Radiation Dosimetry Lecture Notes PDF

Lecture Notes MHP 3: RADIATION DOSIMETRY Introduction Radiation dosimetry deals with the determination, by measurement or calculation, of dosimetric and field quantities resulting from the interaction of ionizing radiation with matter. The most relevant qua...

Lecture Notes MHP 3: RADIATION DOSIMETRY Introduction Radiation dosimetry deals with the determination, by measurement or calculation, of dosimetric and field quantities resulting from the interaction of ionizing radiation with matter. The most relevant quantities are KERMA and absorbed dose. Radiation dosimetry has its origin in the medical application of ionizing radiation starting with the discovery of x-rays by Roentgen in 1895. The need of protection against ionizing radiation and the application in medicine required quantitative methods to determine a “dose of radiation”. The purpose of a quantitative concept of a dose of radiation is to predict associated radiation effect and to reproduce clinical outcomes. Cavity Theory Cavity theory (cavity = detector) relates the mean absorbed dose in the sensitive material of the detector, 𝐷!"# to the absorbed dose at the reference point in the undisturbed medium, 𝐷$"!. Note: Cavity theory factor is determined under reference conditions (incident beam quality, field size, and position of detector in the medium). There is no need for cavity theory factor if the detector is irradiated in the reference condition i.e., field size, incident beam quality (𝑄% ), and position of detector in the medium. a) Bragg-Gray Cavity (for MV photon beam) o The cavity size is much smaller than the 𝑅&'() of secondary electrons o Photon interacts to the medium generating secondary electrons ‘crossers’ that will only cross the cavity. AD (BG cavity) => crossers o Electron fluence inside the cavity = electron fluence at the reference point in the medium (absence of 1  cavity) => iff knock-on equilibrium exists. Cavity acts as an electron fluence sensor (charged-particle sensor). *Equilibrium requirement in charged-particles is knock-on equilibrium which is less demanding than the full charged-particle equilibrium (CPE). The stopping power is electronic (collision) rather than total as we assumed that bremsstrahlung energy losses are assumed to escape from the region of interest. o Spencer-Attix cavity theory takes into account the production of 𝛿-rays (knock-on electrons) and cavity size. Introduce the mean cut-off energy, ∆ which is related to the cavity size. The ∆ is chosen such that an electron of this energy range is sufficient enough to cross the cavity. *Incident electrons all have energies greater than the mean cutoff energy and all energy loss less than the mean cutoff energy are treated as ‘local’ and are assumed to remain in the cavity or in the medium where they are created. o Cavity theory factor (physical quantities): 𝑠$"!,!"# +, 𝑆 § Bragg-Gray stopping power ratio: 𝑠$"!,!"# = [ "-+𝜌]$"! !"# ') 𝐿∆+ $"! § Spencer-Attix stopping power ratio: 𝑠$"!,!"# = [ 𝜌]!"# b) Large Cavity (for kV photon beam) o The mean chord length of the detector is large compared to the 𝑅&'() of secondary electrons. o Photon interacts to the cavity wall and/or cavity volume generating secondary electrons ‘stoppers, starters or insiders’. AD (large cavity => insiders: starts and stop within the cavity) o Electron fluence inside the cavity = electron fluence at the reference point in the medium (absence of cavity) => iff partial charged-particle equilibrium (PCPE) exists. *In a real photon beams, only PCPE can be achieved due to photon attenuation over a distance equal to the ranges of secondary charged particles. c) Intermediate-sized Cavity (in between kV and MV photon beam) o Detector size following two limiting cases of Burlin cavity theory: § Smaller than 𝑅&'() of secondary electrons and acts as a sensor of 𝛷"- in the undisturbed medium § Large compared to 𝑅&'() of secondary electrons and in which PCPE exists o Photon interacts to the medium, cavity wall and/or cavity volume generating secondary electrons ‘crossers, starters, insiders, and stoppers’ § AD (intermediate-size cavity => insiders, stoppers, starters and crossers). *Starters originate in the cavity and stop in the cavity wall, stoppers start in the cavity wall terminating in the cavity o Electron fluence inside the cavity = electron fluence at the reference point in the medium (absence of cavity) => iff charged-particle equilibrium exists 𝜇 o Cavity theory factor (physical quantities): linear combination of [ "/+𝜌]$"! !"# and 𝑠$"!,!"# 2  (!"# (") $"! § General or Burlin Cavity Theory: (#"$ = 𝜔+, [%'&]!"# #"$ + (1 + 𝜔+, )[ ' ]!"#. *𝜔+, is the weighting factor If the media of interest (x), wall (w) and sensitive volume (g) are matched (with respect to atomic composition and density state), the measurement directly provides the dose of the interest If the medium of wall is not matched with the medium of the sensitive volume, matching to the medium of interest depends on the cavity size => General or Burlin Cavity Theory Cavity Type SMALL LARGE INTERMEDIATE Theory Bragg-Gray Photon-Charged-Particle General or Burlin Equilibrium (CPE) Particle Type Photons (MV) and/or Photons (kV) Photons electrons Type justification Detector dimensions Detector dimensions large Detector dimension small relative to relative to secondary between small BG electron ranges electron ranges cavities and very large cavities for which wall influence is negligible Mean Chord Length of Mean Chord Length of Detector < RCSDA at E0 Detector > RCSDA at E0 Photon Interaction Photon interactions in Photon interactions in Photon interactions in medium is significant cavity is significant both cavity and medium significant Photon interactions in Photon interactions in Homogenous photon cavity negligible medium is negligible field exists everywhere in medium and cavity Fluence inside the Fluence of electrons Fluence of photons inside Fluence of electrons cavity inside cavity same with cavity same with entering from medium as undisturbed medium. undisturbed medium. it passes through cavity is attenuated exponentially. Fluence of electrons originating cavity builds up to equilibrium exponentially. KOE exists PCPE exists in cavity and CPE exists at all points in undisturbed medium the medium and cavity 3 How absorbed dose is By secondary electrons By secondary electrons By secondary electrons deposited in the that cross the cavity. that start and stop inside that: detector material (crossers) the cavity. (insiders) -start in cavity and stop in wall (starters) -start in wall and stop in cavity (stoppers), -crossers -insiders Physical quantities Stopping power of Mass-attenuation Both stopping power and needed to determine primary electrons in coefficient of photons in mass attenuation the medium-to- medium and detector. medium and detector coefficient of primary detector absorbed electrons and photons in dose ratio medium and detector Overview of Radiation Detectors and Measurements Detector response refers to the detector reading per unit of a given quantity (M/D => detector sensitivity). Meanwhile, calibration coefficient refers to the inverse of detector response or sensitivity. Absolute, Reference, and Relative Dosimetry: a) Absolute dosimetry: quantity of interest is determined from the fundamental principles consistent with the definition of the quantity and realized with a primary measurement standard. Primary measurement standard: permits the determination of the unit of quantity form its definition b) Reference dosimetry: quantity of interest is determined at the user’s facility using a detector (ionization chamber) that has been calibrated at a standards laboratory, under well-established reference measurement conditions. Reference detector: user’s chamber c) Relative dosimetry: quantity of interest is determined using relevant ratios and/or appropriate corrections, when measurements are made in the user’s beam under non-reference conditions, that is, conditions that are different from those for which the calibration coefficient is strictly applicable. Dosimeter is any device that is capable of providing reading M that is a measure of absorbed dose in the detector deposited in its radiation sensitive volume (RSV) by ionizing radiation. o To be useful as a radiation dosimeter => must possess at least one physical property that changes as a result of exposure in ionizing radiation, and this change must be quantifiable and reproducible o Desirable dosimeter properties: (a) signal that is linearly proportional to the energy imparted to the RSV of the instrument and (b) the smaller the volume of the RSV the greater is the spatial resolution o Undesirable dosimeter properties: dependence on signal on dose or dose rate (non-linearity); energy or radiation quality; and beam direction 4  o General characteristics of a dosimeter: § Reproducibility: indicates how closely it is likely to agree with the expectation value of a quantity being measured. A high-precision dosimeter is capable of excellent measurement reproducibility. Poor reproducibility results from: poor technique; hostile environment (high atmospheric humidity); and faulty associated equipment (ion chamber cables or electrometer) § Dose and dose-rate range: dose sensitivity § Stability (before irradiation): shelf life and time spent in its original place should be stable in time until irradiated § Energy dependence: dependence of the reading M per unit of the quantity to be determined (dose) on the quantum or KE of the radiation Several types of detector can be used for absolute, reference, and relative dosimetry; their design varies depending on their application Radiation detectors are classified into four major groups: (a) calorimeter; (b) ionization chamber; (c) chemical detectors; and (d) solid-state detectors Various Class and Types of Radiation Detectors Primary standard calibrates secondary standards, which in turn is used to calibrate user instruments. It is an instrument used to determine the physical quantity of interest without reference to any other instrument that can measure the same quantity (without the need to be calibrated). Primary Ionization Chamber Calorimeter Fricke Dosimeter Standard Free-air IC Cavity IC Water-type Graphite-Type Dosimeter Design, “Wall less” Solid graphite Vessel of water in Solid with an Made of ferrous Material, With an entrance wall enclosure absorbing sulphate in aerated Position in diaphragm Air-filled cavity stirrer graphite core aqueous solution Beam Inside chamber is Wall electrode With stirrer and separated by Solution placed in air and central heat exchanger vacuum gaps and a cylindrical cell, Collecting electrode Equipped with reflecting flasks, disks, or electrode parallel to (aluminum) rigid and surfaces cuvette made of beam Collecting waterproof Core equipped glass, quartz or - Plate electrodes electrode temperature probe with thermistor plastic (Parallel-Plate perpendicular to Positioned Positioned Positioned FAC) beam perpendicular to perpendicular to perpendicular to - Collecting Rod beam axis beam axis beam axis (Cylindrical FAC) Collecting electrode perpendicular to beam - Film Electrodes (Wide- Angle/Well-Type FAC) 5 Interaction Radiation ionizes Radiation Temperature of Temperature in Irradiation of the with chamber gas. produces water increases the core solution leads to Radiation When polarizing secondary when it is increases when it production of potential is applied electrons in the irradiated is irradiated ferric ions on one of the cavity. Electrons The temperature The The no. of ferric electrodes, electric attach to gas rise is proportional temperature rise ions produced is field is created. molecules to to dose. is proportional to proportional to Gas ions move form ions. dose. dose through the electric When Ferric ions have field and will be polarizing well-defined attracted to the potential is. absorption electrode and the applied on one of spectrum. charge will be the electrodes, Concentration of collected. electric field is ferric ions The amount of created. produced due to charge produced is Gas ions move radiation is proportional to through the quantified by dose. electric field and absorption will be attracted spectroscopy. to the central electrode and the charge will be collected. The amount of charge produced is proportional to dose. Quantity Charge Charge Temperature Temperature Optical Density Measured Measurement Electrometer Electrometer Thermistor Thermistor Spectrophotometer Device Absorbed Dose Determination Correction Ion Recombination Ion Heat Defect (khd) Vacuum Gaps Wall Factors (krecom) Recombination Heat Transfer Effect Perturbation Polarity Effects Polarity Effects (kht) (kpol) Axial Non- Perturbation (kp) Field Distortion uniformity of Photon Scatter Field (kan) (ksc) and Photon Fluorescence (kfl) Attenuation & Electron Loss (ke) Scattering (kwall) Diaphragm Corrections Advantages For determination For Direct For For of air kerma at determination of determination of determination of determination of energies of few keV air kerma and absorbed dose at a absorbed dose to absorbed dose to to few hundred keV absorbed dose to point in water water (by water water (by For photon conversion) Can be used for conversion) energies of 1 MeV For radiotherapy level above few and above radiotherapy Water equivalent hundred keV up standardization Can be used for to 1 MeV radiotherapy level 6  Radiation Reproducible characteristics similar to water Heat exchange with surrounding very low Electrical Heating Heat defect negligible Disadvantages Dependence of Effects of Low sensitvity Dose Insufficient measured current atmospheric Temperature conversion to sensitivity for on air density and conditions disturbance water proces practical use in water vapor content Ion Heat defect radiation recombination protection and polarity effects Ionization chamber (IC) Type Free-air Chamber Cavity Chambers Parallel-Plate Transmission Chamber Chamber Design & “Wall less” Solid wall: Type of cavity Any cavity or parallel- Material With an entrance graphite (air- chamber plate diaphragm equivalent) or With thin entrance With thick PMMA Inside chamber plastic (water- window entrance and exit plates is air equivalent) Have a bulky body coated with colloidal Plate electrodes If use for photon that serves as mini- graphite (Parallel-Plate beam free-in-air, scattering phantom. Electrical contacts FAC) wall thick enough to For electron beams, made by bronze leaf Film Electrodes produce CPE robust window, and spring (Wide- If use in solid or less bulky body Disk collector Angle/Well-Type liquid phantom, With aluminum FAC) thick or thin wall supporting rim Collecting Rod approach and use of In fixed position in the (Cylindrical FAC) build-up cap. head of x-ray generator Air-filled cavity or accelerator where the entire beam passes 7  Wall electrode and With guard ring In medical accelerators, central electrode positioned downstream (aluminum) from primary collimator, With insulator (PS upstream from beam- or Teflon) Shapes: Spherical, Cylindrical, Thimble Axial Symmetry modifying components. Use Primary Use as primary Use in electron For monitoring changes standard for standard above few beams in the beam output of x- determination of hundred keV to 1 Reference chamber ray generators and linear air kerma at few MeV for photon beams in accelerators as it passes keV to few Use as secondary, 10 to 50 kV. through chamber hundred keV reference, or field Low and medium instruments above x-rays 100 kV Co-60 Disadvantages Impractical to Impractical for Less stable at 100 Could interfere with construct an very low photon kV and above. beam and change the instrument where energies (x-rays Compensating effect dose monitored. equilibrium in air below 70 kV) due to of bulky body at this is achieved for energy dependence. energy no longer energies above effective. few hundred keV. Significant polarity (solid material is effect. employed, instead) Chemical Dosimeters Chemical Fricke Alanine Film Gel Dosimeter Radiographic Radiochromic Fricke Polymer Chemical Made of Uses Thin film Ferrous Polymer- Species ferrous organic made of grain Polydiacetyle sulphate in gel based and Material sulphate in crystalline emulsion of ne-based film structure aqueous gel used in aerated amino acid silver bromide with radiation with radiation construction aqueous alanine. with gelatin sensitive sensitive solution In the form protective monomer in monomer in Solution of pellets coating and gelatin matrix the gelatin placed in a acetate base. and coated matrix cylindrical with cell, flasks, polyester disks, or base. cuvette made of glass, quartz, or plastic Interaction Irradiation CH3- CH- Incident or Radiation Irradiation Radiation with of the COOH free secondary induces produces free induces Radiation solution radicals are charged polymerizatio radicals in the polymerizatio produced particles n of gel. n and cross- 8 (Chemical produces free after produce ion diacetylene The free linking of change and radicals. irradiation pairs in or near molecules radicals monomers in chemical The free The no. of grains causing produced will the gel matrix. reactions radicals free radicals converting Ag+ polydiacetyle react to involved) produced will produced is ions to Ag ne dye ferrous ions to react to proportional atoms polymers to produce ferric Concentration ferrous ions to dose. producing a be formed ions. changes in the to produce The no. of latent image. The optical The no. of dosimeter is ferric ions. free radicals The film is characteristic ferric ions quantified The no. of produced is developed s of the dye produced is optically. ferric ions quantified by converting all changes in the proportional to produced is obtaining the Ag+ ions to Ag sensitive dose proportional radical atoms and layer of the The to dose spectrum removing film when production of Ferric ions through bromine. exposed to ferric ions is have well- electron The latent radiation. measured by defined paramagneti image The more spin-lattice absorption c technique produced, or radiation relaxation rate. spectrum. (EPR) the degree of delivered to Concentratio film darkening the film, the n of ferric is related to less ions produced the dose and transmitting due to quantified the film radiation is optically. becomes. quantified by absorption spectroscopy. Quantity Optical Peak-to-peak Optical Density Optical NMR Optical Measured Density height of Density relaxation rate Density central portion of radical spectrum Measuremen Spectrophoto EPR Densitometer Flatbed color NMR Optical CT t Device meter spectromete scanner relaxometry r MRI Absorbed Use Dose sensitometric Determinati curve on Advantages For Biologically For 2D For 2D For 3D For 3D determinatio relevant dosimetry dosimetry dosimetry dosimetry n of absorbed representati High spatial High spatial Energy dose to water on of resolution resolution independent in For radiation Relative megavoltage reference damage in energy range. therapy living dependence radiation materials dosimetry Near and radiation water- biology equivalence Water- Stable equivalent response, suitable as 9  audit dosimeter For high- energy radiation Small energy dependence over wide energy range Small size Disadvantag Insufficient Low Elaborate Elaborate Involved Involved es sensitivity for sensitivity Processing Read-out processing and processing practical use Costly Nonlinear Nonlinear instrumentatio and in radiation equipment Response Response n instrumentati protection Strong UV-sensitive on Temperature Energy Energy Temperature dependence, Dependence Temperature dependence in Energy dependent Humidity Light sensitive kilovoltage dependence Costly read- effects Sensitive region in kilovoltage out device region Purity of solution Wall perturbation Solid-State Detectors Solid-State Thermoluminescence Optically Scintillation Semiconductor Dosimetry Dosimetry Dosimetry Stimulated Dosimetry (TLD) Luminescence Diode MOSFET Diamond Dosimetry Dosimeter Dosimeter Detector (OSL) Design, Lithium fluoride Aluminum Plastic or Diode in p-n Special type Diamond Components doped with oxide crystal organic junction made of FET made crystal with and Material magnesium and doped with scintillator with from doped of n-type titanium phosphor carbon. silicon For personal Powdered With 3 dosimetry, in powder and formed terminals, 2 p- electrode form made in disk into thin layer terminals shape attached to over a MgO reflector Teflon substrate polymer connected to a silicon or substrate. PMT via light germanium pipe or fiber. substrate type and 1 n- type and SiO2 insulating layer. Interaction As the radiation As the During Ionizing When Ionizing with cause ionization in radiation irradiation, light radiation MOSFET is radiation Radiation the material cause is emitted from incident on irradiated the produces electrons and holes ionization in the scintillating the diode will ff. occurs: electrons and will be trap on the the material material. produce holes in the phosphor. electrons and diamond. 10  The no. of charged holes will be The amount of electron and - build-up of Electrons are carriers trapped is trap on the light emitted is hole pairs trapped trapped in proportional to dose. lattice of the proportional to The no. of charges forbidden gaps crystal. dose. charged - increase in The no. of carriers number of charged produced is interface traps carriers proportional - increase in trapped is to dose. no. of bulk proportional oxide traps. to dose. Absorbed Heating the Exposing the The light is Applying Electron As radiation Dose phosphor at a certain crystal to optically reverse-biased hole pairs increases, the Determination temperature causes green light transmitted to voltage in the generated by trapped the electron and causes the the PMT n-terminal of radiation will electrons will holes to be released electron and through the the diode be trapped on reach and recombine. holes to be optical fiber causes the the interface equilibrium. The recombination released and The PMT electrons and between Si At that point, process causes an recombine. produces holes in the and SiO2 ionization emission of light. The current related diode to be causing a shift current is The intensity of recombination to the pulled out the in threshold proportional light emitted is process transmitted other side voltage to dose related to dose. causes an light signals. creating a The shift is emission of depletion proportional light. layer where to the no. of The current cannot trapped intensity of flow. charges which light emitted When is is related to radiation hits proportional dose. the depletion to dose. layer and generates electron and holes, current will flow the junction. Radiation- induced current is proportional to dose rate. Quantity Light Light Current Current Voltage Current Measured Measurement TLD Reader OSLD Reader Electrometer Electrometer Voltmeter Electrometer Device Advantages Linearity over High Very fast Instant read- Small size, Soft tissue- wide dose range sensitivity decay times out high spatial equivalence dose rate over wide For energy resolution Insensitive independent dose range spectrometry Linear to radiation reproducible linearity Linear response damage response reproducible response for High small size response wide range of sensitivity commercially small size doses Almost available commercially energy reusable & economic available independent 11  convenient readout reusable & economic response can be re- read. Disadvantages Supra linearity at 1 Light and Elaborate Energy and Energy and Dose rate -2 Gy heat sensitive instrumentation temperature directional dependence response can be elaborate Unknown LET dependence dependence Non- read once and processing dependence Angular Short life reproducibility annealed right after Cerenkov dependence of natural lack of uniformity light generation Degradation diamonds storage instability in optical fiber of sensitivity fading of response Degradation throughout light sensitive of response use change in sensitivity with a throughout use ccumulated dose 12