Frequency (Hz) Radiation Biologic Effects

1–50 Electric power ?

106 –1011 Radio waves and radar Thermal effects, cataracts

109 –1010 Microwaves Lens opacities

1011 –1014 Infrared Cataracts

1015 Visible light Retinal burns (lasers)

1015 –1018 Ultraviolet light Skin burns, cancer

1018 –1020 X-rays and gamma rays Acute and delayed injury; cancer

1027 Cosmic radiation ?

by patients undergoing diagnostic procedures such as mammography or chest radiography, and by nuclear power plant workers. Unfortunately, fear of widespread radiation exposure

following a terrorist attack reinforces the importance of understanding the mechanisms and clinical manifestations of radiation injury.[52] Despite our understanding of the health effects of

high doses of radiation, the potential adverse effects of low doses are controversial. Furthermore, accidents at nuclear power plants in Windscale, England, in 1957, at Three Mile Island in

Pennsylvania in 1979, and at Chernobyl in the former Soviet Union in 1986 perpetuate public anxiety about excess cancers associated with the medical, commercial, and military uses of

radioactivity.[53]

Electromagnetic radiation characterized by long wavelengths and low frequencies is described as nonionizing radiation. Electric power, radio waves and microwaves, infrared, and

ultraviolet light are examples of nonionizing radiation. They produce vibration and rotation of atoms in biologic molecules. Radiation energy of short wavelengths and high frequency can

ionize biologic target molecules and eject electrons. X-rays, gamma rays, and cosmic rays are forms of ionizing radiation. Ionizing radiation can be in the form of electromagnetic waves,

such as x-rays produced by a roentgen tube or gamma rays emitted from natural sources, or particles that are released by natural decay of radioisotopes or by artificial acceleration of

subatomic particles. Particulate radiation is classified by the type of particles emitted: alpha particles, beta particles or electrons, protons, neutrons, mesons, or deuterons. The energy of

these particles is measured in million electron volts (MeV). Radioisotopes decay by emission of alpha or beta particles or by capture of electrons. In the case of radon gas, unstable

daughter nuclei are produced that subsequently disintegrate, releasing alpha particles. Alpha particles consist of two neutrons and two protons; they have strong ionizing power but low

penetration because of their large size. In contrast, beta particles are electrons emitted from the nucleus of an atom; these have weaker ionizing power but higher penetration than alpha

particles. The decay of radioisotopes is expressed by the curie (Ci), 3.7 × 1010 disintegrations per second, or the becquerel (Bq), 1 disintegration per second. The rate of decay of

radioisotopes is usually expressed as the half-life (t1/2 ) and ranges from a few seconds to centuries. Internal deposition of radioisotopes with long half-lives is especially dangerous because

it results in continuous release of radioactive particles and gamma rays. For example, radium was used to paint watch dials and treat cancer in the first half of the 20th century; its long halflife

of 1638 years and ability to be concentrated in the skeleton result in delayed appearance of bone tumors.

Ionizing Radiation

The dose of ionizing radiation is measured in several units:

• roentgen: unit of charge produced by x-rays or gamma rays that ionize a specific volume of air

• rad: the dose of radiation that will produce absorption of 100 ergs of energy per gram of tissue; 1 gm of tissue exposed to 1 roentgen of gamma rays is equal to 93 ergs

• gray (Gy): the dose of radiation that will produce absorption of 1 joule of energy per kilogram of tissue; 1 Gy corresponds to 100 rad

• rem: the dose of radiation that causes a biologic effect equivalent to 1 rad of x-rays or gamma rays

• sievert (Sv): the dose of radiation that causes a biologic effect equivalent to 1 Gy of x-rays or gamma rays; 1 Sv corresponds to 100 rem.[53]

These measurements do not directly quantify energy transferred per unit of tissue and therefore do not predict the biologic effects of radiation. The following terms provide a better

approximation of such information.

• Linear energy transfer (LET) expresses energy loss per unit of distance traveled as electron volts per micrometer. This value depends on the type of ionizing radiation. LET is

high for alpha particles, less so for beta particles, and even less for gamma rays and x-rays. Thus, alpha and beta particles penetrate short distances and interact with many

molecules within that short distance. Gamma rays and x-rays penetrate deeply but interact with relatively few molecules per unit distance. It should be evident that if equivalent

amounts of energy entered the body in the form of alpha and gamma radiation, the alpha particles would induce heavy damage in a restricted area, whereas gamma rays would

dissipate energy over a longer course and produce considerably less damage per unit of tissue.

• Relative biologic effectiveness (RBE) is simply a ratio that represents the relationship of the LETs of various forms of irradiation to cobalt gamma rays and megavolt x-rays, both

of which have an RBE of unity (1).

In addition to the physical properties of the radioactive material and the dose, the biologic effects of ionizing radiation depend on several factors:

• Dose rate: a single dose can cause greater injury than divided or fractionated doses that allow time for cellular repair.

• Since DNA is the most important subcellular target of ionizing radiation, rapidly dividing cells are more radiosensitive than are quiescent cells. Hematopoietic cells, germ cells,

gastrointestinal epithelium, squamous epithelium, endothelial cells, and lymphocytes are highly susceptible to radiation injury; bone, cartilage, muscle, and peripheral nerves are

more resistant.

• A single dose of external radiation administered to the whole body is more lethal than regional doses with shielding. For example, the median lethal dose (LD50 ) of ionizing

radiation is 2.5 to 4.0 Gy (250 to 400 rad), whereas doses of 40 to 70 Gy (4000 to 7000 rad) can be delivered in a fractionated manner during several weeks for cancer therapy.

• Cells in the G2 and mitotic phases of the cell cycle are most sensitive to ionizing radiation.

• Different cell types differ in the extent of their adaptive and reparative responses.

• Since ionizing radiation produces oxygen-derived radicals from the radiolytic cleavage of water ( Chapter 1 ), cell injury induced by x-rays and gamma rays is enhanced by

hyperbaric oxygen. Halogenated pyrimidines can also increase radiosensitivity to tumor cells. Conversely, free radical scavengers and antioxidants protect against radiation injury.

Date: 2016-04-22; view: 847