# Tissue weight factor (weighting factor)

Exposure dose

The main characteristic of the interaction of ionizing radiation and the medium is the ionization effect. In the initial period of the development of radiation dosimetry, one most often had to deal with X-rays propagating in the air. Therefore, the degree of air ionization of X-ray tubes or apparatuses was used as a quantitative measure of the radiation field. A quantitative measure based on the amount of ionization of dry air at normal atmospheric pressure, which is fairly easy to measure, is called the exposure dose.

The exposure dose determines the ionizing capacity of X-rays and gamma rays and expresses the radiation energy converted into the kinetic energy of charged particles per unit mass of atmospheric air. Exposure dose is the ratio of the total charge of all ions of the same sign in an elementary volume of air to the mass of air in this volume.

In the SI system, the unit of exposure dose is the coulomb divided by the kilogram (C/kg). The off-system unit is the roentgen (R). 1 C/kg = 3876 R.

Dose rate (irradiation intensity) is the increment of the corresponding dose under the influence of a given radiation per unit of time. It has the dimension of the corresponding dose (absorbed, exposure, etc.) divided by a unit of time. It is allowed to use various special units (for example, Sv/h, rem/min, cSv/year, etc.).

The power of exposure

1Р/s=2.58 10−4 C/kg*s

Absorbed dose

With the expansion of the range of known types of ionizing radiation and the scope of its application, it turned out that the measure of the effect of ionizing radiation on a substance cannot be simply determined due to the complexity and diversity of the processes occurring in this case. An important of them, giving rise to physicochemical changes in the irradiated substance and leading to a certain radiation effect, is the absorption of the energy of ionizing radiation by the substance. As a result, the concept of absorbed dose arose. The absorbed dose shows how much radiation energy is absorbed per unit mass of any irradiated substance and is determined by the ratio of the absorbed ionizing radiation energy to the mass of the substance.

The SI unit of absorbed dose is the gray (Gy). 1 Gy is such a dose at which the energy of ionizing radiation of 1 J is transferred to a mass of 1 kg. The non-systemic unit of absorbed dose is rad. 1 Gy=100 rad.

A value by which the absorbed dose in a tissue or organ is multiplied to account for the relative biological effectiveness of radiation in inducing stochastic effects at low doses, resulting in an equivalent dose value.

Type of radiation and energy range

Weight multiplier

Photons of all energies

Electrons and muons of all energies

Neutrons with energies < 10 keV

Neutrons from 10 to 100 keV

Neutrons from 100 keV to 2 MeV

Neutrons from 2 MeV to 20 MeV

Neutrons > 20 MeV

Protons with energies > 2 MeV (except recoil protons)

alpha particles, fission fragments and other heavy nuclei

Equivalent dose (N). To assess the possible damage to human health under conditions of chronic exposure in the field of radiation safety, the concept of an equivalent dose H is introduced, which is equal to the product of the absorbed dose Dr created by exposure – r and averaged over the analyzed organ or throughout the body, by the weight factor wr (also called the quality factor radiation) The unit of equivalent dose is Joule per kilogram. It has a special name Sievert (Sv).

Fabric weight factor (weighting factor)

Multipliers of the equivalent dose per organ or tissue used for the purposes of radiation protection, which allow taking into account the different sensitivity of different organs and tissues to the induction of stochastic effects of radiation.

Therefore, to determine the total effect of ionizing radiation on a person, a tissue weight factor (or weighting factor) (wT) is applied. At its core, this factor takes into account the difference in the radiosensitivity of organs and tissues and the severity of the oncological disease that has arisen.

The effective dose is equal to the sum of the weighted equivalent doses in all organs and tissues: where wt is the tissue weight factor (Table 12) and Ht is the equivalent absorbed dose in

fabrics – t. The unit of effective equivalent dose is Sievert.

Effective dose (E, Effective equivalent dose) is a value used in radiation protection as a measure of the risk of long-term consequences of irradiation (stochastic effects) of the entire human body and its individual organs and tissues, taking into account their radiosensitivity.

Collective effective dose – the effective dose received by a group of people from any source of radiation; it is equal to the sum of individual effective doses. The unit of effective collective dose is man-sievert (man-Sv).