Radiation quality factor

Tvne of radiation Quality factor (O) Absorbed dose equal to a unit dose equivalent ... 

Quality Factor (Q)—The linear-energy-transfer-dependent factor by which absorbed doses are multiplied to obtain (for radiation protection purposes) a quantity that expresses - on a common scale for all ionizing radiation - the approximate biological effectiveness of the absorbed dose. 

Relative Biological Effectiveness (RBE)—The RBE is a factor used to compare the biological effectiveness of absorbed radiation doses (i.e., rad) due to different types of ionizing radiation. More specifically, it is the experimentally determined ratio of an absorbed dose of a radiation in question to the absorbed dose of a reference radiation (typically 60Co gamma rays or 200 keV x rays) required to produce an identical biological effect in a particular experimental organism or tissue (see Quality Factor). 

Rem—A unit of dose equivalent that is used in the regulatory, administrative, and engineering design aspects of radiation safety practice. The dose equivalent in rem is numerically equal to the absorbed dose in rad multiplied by the quality factor (1 rem is equal to 0.01 sievert). 

Dose equivalent or rem is a special radiation protection quantity that is used, for administrative and radiation safety purposes only, to express the absorbed dose in a manner which considers the difference in biological effectiveness of various kinds of ionizing radiation. The ICRU has defined the dose equivalent, H, as the product of the absorbed dose, D, and the quality factor, Q, at the point of interest in biological tissue. This relationship is expressed as H = D x Q. The dose equivalent concept is applicable only to doses that are not great enough to produce biomedical effects. 

RBE is used to denote the experimentally determined ratio of the absorbed dose from one radiation type to the absorbed dose of a reference radiation required to produce an identical biologic effect under the same conditions. Gamma rays from cobalt-60 and 200-250 keV x-rays have been used as reference standards. The term RBE has been widely used in experimental radiobiology, and the term quality factor used in calculations of dose equivalents for radiation safety purposes (ICRP 1977 NCRP 1971 UNSCEAR 1982). RBE applies only to a specific biological end point, in a specific exposure, under specific conditions to a specific species. There are no generally accepted values of RBE. 

The formal definition of this quality factor, Q, is the amount of power stored in the resonator divided by the amount of power dissipated per cycle (at 9.5 GHz a cycle time is l/(9.5 x 109) 100 picoseconds). The dissipation of power is through the resonator walls as heat, in the sample as heat, and as radiation reflected out of the resonator towards the detector. The cycle time is used in the definition because the unit time of one second would be far too long for practical purposes within one second after the microwave source has been shut off, all stored power has long been dissipated away completely. 

The quality factor is a dimensionless quantity that depends in part on the stopping power for charged particles, and it accounts for the differences in biological effectiveness found among the types of radiation. By definition it is independent of tissue and biological end point and, therefore, of little use... 

In hadron therapy, a similar approach and symbolism are presented to take into account the RBE differences. RBE values for neutrons are well documented, and reliable data are now becoming available for protons and heavy ions, on which safe weighting factors IErbe can be based. Besides radiobiological determinations, microdosimetry brings independently additional information on the radiation quality and improves confidence in both sets of data. Microdosimetry provides an objective description of the radiation quality at the point of interest under the actual irradiation conditions. Correlation between this set of information and the experimental RBE results is of great scientific and clinical value in hadron therapy [24,25]. 

Contemporary radiation protection systems (ICRP, 1977a 1991 NCRP, 1987 1993) include dose limits expressed in such a quantity. To obtain the quantity, absorbed doses are first multiplied by a quality factor (ICRP, 1977a) or a radiation weighting factor (ICRP, 1991), selected for the type and energy of the radiation incident upon the body, yielding, respectively, the dose equivalent in the tissue (ICRP, 1977a) or equivalent dose in the tissue (ICRP, 1991). Therefore ... 

For low-LET radiation, the quality factor and radiation weighting factor have the value of one. Therefore, dose equivalent and equivalent dose have the same numerical value. 

Dose Equivalent The amount of effective radiation when modifying factors have been taken into account. The product of absorbed dose multiplied by a quality factor multiplied by a distribution factor, expressed numerically in units of REMS. 

The biological effectiveness of dose depends on the type of radiation and also on the mass and sensitivity of the irradiated tissue. For alpha irradiation, a quality factor of 20 is assumed (ICRP, 1981), and the dose in Sieverts is 20 times the dose in Grays. In addition, ICRP recommends a weighting factor of 0.12 for irradiation of the whole lung and 0.06 for irradiation of bronchial epithelium only. Thus the effective dose equivalent , symbol HE, is defined as the dose to the whole body which carries the same risk as the given dose to the organ or tissue. This, for irradiation of bronchial tissue is 20 x 0.06 = 1.2 times the dose to the organ in Gy. 

Natural radionuclides contaminate air, food, and water. The annual per capita intake of natural radionuclides has been estimated to range from 2 Becquerels (Bq) for 232Th to about 130 Bq for 4 K (Sinclair 1988). The Bq is the International System of Units (SI) unit of radioactivity 1 Bq = 1 radioactive disintegration per second. The previously used unit of radioactivity is the Curie (Ci) 1 Ci = 3.7 x 1010 disintegrations per second, and 1 Bq = 27 x 10-12 Ci. The quantity of radiation or energy absorbed is expressed in Sievert (Sv), which is the SI unit of dose equivalent. The absorbed dose (in Gy) is multiplied by a quality factor for the particular type of radiation. Rem is the previously used unit for dose equivalent 100 rem = 1 Sv. 

Rem This older term is the short abbreviation for roentgen equivalent man. It is defined as the dose of radiation absorbed, multiplied by a modifying factor called quality factor Q, or relative biological effectiveness, RBE that takes into account the type of radiation (of any kind) and how the radiation is absorbed. Thus, for beta, gamma and X-rays, the quality factor is 1 for alpha radiation it may be as high as 20, and for neutrons it varies from 3 to 10. 

Sievert (Sv) It is a more modem unit that substitutes the rem, as it reflects the biological effect of radiation. It is defined as the absorbed dose in Gy multiplied by a somewhat complicated factor that takes into account for example the quality factor defined above, the part of the body irradiated and the time and volume of exposure. 1 Sv = 100 rems. 

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