem of response. The gray and the sievert are international units equivalent to 100 rads and rems, respectively. A curie, named after French physicists who experimented with radiation, is a measure of actual radioactivity given off by a radioactive element, not a measure of its effect. The average annual human exposure to natural background radiation is roughly 3 milliSieverts (mSv).

It is reasonable to presume that any amount of ionizing radiation will produce some damage. However, there

is radiation everywhere, from the sun (cosmic rays) and from traces of radioactive elements in the air (radon) and the ground (uranium, radium, carbon-14, potassium-40 and many others). Earth's atmosphere protects us from most of the sun's radiation. Living at 5,000 feet altitude in Denver, Colorado, doubles exposure to radiation, and flight in a commercial airliner increases it 150-fold by lifting us above 80% of that atmosphere. Because no amount of radiation is perfectly safe and because radiation is ever present, arbitrary limits have been established to provide some measure of safety for those exposed to unusual amounts. Less than 1% of them reach the current annual permissible maximum of 50 mSv.

One of the most remarkable bits of information to come out of studies of Japanese people exposed to atomic bomb irradiation in 1945 is the absence of genetic damage to survivors. Forty years of studying 76,000 children has detected no increase in abnormal pregnancies or chromosomes. This evidence suggests that it takes about 1 Sv of gonad radiation to double human mutations caused by background radiation, and that background radiation causes less than 1% of these mutations. Other organisms are much more susceptible than humans to mutations from radiation.

Ionizing radiation has many uses in medicine, both in diagnosis and in treatment. X rays and CT scanners use it to form images of the body's insides. Nuclear medicine uses radioactive isotopes to diagnose and to treat medical conditions. In the body, radioactive elements localize to specific tissues and give off tiny amounts of radiation. Detecting that radiation provides information on both anatomy and function. Radioactive chemicals are also used to treat certain conditions, most commonly an overactive thyroid. Because the thyroid is the only gland in the body to utilize iodine, all iodine in the body is concentrated there. A radioactive isotope of iodine (I-131) will gradually destroy overactive thyroid tissue, curing the disease. The dosage must be carefully measured and even then sometimes does too good a job. Since it is easier to replace inadequate thyroid than to deal with too much, this treatment is very acceptable.

Early workers with x rays frequently died from its long term effects, notably leukemia. Wrist watches used to glow in the dark due to radium that was painted on the dial and watch hands. This work was done by workers who moistened their brushes with their tongues. Many of them developed cancer of the tongue. After lessons like these, most sources of man-made radioactivity have been eliminated from the environment. Watches no longer glow in the dark from radium. Shoe salesmen no longer use fluoroscopes to check shoes for a proper fit. Today, doses used for medical examinations are ordinarily too small to be of concern. Methods of magnification, lead shielding, and a greater awareness of the risks have all but eliminated the danger from diagnostic radiation. It adds on average only 0.6 Sv a year, or 20% of the background radiation. Nevertheless, people who work around x rays monitor their exposure, because there is no such thing as a completely safe dose.

It is therapeutic, accidental, and deliberate radiation that does the obvious damage. There has not been much in the way of deliberate radiation damage since Nagasaki, but accidental radiation exposure happens periodically. Between 1945 and 1987, there were 285 nuclear reactor accidents, injuring over 1,550 people and killing 64. The most striking example, and the only one to endanger the public, was the meltdown of the graphite core nuclear reactor at Chernobyl in 1986, which spread a cloud of radioactive particles across the entire continent of Europe. Information about radiation effects is still being gathered from that disaster. There have also been a few accidents with medical and industrial radioactivity.

Nevertheless, it is believed that radiation is responsible for less than 1% of all human disease and for about 3% of all cancers. This figure does not include lung cancer from environmental radon, because that information is unknown. The figure could be significant, but it is greatly confounded by the similar effects of tobacco.

Because cancers are usually faster growing than their host tissues, they can be selectively killed by carefully measured radiation. This is most true of the lymphomas. Other cancers are less radiosensitive. Whenever radiation is used to treat cancer, care must be taken to measure the dose carefully and aim it as accurately as possibly. Even so, many cancers differ so little from the surrounding tissue in their sensitivity that undesirable damage is unavoidable. Skin will become thin making the blood vessels very visible. Bowels will become irritated and cause vomiting and diarrhea. Other organs may scar and decrease their function. Bone marrow is always at risk of damage. Fortunately, the bone marrow is so widely spread throughout the body that localized treatment damages only a part of it. A typical therapeutic dose of radiation to a localized area is about 2 Gy (grays) per day, repeated at intervals to a total dose that varies with the type of cancer being treated.

Newer techniques of directing radiation are providing greater safety for equivalent tumor doses of radiation. One method uses several different beams of radiation so that only the point of their convergence receives the full dose. A gamma knife is a new surgical tool that focuses radiation with extreme accuracy in three dimensions, sparing closely surrounding tissue from the radiation effects. It focuses 201 Cobalt-60 or linear accelerator sources without need for a surgical incision.