Medical Radiology - Lazar A.P. 2008

Radiation Therapy
Dosimetry of Ionizing Radiation

Dosimetry emerged from the practical need to obtain quantitative and qualitative characteristics of ionizing radiation. Dosimetry is the measurement of the quantity and quality of ionizing radiation.

Dosimetry addresses the following key tasks:

1. Locating the radiation source and determining the type, quantity, and energy of the radiation.

2. Determining the extent of the radiation's effect on the irradiated object.

3. Monitoring protective equipment and devices used to ensure the radiation safety of staff and patients.

Dosimetry distinguishes between exposure dose, absorbed dose, equivalent dose, Skin dose, depth dose, integral dose, isodose, and several others.

As a rule, the exposure dose is measured near the radiation source in the air of an ionization chamber used to detect the given radiation. The exposure dose can be determined at any distance from the radiation source, for example, in adjacent rooms or behind protective shields. As the distance from the radiation source increases, the dose in air decreases in accordance with the inverse-square law, meaning its reduction is proportional to the square of the distance.

Table 1. Main dosimetric units and their designations.

The units of exposure dose are 1 R (Roentgen, a non-system unit) and 1 C/kg (Coulomb per kilogram, the SI international system unit) (Table 1). The exposure dose is determined by the magnitude of the charge of the ions formed per unit mass.

The exposure dose rate is defined as the exposure dose received per unit of time. The units of dose rate are: 1 Roentgen per second (R/s, a non-system unit) and 1 Ampere per kilogram (A/kg, a system unit).

Absorbed dose is the energy of ionizing radiation absorbed by the irradiated matter (air, body Tissues) per unit mass (D = E/M).

The unit of absorbed dose is 1 Gy, which is equal to 1 J/kg; that is, one gray is 1 joule of energy imparted to 1 kilogram of matter. The non-system unit is 1 rad (radiation absorbed dose) — the energy of 1 erg absorbed in a mass of 1 g (1 Gy = 100 rad).

Equivalent dose (H). To assess potential health risks under chronic exposure, METABOLISM/2.html">THE CONCEPT OF equivalent dose H is used, which is equal to the product of the absorbed dose Dr in an organ or tissue caused by radiation r and the radiation weighting factor Wr: H = DrxWr.

The unit of measurement for equivalent dose is the Joule per kilogram, which has the special name Sievert (Sv). The unit is named after the Swedish scientist Rolf Sievert, who worked on radiation protection problems. The non-system unit of equivalent dose is the rem, which stands for "roentgen equivalent man". Therefore, for photon radiation of medium and high energy in practical radiation protection, it can be assumed that the following quantities are approximately equal (1 rad = 1 rem = 1 R, 1 Sv = 1 Gy). Thus, 1 rad = 1 cGy, 1 rem = 1 cSv, 1 Sv = 100 rem.

The skin dose is measured directly on the skin surface. It should be noted that it is slightly higher than the dose just above the skin because it consists of the energy absorbed by the skin plus the energy of secondary radiation from excited skin atoms. As the radiation field size increases, The amount of secondary scattered radiation from the skin itself grows, since secondary radiation from a larger tissue volume reaches the dose measurement zone. The severity of the skin reaction to Radiation therapy depends on the magnitude of the skin dose.

The depth dose is the dose at a depth within the irradiated object. Since part of the ionizing radiation energy is spent on atomic excitation, ionization, and scattering as it passes through tissues, the radiation dose decreases with depth. The degree of dose attenuation within The Human Body depends mainly on two factors: the energy of the ionizing radiation and the density of the tissues and Organs through which it passes. The higher the ionizing radiation energy, the less the depth dose decreases. For example, at a gamma-ray energy of 160 keV, the dose at a depth of 10 cm from the surface is about 30% of the skin dose, whereas at a gamma-ray energy of 2 MeV, the depth dose at the same distance is already about 70% of the skin dose. The dependence of the depth dose on tissue density can be correlated with the densitometric density value on a computed tomography scan, which depends on the degree of X-ray penetration through the tissues.

The relative depth dose is The ratio of the dose at a depth within the body to the skin dose, usually expressed as a percentage. During radiation therapy for deep-seated pathological processes, It is important that the relative depth dose be as high as possible. The relative depth dose depends primarily on the radiation energy. A radiation beam contains a soft (low-energy) component and a hard (high-energy) component, located in the long-wave and short-wave Regions of the electromagnetic spectrum, respectively. Therefore, measures are taken to reduce the "soft component" of the radiation, which only increases the non-therapeutic radiation burden on the patient. Screening out soft radiation is achieved, firstly, by increasing the distance from the source to the skin surface (the source-to-skin distance is designated as SSD). In this process, the softest part of its energy is spent on exciting air atoms and ionizing them, and thus it does not reach the skin surface. In addition, aluminum and copper filters (plates) of various thicknesses are used, which also absorb the soft radiation. The relative depth dose can also be increased by reducing the size of the irradiation field, as this decreases the skin dose.

The amount of absorbed radiation energy in a pathological focus (e.g., a tumor) during radiation therapy is called the tumor dose (focal dose). The value of the focal dose depends on several factors: 1) the depth of the pathological lesion; 2) the relative depth dose; 3) the atomic number and thickness of the filter used; 4) the radiation energy; 5) the size of the irradiation field (on the skin).

During radiation therapy, not only the lesion itself is irradiated, but also the surrounding organs and tissues along the entire beam path. Therefore, to assess its effect on the Organism, the integral dose is determined, which characterizes the total energy absorbed by the entire irradiated organism. The unit of the integral dose is the gray-kilogram (or gram-rad).

In medical facilities, when radioactive elements are used for diagnostic and therapeutic purposes, not only the unit of energy absorbed by matter is used, but also the unit of activity. Activity (radioactivity) is the ability of atoms to undergo spontaneous transformation. Absolute, relative, and specific activities are distinguished. The absolute activity of a radioactive element is the number of nuclei decaying in it per unit of time. The unit of absolute activity is 1 curie (Ku, Ci, abbreviated as C). A curie is the quantity of a radioactive substance in which 3.7 × 10¹⁰ atoms decay per second. For practical use, this unit of activity is too large, so submultiples of the curie are used: millicurie (mCi) and microcurie (µCi). Relative activity refers to the counting rate of radiation pulses from a source recorded by a measuring device per unit of time. Specific activity is The activity of 1 g of a radioactive substance, expressed in curies per gram.

Ionizing radiations are not perceived by human Senses; therefore, specially designed instruments are used for their detection and measurement. Methods for detecting and measuring ionizing radiation depend on its fundamental properties—its physical, biological, and photochemical effects. Consequently, the Main methods of dosimetry are physical, biological, and photochemical.

The most common Physical Methods of dosimetry are ionization, scintillation, and thermoluminescent methods.

The ionization method of dosimetry has become widespread in practical medicine because it allows for highly accurate measurement of the dose of any type of ionizing radiation spent on The formation of ion pairs. Regardless of the type of radiation, the energy expended to form a single ion pair averages 34 eV. The ionization method makes it possible to precisely measure the energy of ion pairs produced under The Influence of radiation and, consequently, to calculate their number. Special ionization chambers or gas-discharge counters are used for this method, and galvanometers or milliammeters calibrated in roentgens are employed to record the energy of the ion pairs.

In many modern instruments that detect ionizing radiation, Geiger-Müller gas-discharge counters are used instead of ionization chambers. Geiger-Müller counters are capacitors designed as a hermetically sealed cylinder, but instead of air, the cavity is filled with gas, alcohol vapor, or ether. The walls of the counter are made of Glass or a thin layer of metal. A metal wire is stretched along the center of the cylinder, serving as one of the capacitor electrodes with an external lead. The second electrode of the counter is a very thin metal plate located on the inner surface of the glass cylinder, or the cylinder wall itself if it is metallic.

Under the action of ionizing radiation, ionization of the gas present occurs within the counter. Because the electrical voltage across the counter's electrodes is quite high, the generated ions rush toward the electrodes at extremely high speeds, triggering secondary ionization along their path. Driven by the strong electric field, these secondary ions also move with greater kinetic energy, producing tertiary ionization, and so on. As a result, an avalanche of ions—known as impact ionization—develops. This process causes the electrical circuit to close through the counter, producing a gas discharge (which is why these devices are called gas-discharge counters). Gas discharge within the counter cavity leads to a momentary surge of current in the circuit, which is then registered by a measuring instrument.

Gas-discharge counters are suitable for measuring any type of ionizing radiation, though they feature certain design specifics for each type. For instance, when measuring X-ray or gamma radiation, the inner wall of a glass counter is coated with a thin layer of copper; for beta particles, a layer of aluminum is used, while metal counters feature walls made from an alloy of these metals. End-window counters are utilized for alpha particles. The end of such a counter is sealed with a thin layer of mica that readily allows alpha particles to pass through.

The scintillation method of dosimetry (from Latin scintillatio meaning flashing or sparkling) is based on the ability of ionizing radiation to induce luminescence in certain salts. This method is distinguished by high measurement precision because it registers every single particle of corpuscular or quantum wave ionizing radiation. Crystals (such as sodium iodide and potassium iodide), special plastics, and scintillation liquids are commonly employed as scintillation Materials.

Because the light flash produced by individual alpha or beta particles is extremely faint and cannot be seen with the naked eye, photomultiplier tubes (PMTs) are attached to the crystals. When light flashes from the crystals strike the photocathode of the photomultiplier, they knock out electrons. These electrons then strike intermediate electrodes (dynodes), dislodging an even greater number of electrons from their atoms. Thus, the electron count multiplies from stage to stage until it reaches the anode of the photomultiplier. From there, the electrons pass into electronic amplifiers (vacuum-tube or semiconductor-based) and subsequently to measuring and recording devices.

The scintillation dosimetry method can be implemented using a scintillator (crystal) paired with a photomultiplier, designed as an attachment for any standard counting setup. Stationary scintillation systems (such as gamma topographs or scanners) make it possible to determine the accumulation of radioactive substances in specific human organs.

Thermoluminescent dosimetry is conceptually similar to scintillation dosimetry. Today, the method is widely used for assessing personnel radiation exposure levels, calibrating the radiation output of therapeutic sources, and measuring body dose distributions, among other Applications. The detector used in this type of dosimetry is a small pellet or rod (3-10 mm in diameter) made of a lithium fluoride (LiF) single crystal or another thermoluminescent material (CaF₂, AlO₂, CaSO₄, etc.) capable of storing the energy of ionizing radiation within its electron shells and then releasing it as a light flash upon heating. Such dosimeters allow for the measurement of doses across a wide range (up to 8 orders of magnitude).

The biological method of dosimetry is not used in clinical practice and holds purely historical significance. It is based on detecting changes occurring throughout the entire body, as well as in specific human tissues and organs, under the influence of ionizing radiation—such as the appearance of erythema, Metabolic Disorders, alterations in peripheral Blood composition, and more.

The photochemical method of dosimetry relies on the ability of ionizing radiation to induce the decomposition (dissociation) of salts within the light-sensitive layer of photographic film. Upon exposure and subsequent development, silver is reduced from its halide form to metallic silver in the areas where dissociation occurred, leading to film blackening. By comparing the degree of film darkening against standard References, the intensity of the radiation can be determined.