BIOLOGICAL IMPACT

It is sometimes desirable to express the actual or relative biological impact of radiation. It is necessary to develop a distinction between the biological impact and the physical quantity of radiation because all types of radiation do not have the same potential for producing biological change. For example, one rad of one type of radiation might produce significantly more radiation damage than one rad of another type. In other words, the biological impact is determined by both the quantity of radiation and its ability to produce biological effects. Two radiation quantities are associated with biological impact.

RELATIVE BIOLOGICAL EFFECTIVENESS (RBE)

When specific radiation effects rather than general risk are being considered, the relative biological effectiveness (RBE) of the radiation must be taken into account. The value of the RBE depends on characteristics of the radiation and the specific biological effects being considered. This radiation characteristic is generally not used in association with diagnostic procedures.

EFFECTIVE DOSE

The Concept of Effective Dose

Effective dose is a very useful radiation quantity for expressing relative risk to humans, both patients and other personnel.  It is actually a simple and very logical concept.  It takes into account the specific organs and areas of the body that are exposed.  The point is that all parts of the body and organs are not equally sensitive to the possible adverse effects of radiation, such as cancer induction and mutations.

For the purpose of determining effective dose, the different areas and organs have been assigned tissue weighting factor (w_T) values.  For a specific organ or body area the effective dose is:

Effective Dose (Gy) = Absorbed Dose (Gy) x W_T

 If more than one area has been exposed, then the total body effective dose is just the sum of the effective doses for each exposed area.

LIGHT

The basic light quantities and units encountered in radiology can be conveniently divided into two categories:

• those that express the amount of light emitted by a source,
•  those that describe the amount of light falling on a surface, such as a piece of film.

The relationships of several light quantities and units are shown below:

Light Quantities and Units Encountered in Radiology

LUMINANCE

Luminance is the light quantity generally referred to as brightness. It describes the amount of light being emitted from the surface of the light source. The basic unit of luminance (brightness) is the nit, which is equivalent to 1 candela per m² of source area or foot lambert, which is equivalent to to 3.426 nits.

The concept of luminance is somewhat easier to understand if it is related to the number of light photons involved. The quantity for specifying an amount of light is the lumen. One lumen of light with wavelengths encountered in x-ray imaging systems (540 nm) is equivalent to 3.8 x 10¹⁵ photons per second. Another factor that determines luminance is the concentration of light in a given direction. This can be described in terms of a cone or solid angle that is measured in units of steradians (sr).

ILLUMINANCE

Illuminance is a specification of the quantity of light falling on or illuminating a surface. The basic unit is the lux. A surface has an illuminance of 1 lux when it receives 1 lumen/m² of surface area. Consider a small area on a piece of film that is 1 mm². For light with a wavelength of 540 nm, there are 3.8 x 10¹⁵ photons per second per lumen. An illuminance to the film of 1 lux would be equivalent to 3.8 x 10² photons per sec to a 1-mm² area. The total light exposure to a film is found by multiplying the illuminance, in lux, by the exposure time, in seconds, and is expressed in units of lux-seconds. Another unit used in some literature for specifying illuminance is the footcandle, which is equivalent to 10.764 lux.

RADIO FREQUENCY (RF) RADIATION

Radio frequency (RF) radiation is used in magnetic resonance imaging (MRI). During an imaging procedure, pulses of RF energy are applied to the patient’s body where most of it is absorbed. Conventional energy units are used to express the amount of RF energy imparted to the body.

Power is the rate at which energy is transferred. The unit for power is the watt, which is equivalent to an energy transfer at the rate of 1 joule/second. During the acquisition phase of MRI, the system transfers energy to the patient’s body at some specific power level. The actual power (watts) used depends on many factors associated with the examination.

SPECIFIC ABSORPTION RATE (SAR)

From the standpoint of effect on the patient’s body a more significant quantity is the concentration of power in the tissue. This is expressed in the units of watts per kilogram of tissue and is designated the specific absorption rate (SAR). Two quantities considered in SAR include:

• the SAR in a specific location,
• the average SAR within the total body.

The RF energy absorbed by the tissue is converted into heat. Therefore, the power concentration is an indication of the rate at which heat is produced within specific tissue.

RADIATION DETECTOR METHODS

Radiatio detectors are devices which senses and relay information about incoming radiation. A foundamental features of nuclear processes is that the energy released is larger than the binding energies of atomic electrons. Any emitted particles will have sufficient energy to ionize atoms. Nuclear            radiation is called ionizing radiation, and detecting          this ionization allows us to observe nuclear processes. Radiations  that interact with matter via the electromagnetic force, i.e., electrons, charged  particles and photons, can directly ionize or            excite atoms. These radiations are readily detected. Neutrons interact with nuclei only via the    nuclear force and are detected through indirect or secondary ionization processes.

            Although the various types of radiation detectors differ in many respects, several common criteria are used to evaluate the performance of any detector type.  The criteria used for this purpose are as follows:          

 1. The sensitivity of the detector. What types of radiation will the detector detect? For example, solid scintillation detectors are normally not used to detect         α-­‐particles from radioactive decay because the α-particles cannot penetrate the detector covering.     

2. The energy resolution of the detector. Will the detector  measure the energy of the radiation striking it, and if so, how precisely does it do this?   If two  γ-rays of energies 1.10MeV and 1.15MeV strike the detector,            can it distinguish between them?      

3. The time resolution of the detector or its  pulse resolving time.  How high a counting  rate will be            measured by the detector without error? How accurately and precisely can one measure the time of arrival of a particle at the       detector?       

4. The  detector efficiency. If 100 γ‐rays strike a detector, exactly  how many will be detected? 

 The detection methods can be classified as: (i) collection of the ionization produced in a gas or solid (ii) detection of secondary electronic excitation in a solid or liquid scintillation or (iii) detection of specific chemical changes induced in sensitive emulsions.

GAS IONIZATION

Several            detector types take advantage of the ionizing effect of radiation on gases. The ion pairs so produced can be          separately collected. When a potential gradient is applied between the two electrodes in a gas filled ion   chamber, the positively charged molecules move to the cathode and the negative ions        (electrons) move swiftly to the anode, thereby creating a measurable pulse. Such pulses can be readily measured by the associated devices        as individual events    or integrated current.

IONIZATION IN A SOLID (SEMICONDUCTOR DETECTORS).

In a semi-conductor radiation detector,  incident radiation interacts with the detector         material, a semi-conductor such as Si or Ge, to create hole-electron       pairs. These hole        electron pairs are collected by charged electrodes with the electrons migrating to the positive electrode and the holes to the negative electrode, thereby creating an electrical pulse. Such pulses contain information on the type, energy, time of arrival, and number of particles arriving per unit time. The important features of semiconductor detectors       are their superior energy resolution due to a lower      ionization potential and compact size.

 SOLID SCINTILLATORS  

Some of the energy of ionizing radiation can be transferred to fluor molecules (i.e., compounds that can produce           fluorescence) in a crystalline solid. The absorbed energy causes excitation of orbital electrons in the fluor. De-excitation causes the emission of the absorbed energy as EM radiation in thr visible or near UV region (scintillations). Observing these weak scintillations visualy under cerain circumstances is possible, but visual observation is normal not a feasible detection method. Instead a photomultiplier tube close to the solid fluor is employed. In the photomultiplier, the photons are converted to photoelectrons, which are greatly amplified by secondary electron emission through a series of electrons (dryodes) to cause a sizable electrical pulse. Thus, the original excitation energy is transformed into a measurable pulse.

LIGUID SCINTILLATORS

This detection mechanism is quite similar in principle to the preceding one. Here, howver, the radioactive sample and the fluor are the solute in a liquid medium, usually a nonpolar solvent. The energy of nuclear radiation first excites the solvent molecules. This excitation energy eventually appears as photons emitted from the fluor following an intermediate transfer stage. The photons are detected by means of a photomultiplier arrangement.

NUCLEAR EMULSIONS

The process involved here is a chemical one. Ionizing radiation from a sample interacts with the silver halide grains in a phoographic emuision to cause a chemical reaction. Subsequent development of the film produces an image and so permits a semi-quantitative estimate of the radiation coming from the sample.

RADIATION DOSIMETER

A radiation dosimeter is a device, instrument or system that measures or evaluates, either directly or indirectly, the quantities exposure, kerma, absorbed dose or equivalent dose, or their time derivatives (rates), or related quantities of ionizing radiation. Dosimetry is the act of measuring or estimating radiation doses and assigning those doses to individuals. A dosimeter along with its reader is referred to as a dosimetry system. Measurement of a dosimetric quantity is the process of finding the value of the quantity experimentally using dosimetry systems. The result of a measurement is the value of a dosimetric quantity expressed as the product of a numerical value and an appropriate unit. To function as a radiation dosimeter, the dosimeter must possess at least one physical property that is a function of the measured dosimetric quantity and that can be used for radiation dosimetry with proper calibration. In order to be useful, radiation dosimeters must exhibit several desirable characteristics. For example, in radiotherapy exact knowledge of both the absorbed dose to water at a specified point and its spatial distribution are of importance, as well as the possibility of deriving the dose to an organ of interest in the patient. In this context, the desirable dosimeter properties will be characterized by accuracy and precision, linearity, dose or dose rate dependence, energy response, directional dependence and spatial resolution. Obviously, not all dosimeters can satisfy all characteristics. The choice of a radiation dosimeter and its reader must therefore be made judiciously, taking into account the requirements of the measurement situation; for example, in radiotherapy ionization chambers are recommended for beam calibrations and other dosimeters are suitable for the evaluation of the dose distribution (relative dosimetry) or dose verification.

IONIZATION CHAMBER DOSIMETRY SYSTEMS

CHAMBERS AND ELECTROMETER

Ionization chambers are used in radiotherapy and in diagnostic radiology for the determination of radiation dose. The dose determination in reference irradiation conditions is also called beam calibration. Ionization chambers come in various shapes and sizes, depending upon the specific requirements, but generally they all have the following properties:

● An ionization chamber is basically a gas filled cavity surrounded by a conductive outer wall and having a central collecting electrode. The wall and the collecting electrode are separated with a high quality insulator to reduce the leakage current when a polarizing voltage is applied to the chamber.

● A guard electrode is usually provided in the chamber to further reduce chamber leakage. The guard electrode intercepts the leakage current and allows it to flow to ground, bypassing the collecting electrode. It also ensures improved field uniformity in the active or sensitive volume of the chamber, with resulting advantages in charge collection.

● Measurements with open air ionization chambers require temperature and pressure correction to account for the change in the mass of air in the chamber volume, which changes with the ambient temperature and pressure. Electrometers are devices for measuring small currents, of the order of 10–9 A or less. An electrometer used in conjunction with an ionization chamber is a high gain, negative feedback, operational amplifier with a standard resistor or a standard capacitor in the feedback path to measure the chamber current or charge collected over a fixed time interval.

FILM DOSIMETRY

RADIOGRAPHIC FILM

Radiographic X ray film performs several important functions in diagnostic radiology, radiotherapy and radiation protection. It can serve as a radiation detector, a relative dosimeter, a display device and an archival medium. Unexposed X ray film consists of a base of thin plastic with a radiation sensitive emulsion (silver bromide (AgBr) grains suspended in gelatin) coated uniformly on one or both sides of the base.

● Ionization of AgBr grains, as a result of radiation interaction, forms a latent image in the film. This image only becomes visible (film blackening) and permanent subsequently to processing.

● Light transmission is a function of the film opacity and can be measured in terms of optical density (OD) with devices called densitometers.

● The OD is defined as OD = log10 (I0/I) and is a function of dose. I0 is the initial light intensity and I is the intensity transmitted through the film.

● Film gives excellent 2-D spatial resolution and, in a single exposure, provides information about the spatial distribution of radiation in the area of interest or the attenuation of radiation by intervening objects.

● Τhe useful dose range of film is limited and the energy dependence is pronounced for lower energy photons. The response of the film depends on several parameters, which are difficult to control. Consistent processing of the film is a particular challenge in this regard.

● Typically, film is used for qualitative dosimetry, but with proper calibration, careful use and analysis film can also be used for dose evaluation.

● Various types of film are available for radiotherapy work (e.g. direct exposure non-screen films for field size verification, phosphor screen films used with simulators and metallic screen films used in portal imaging).

● Unexposed film would exhibit a background OD called the fog density (ODf). The density due to radiation exposure, called the net OD, can be obtained from the measured density by subtracting the fog density.

● OD readers include film densitometers, laser densitometers and automatic film scanners. The principle of operation of a simple film densitometer is shown in Fig. 3.6.

Ideally, the relationship between the dose and OD should be linear, but this is not always the case. Some emulsions are linear, some are linear over a limited dose range and others are non-linear. The dose versus OD curve, known as the sensitometric curve (also known as the characteristic or H&D curve, in honour of Hurter and Driffield, who first investigated the relationship) must therefore be established for each film before using it for dosimetry work. A typical H&D curve for a radiographic film is shown in the diagram below. It has four regions: (i) fog, at low or zero exposures; (ii) toe; (iii) a linear portion at intermediate exposures; and (4) shoulder and saturation at high exposures. The linear portion is referred to as optimum measurement conditions, the toe is the region of underexposure and the shoulder is the region of overexposure.

Basic film densitometer.

Important parameters of film response to radiation are gamma, latitude and speed:

● The slope of the straight line portion of the H&D curve is called the gamma of the film.

● The exposure should be chosen to make all parts of the radiograph lie on the linear portion of the H&D curve, to ensure the same contrast for all ODs.

● The latitude is defined as the range of exposures over which the ODs will lie in the linear region.

● The speed of a film is determined by giving the exposure required to produce an OD of 1.0 greater than the OD of fog.

Typical applications of a radiographic film in radiotherapy are qualitative and quantitative measurements, including electron beam dosimetry, quality control of radiotherapy machines (e.g. congruence of light and radiation fields and the determination of the position of a collimator axis, the so called star test), verification of treatment techniques in various phantoms and portal imaging.

Typical sensitometric (characteristic H&D) curve for a radiographic film.

Radiochromic film

Radiochromic film is a new type of film in radiotherapy dosimetry. The most commonly used is a GafChromic film. It is a colourless film with a nearly tissue equivalent composition (9.0% hydrogen, 60.6% carbon, 11.2% nitrogen and 19.2% oxygen) that develops a blue colour upon radiation exposure. Radiochromic film contains a special dye that is polymerized upon exposure to radiation. The polymer absorbs light, and the transmission of light through the film can be measured with a suitable densitometer. Radiochromic film is self-developing, requiring neither developer nor fixer. Since radiochromic film is grainless, it has a very high resolution and can be used in high dose gradient regions for dosimetry (e.g. measurements of dose distributions in stereotactic fields and in the vicinity of brachytherapy sources). Dosimetry with radiochromic films has a few advantages over radiographic films, such as ease of use; elimination of the need for darkroom facilities, film cassettes or film processing; dose rate independence; better energy characteristics (except for low energy X rays of 25 kV or less); and insensitivity to ambient conditions (although excessive humidity should be avoided). Radiochromic films are generally less sensitive than radiographic films and are useful at higher doses, although the dose response non-linearity should be corrected for in the upper dose region.

● Radiochromic film is a relative dosimeter. If proper care is taken with calibration and the environmental conditions, a precision better than 3% is achievable. ● Data on the various characteristics of radiochromic films (e.g. sensitivity, linearity, uniformity, reproducibility and post-irradiation stability) are available in the literature.

 LUMINESCENCE DOSIMETRY

Some materials, upon absorption of radiation, retain part of the absorbed energy in metastable states. When this energy is subsequently released in the form of ultraviolet, visible or infrared light, the phenomenon is called luminescence. Two types of luminescence, fluorescence and phosphorescence, are known, which depend on the time delay between stimulation and the emission of light. Fluorescence occurs with a time delay of between 10–10 and 10–8 s; phosphorescence occurs with a time delay exceeding 10–8 s. The process of phosphorescence can be accelerated with a suitable excitation in the form of heat or light.

● If the exciting agent is heat, the phenomenon is known as thermoluminescence and the material is called a thermoluminescent material, or a TLD when used for purposes of dosimetry. ● If the exciting agent is light, the phenomenon is referred to as optically stimulated luminescence (OSL). 

As discussed in Section 1.4, the highly energetic secondary charged particles, usually electrons, that are produced in the primary interactions of photons with matter are mainly responsible for the photon energy deposition in matter. In a crystalline solid these secondary charged particles release numerous low energy free electrons and holes through ionizations of atoms and ions. The free electrons and holes thus produced will either recombine or become trapped in an electron or hole trap, respectively, somewhere in the crystal. The traps can be intrinsic or can be introduced in the crystal in the form of lattice imperfections consisting of vacancies or impurities. Two types of trap are known in general: storage traps and recombination centres.

● A storage trap merely traps free charge carriers and releases them during the subsequent (a) heating, resulting in the thermoluminescence process, or (b) irradiation with light, resulting in the OSL process. ● A charge carrier released from a storage trap may recombine with a trapped charge carrier of opposite sign in a recombination centre (luminescence centre). The recombination energy is at least partially emitted in the form of ultraviolet, visible or infrared light that can be measured with photodiodes or photomultiplier tubes (PMTs).

THERMOLUMINESCENCE

Thermoluminescence is thermally activated phosphorescence; it is the most spectacular and widely known of a number of different ionizing radiation induced thermally activated phenomena. Its practical applications range from archaeological pottery dating to radiation dosimetry. In 1968 Cameron, Suntharalingam and Kenney published a book on the thermoluminescence process that is still considered an excellent treatise on the practical aspects of the thermoluminescence phenomenon. A useful phenomenological model of the thermoluminescence mechanism is provided in terms of the band model for solids. The storage traps and recombination centres, each type characterized with an activation energy (trap depth) that depends on the crystalline solid and the nature of the trap, are located in the energy gap between the valence band and the conduction band. The states just below the conduction band represent electron traps, the states just above the valence band are hole traps. The trapping levels are empty before irradiation (i.e. the hole traps contain electrons and the electron traps do not). During irradiation the secondary charged particles lift electrons into the conduction band either from the valence band (leaving a free hole in the valence band) or from an empty hole trap (filling the hole trap). The system may approach thermal equilibrium through several means such as

● Free charge carriers recombine with the recombination energy converted into heat;

● A free charge carrier recombines with a charge carrier of opposite sign trapped at a luminescence centre, the recombination energy being emitted as optical fluorescence;

● The free charge carrier becomes trapped at a storage trap, and this event is then responsible for phosphorescence or the thermoluminescence and OSL processes.

THERMOLUMINESCENT DOSIMETER SYSTEMS

The TLDs most commonly used in medical applications are LiF:Mg,Ti, LiF:Mg,Cu,P and Li2B4O7:Mn, because of their tissue equivalence. Other TLDs, used because of their high sensitivity, are CaSO4:Dy, Al2O3:C and CaF2:Mn.

● TLDs are available in various forms (e.g. powder, chips, rods and ribbons).

● Before they are used, TLDs need to be annealed to erase the residual signal. Well established and reproducible annealing cycles, including the heating and cooling rates, should be used.

A basic TLD reader system consists of a planchet for placing and heating the TLD, a PMT to detect the thermoluminescence light emission and convert it into an electrical signal linearly proportional to the detected photon fluence and an electrometer for recording the PMT signal as a charge or current. A basic schematic diagram of a TLD reader is shown below.

• The thermoluminescence intensity emission is a function of the TLD temperature T. Keeping the heating rate constant makes the temperature T proportional to time t, and so the thermoluminescence intensity can be plotted as a function of t if a recorder output is available with the TLD measuring system.

SEMICONDUCTOR DOSIMETRY

SILICON DIODE DOSIMETRY SYSTEM

A silicon diode dosimeter is a p–n junction diode. The diodes are produced by taking n type or p type silicon and counter-doping the surface to produce the opposite type material. These diodes are referred to as n–Si or p– Si dosimeters, depending upon the base material. Both types of diode are commercially available, but only the p–Si type is suitable for radiotherapy dosimetry, since it is less affected by radiation damage and has a much smaller dark current. Radiation produces electron–hole (e–h) pairs in the body of the dosimeter, including the depletion layer. The charges (minority charge carriers) produced in the body of the dosimeter, within the diffusion length, diffuse into the depleted region. They are swept across the depletion region under the action of the electric field due to the intrinsic potential. In this way a current is generated in the reverse direction in the diode.

Diodes are used in the short circuit mode, since this mode exhibits a linear relationship between the measured charge and dose. They are usually operated without an external bias to reduce leakage current.

● Diodes are more sensitive and smaller in size than typical ionization chambers. They are relative dosimeters and should not be used for beam calibration, since their sensitivity changes with repeated use due to radiation damage.

● Diodes are particularly useful for measurement in phantoms, for example of small fields used in stereotactic radiosurgery or high dose gradient areas such as the penumbra region. They are also often used for measurements of depth doses in electron beams. For use with beam scanning devices in water phantoms, they are packaged in a waterproof encapsulation. When used in electron beam depth dose measurements, diodes measure directly the dose distribution (in contrast to the ionization measured by ionization chambers).

● Diodes are widely used in routine in vivo dosimetry on patients or for bladder or rectum dose measurements. Diodes for in vivo dosimetry are provided with buildup encapsulation and hence must be appropriately chosen, depending on the type and quality of the clinical beams. The encapsulation also protects the fragile diode from physical damage.

● Diodes need to be calibrated when they are used for in vivo dosimetry, and several correction factors have to be applied for dose calculation. The sensitivity of diodes depends on their radiation history, and hence the calibration has to be repeated periodically.

● Diodes show a variation in dose response with temperature (this is particularly important for long radiotherapy treatments), dependence of signal on the dose rate (care should be taken for different source to skin distances), angular (directional) dependence and energy dependence even for small variations in the spectral composition of radiation beams (important for the measurement of entrance and exit doses).

MOSFET DOSIMETRY SYSTEMS

A metal-oxide semiconductor field effect transistor (MOSFET), a miniature silicon transistor, possesses excellent spatial resolution and offers very little attenuation of the beam due to its small size, which is particularly useful for in vivo dosimetry. MOSFET dosimeters are based on the measurement of the threshold voltage, which is a linear function of absorbed dose. Ionizing radiation penetrating the oxide generates charge that is permanently trapped, thus causing a change in threshold voltage. The integrated dose may be measured during or after irradiation. MOSFETs require a connection to a bias voltage during irradiation. They have a limited lifespan.

● A single MOSFET dosimeter can cover the full energy range of photons and electrons, although the energy response should be examined, since it varies with radiation quality. For megavoltage beams, however, MOSFETs do not require energy correction, and a single calibration factor can be used.

● MOSFETs exhibit small axial anisotropy (±2% for 360º) and do not require dose rate corrections.

● Similarly to diodes, single MOSFETs exhibit a temperature dependence, but this effect has been overcome by specially designed double detector MOSFET systems. In general, they show non-linearity of response with the total absorbed dose; however, during their specified lifespan, MOSFETs retain adequate linearity. MOSFETs are also sensitive to changes in the bias voltage during irradiation (it must be stable), and their response drifts slightly after the irradiation (the reading must be taken in a specified time after exposure).

● MOSFETs have been in use for the past few years in a variety of radiotherapy applications for in vivo and phantom dose measurements, including routine patient dose verification, brachytherapy, TBI, intensity modulated radiotherapy (IMRT), intraoperative radiotherapy and radiosurgery. They are used with or without additional buildup, depending on the application.

 ASSIGNMENT

1. How you can protect yourself from radiation?
2. Mention the four most commonly used radiation dosimeters. What are their advantages and disadvantages?
3. A radioactive sample is counted for 1 minute and produces 900 counts. The background is counted for 10 minutes and produces 100 c ounts. The net count rate and net standard deviation are about ____ and ____ counts/min
4. How many counts must be collected in an instrument with zero background to obtain an error limit of 1% with a confidence interval of 95%?
5. The window setting used for Tc-99m is set with the center at 140 keV with a width of +/-14 keV i.e., 20%. What is the reason for this?