radiation(redirected from extrafocal radiation)
Also found in: Dictionary, Thesaurus, Encyclopedia.
Another type is the radiation emitted by radioactive materials. alpha particles are high-energy helium-4 nuclei consisting of two protons and two neutrons, emitted by radioisotopes of heavy elements such as uranium. beta particles are high-energy electrons emitted by radioisotopes of lighter elements. gamma rays are high-energy photons emitted along with alpha and beta particles and also emitted alone by metastable radionuclides, such as technetium-99m. Gamma rays have energies in the x-ray region of the spectrum and differ from x-rays only in that they are produced by radioactive decay rather than by x-ray machines.
Radiation with enough energy to knock electrons out of atoms and produce ions is called ionizing radiation and includes alpha particles, beta particles, x-rays, and gamma rays. This kind of radiation can produce tissue damage directly by striking a vital molecule, such as DNA, or indirectly by striking a water molecule and producing highly reactive free radicals that chemically attack vital molecules. The effects of radiation can kill cells, make them unable to reproduce, or cause nonlethal mutations, producing cancer cells or birth defects in offspring. The radiosensitivity of normal tissues or cancer cells increases with their rate of cell division and decreases with their rate of cell specialization. Highly radiosensitive cells include lymphocytes, bone marrow hematopoietic cells, germ cells, and intestinal epithelial cells. Radiosensitive cancers include leukemias and lymphomas, seminoma, dysgerminoma, granulosa cell carcinoma, adenocarcinoma of the gastric epithelium, and squamous cell carcinoma of skin, mouth, nose and throat, cervix, and bladder.
The application of radiation, whether by x-ray or radioactive substances, for treatment of various illnesses is called radiation therapy or radiotherapy.
Three types of units are used to measure ionizing radiation. The roentgen (R) is a unit of exposure dose applicable only to x-rays and gamma rays. It is the amount of radiation that produces 2.58 × 10−4 coulomb of positive and negative ions passing through 1 kilogram of dry air. The rad is a unit of absorbed dose equal to 100 ergs of energy absorbed per 1 g of absorbing material; the absorbed dose depends both on the type of radiation and on the material in which it is absorbed. The rem is a unit of absorbed dose equivalent that produces the same biologic effect as 1 rad of high-energy x-rays. For beta and gamma radiation, 1 rem is approximately equal to 1 rad; for alpha radiation, 1 rad is approximately 20 rem.
Previously, doses administered in radiation therapy were commonly specified as measured exposure doses in roentgens. The current practice is to specify the absorbed dose in the tissue or organ of interest in rads. Many personnel monitoring devices read out in rems. Eventually, the rad and rem may be replaced by the new SI units, the gray and sievert; 1 gray equals 100 rad, and 1 sievert equals 100 rem.
Exposure to large doses of radiation over a short period of time produces a group of symptoms known as the acute radiation syndrome. These symptoms include general malaise, nausea, and vomiting, followed by a period of remission of symptoms. Later, the patient develops more severe symptoms such as fever, hemorrhage, fluid loss, anemia, and central nervous system involvement. The symptoms then gradually subside or become more severe, and may lead to death.
Shielding is of special importance when time and distance cannot be completely utilized as safety factors. In such instances lead, which is an extremely dense material, is used as a protective device. The walls of diagnostic x-ray rooms are lined with lead, and lead containers are used for radium, cobalt-60, and other radioactive materials used in radiotherapy.
Monitoring devices such as the film badge, thermoluminescent dosimeter, or pocket monitor are worn by persons working near sources of radiation. These devices contain special detectors that are sensitive to radiation and thus serve as guides to the amount of radiation to which a person has been exposed. For monitoring large areas in which radiation hazards may pose a problem, survey meters such as the Geiger counter may be used. The survey meter also is useful in finding sources of radiation such as a radium implant, which might be lost.
Sensible use of these protective and monitoring devices can greatly reduce unnecessary exposure to radiation and allow for full realization of the many benefits of radiation.
Medspeak The direct, band-like extension of a sensation, in particular of pain, from a point of origin to another region of the body
Oncology The administration of ionizing radiation to kill malignant tumour cells
Physics An emission of energy in the form of photons—gamma, x-rays; particles—alpha, beta, positrons, neutrons or waves—heat, light, radio waves and microwaves
radiationThe combined processes of emission, transmission and absorption of highly energetic waves and particles on the electromagnetic spectrum treatment to kill cancer cells. See Acute radiation injury, Alpha radiation, Background radiation, Chemoradiation, Coherent radiation, Corpuscular radiation, Definitive radiation, Electromagnetic radiation, External radiation, Gamma radiation, Grenz radiation, Implant radiation, Internal radiation, Ionizing radiation, Non-ionizing radiation, Remnant radiation, Scattered radiation, Synchrotron radiation, Total body irradiation Clinical practice The direct, band-like extension of a sensation, in particular of pain, from a point of origin to another region of the body. Cf Referred pain Oncology The administration of ionizing radiation to kill malignant tumor cells. See Radiation fibrosis, Radiation therapy.
Compare: irradiation (2)
radiationThe emission and almost instantaneous propagation of electromagnetic waves ranging in wavelength from thousands of metres (radio waves) to millionths of millionths of millimetres (gamma rays). Radiation of long wavelength may cause body atoms and molecules to vibrate but does not, so far as is known, significantly damage them (non-ionizing radiation). Very short wavelength radiation, such as X-rays and gamma rays (ionizing radiation), however, can knock out linking electrons from molecules, causing them to separate into smaller charged bodies or chemical groups called ions, or FREE RADICALS. Ionizing radiation can damage any body molecules, including DNA, and this may kill cells or alter their genetic structure. Such mutations in surviving cells may lead to cancer. At the same time, rapidly dividing cancer cells are more susceptible to the effects of ionizing radiation than normal cells. This is the basis of RADIOTHERAPY.
radiationthe electromagnetic energy that travels through empty space with the speed of light (2×108 ms -1). All objects emit radiation, at room temperature mostly in the infrared range, whereas at high temperatures visible radiation is produced. See ELECTROMAGNETIC SPECTRUM, ULTRAVIOLET LIGHT, X-RAY.
radiationemission of electromagnetic impulses from a source; e.g. visible light, ultraviolet light, infrared light, X-rays, gamma rays
n 2. the combined processes of emission, transmission, and absorption of radiant energy.
Bremsstrahlung translates to “braking radiation,” referring to the sudden slowing of electrons that occurs when they encounter nuclei with a high positive charge.
Patient discussion about radiation
Q. Is an X- Ray dangerous to my fetus? I fell down while I am pregnant and was sent to the ER. I was given an x- ray there, is the radiation dangerous to my fetus?
Q. What does radiation do for cancer patients? We found out today that my grandmother has cancer and my mother said that the oncologist is planning on using radiation to ease her pain. My question is, what does radiation do? I know, eases pain, but how?
Q. Is it proven that cellular radiation can damage health?
could be that in 15 years from now the amount of brain cancer will increase and they will know for sure it's from cellular phones. , why take a chance- use as less as possible, use an earphone and don't give a cellular phone to your children until they are 17.
you can never know what will they find next...