Medical Physics Lecture 04 Dosimetry Systems PDF

M.Sc. MEDICAL PHYSICS Principles of Physics in Radiation Oncology - RAD 6135 Dosimetry Equipment Dr. KHALID IBRAHIM HUSSEIN INTRODUCTION ❑ Accurate dose delivery to the target with external photon or electron beams is governed by a chain consisting of the following main links:...

M.Sc. MEDICAL PHYSICS Principles of Physics in Radiation Oncology - RAD 6135 Dosimetry Equipment Dr. KHALID IBRAHIM HUSSEIN INTRODUCTION ❑ Accurate dose delivery to the target with external photon or electron beams is governed by a chain consisting of the following main links: ❑Basic output calibration of the beam ❑Procedures for measuring the relative dose data. ❑Equipment commissioning and quality assurance. ❑Treatment planning ❑Patient set-up on the treatment machine. M.Sc. Medical Physics Dr. Khalid I Hussein  INTRODUCTION ❑The basic output for a clinical beam is usually stated as: ❑ Dose rate for a point P in G/min or Gy/MU. ❑ At a reference depth zref (often the depth of dose maximum zmax). ❑ In a water phantom for a nominal source to surface distance (SSD) or source to axis distance (SAD). ❑ At a reference field size on the phantom surface or the isocentre (usually 10×10 cm2). M.Sc. Medical Physics Dr. Khalid I Hussein INTRODUCTION ❑Machine basic output is usually given in: ❑ Gy/min for kilovoltage x-ray generators and teletherapy units. ❑ Gy/MU for clinical linear accelerators. A monitor unit (MU) is a measure of machine output from a clinical accelerator for radiation therapy. Monitor units are measured by monitor chambers, which are ionization chambers that measure the dose delivered by a beam and are built into the treatment head of radiotherapy linear accelerators ❑For superficial and orthovoltage beams and occasionally for beams produced by teletherapy machines, the basic beam output may also be stated as the air kerma rate in air (in Gy/min) at a given distance from the source and for a given nominal collimator or applicator setting. M.Sc. Medical Physics Dr. Khalid I Hussein INTRODUCTION ❑Basic output calibration for photon and electron beams is carried out with: ❑ Radiation dosimeters ❑ Special dosimetry techniques. ❑Radiation dosimetry refers to a determination by measurement and/or calculation of: ❑ Absorbed dose or ❑ Some other physically relevant quantity, such as air kerma, fluence or equivalent dose at a given point in the medium. M.Sc. Medical Physics Dr. Khalid I Hussein INTRODUCTION ❑Radiation dosimeter is defined as any device that is capable of providing a reading M that is a measure of the dose D deposited in the dosimeter’s sensitive volume V by ionizing radiation. ❑Two categories of dosimeters are known: ❑ Absolute dosimeter the dose determined form the first principles- without reference to a another dosimeter. Example: Calorimeter ❑ Secondary /Relative dosimeter is the dosimeters that require calibration against a primary standard. Example: thimble chambers and plane-parallel ion chambers. TLD, diode and films are also secondary dosimeters. They required calibration against a calibration ion chamber and may require addition corrections for energy dependence and other conditions.  Ionization Chamber Based Dosimetry Systems Two types of ionization chamber are used for beam calibration: ❑Cylindrical (also called thimble) chambers are used in calibration o ❑Orthovoltage x-ray beams ❑Megavoltage x-ray beams ❑Electron beams with energies of 10 MeV and above ❑Parallel-plate (also called end window or plane-parallel) chambers are used in calibration of: ❑Superficial x-ray beams ❑Electron beams with energies below 10 MeV ❑Photon beams in the buildup region and surface dose M.Sc. Medical Physics Dr. Khalid I Hussein Reference dosimetry with ionization chambers ❑Ionization chamber is the most practical and most widely used type of dosimeter for accurate measurement of machine output in radiotherapy. ❑It may be used as an absolute or relative dosimeter. ❑Its sensitive volume is usually filled with ambient air and: ❑ The dose related measured quantity is charge Q, ❑ The dose rate related measured quantity is current I, ❑ produced by radiation in the chamber sensitive volume. Reference dosimetry with ionization chambers ❑ Measured charge Q and sensitive air mass mair are related to absorbed dose in air Dair by: Q  W air  Dair =   mair  e  ❑ W air /e is the mean energy required to produce an ion pair in air per unit charge e. ❑Currently, the value of W air /e for dry air is 33.97 eV/ion pair or 33.97 J/C. M.Sc. Medical Physics Dr. Khalid I Hussein Reference dosimetry with ionization chambers ❑Sensitive air volume or sensitive mass of air in ionization chamber is determined: ❑Directly by measurement (the chamber becomes an absolute dosimeter under special circumstances). ❑Indirectly through calibration of the chamber response in a known radiation field (the chamber is then used as a relative dosimeter). M.Sc. Medical Physics Dr. Khalid I Hussein Reference dosimetry with ionization chambers ❑It is generally assumed that a constant value of W air /e can be used for the complete photon and electron energy range used in radiotherapy dosimetry. ❑There is no direct experimental support for such an assumption, as the data available have been obtained only from measurements with Co-60 and Cs-137 gamma ray beams and 2 MV x ray beams. M.Sc. Medical Physics Dr. Khalid I Hussein Reference dosimetry with ionization chambers ❑Three types of ionization chamber may be used in reference dosimetry as absolute dosimeter: ❑Standard free air ionization chamber: for superficial and orthovoltage x rays (up to 300 kV). ❑Cavity ionization chamber: for energies in the range from 0.6 to 1.5 ❑Extrapolation chamber: for megavoltage photon and electron beams. ❑The “absoluteness” of dose determination with ionization chambers depends on the accurate knowledge of W air /e, the mean energy required to produce an ion pair in air. M.Sc. Medical Physics Dr. Khalid I Hussein Clinical beam calibration and measurement chain ❑Clinical photon and electron beams are most commonly calibrated with ionization chambers that ❑Are used as relative dosimeters. ❑Have calibration coefficients determined either in air or in water and are traceable to a national primary standards dosimetry laboratory (PSDL). ❑The chamber calibration coefficient essentially avoid the need for an accurate knowledge of the chamber sensitive volume. M.Sc. Medical Physics Dr. Khalid I Hussein Clinical beam calibration and measurement chain ❑Traceability of a calibration coefficient to a national PSDL implies that: ❑Either the chamber was calibrated directly at the PSDL in terms of: ❑Air kerma in air ❑Absorbed dose in water ❑Or the chamber was calibrated directly at an accredited dosimetry calibration laboratory (ADCL) or at a secondary standards dosimetry laboratory (SSDL) that traces its calibration to a PSDL. ❑Or the chamber calibration coefficient was obtained through a cross-calibration with another ionization chamber, the calibration coefficient of which was measured directly at a PSDL, an ADCL or an SSDL. IONIZATION CHAMBER BASED DOSIMETRY SYSTEMS Examples of typical ionization chambers: (a) Cylindrical chambers used for relative dosimetry. (b) Pinpoint mini-chamber and Co-60 buildup cap. (c) Farmer type cylindrical chamber and cobalt-60 buildup cap. (d) Parallel-plate Roos type electron beam chamber. M.Sc. Medical Physics Dr. Khalid I Hussein  IONIZATION CHAMBER BASED DOSIMETRY SYSTEMS ❑Ionization chamber is essentially a capacitor in which leakage current or leakage charge is induced through the action of a radiation beam. ❑Charge or current induced in the chamber are very small and are measured by a very sensitive charge or current measuring device called an electrometer. M.Sc. Medical Physics Dr. Khalid I Hussein  IONIZATION CHAMBER DOSIMETRY ❑An ionization chamber is basically a gas filled cavity surrounded by a conductive outer wall and having a central collecting electrode. Basic design of a cylindrical Farmer-type ionization chamber.  IONIZATION CHAMBER DOSIMETRY ❑ 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 directly, bypassing the collecting electrode. ❑ The guard electrode ensures improved field uniformity in the active or sensitive volume of the chamber (for better charge collection). IONIZATION CHAMBER DOSIMETRY  IONIZATION CHAMBER DOSIMETRY Cylindrical (thimble type) ionization chamber ❑Most popular design ❑Independent of radial beam direction ❑Typical volume between 0.05 − 1.00 cm3 ❑Typical radius ~27 mm ❑Length ~4 − 25 mm ❑Thin walls: ~0.1 g/cm2 ❑Used for: ❑electron, photon, proton, or ion beams.  IONIZATION CHAMBER DOSIMETRY Parallel-plate (plane-parallel) ionization chamber ❑ Parallel-plate chambers have the two electrodes in the shape of flat plates to each other. The air gap between the two electrode constitutes the sensitive volume. ❑The parallel-plate chamber is recommended for dosimetry of electron beams with energies below 10 MeV. ❑It is useful for depth dose measurements. ❑It is also used for surface dose and depth dose measurements in the build-up region of megavoltage photon beams. M.Sc. Medical Physics Dr. Khalid I Hussein  Other Ionization Chamber Dosimetry ❑Brachytherapy chamber : Is a Well type chamber. ❑High sensitivity (useful for low rate sources as used in brachytherapy) ❑Large volumes (about 250 cm3) ❑Extrapolation chambers: Are parallel-plate chambers with a variable electrode separation. ❑Used in absolute radiation dosimetry (when embedded into a tissue equivalent phantom). ❑Segmented chamber ❑Calibrated in terms of absorbed dose ❑Volume of each: 5 mm x 5 mm x 4 mm  Film Dosimetry Film DOSIMETRY Radiographic X ray film performs important functions, e.g. in: ❑Diagnostic Radiology ❑Radiation protection. ❑Radiotherapy: ❑Qualitative and quantitative dose measurements (including electron beam dosimetry) ❑Quality control of radiotherapy machines ❑Verification of treatment techniques in various phantoms ❑Portal imaging. ❑Staff and patient dose measurements  LUMINESCENCE DOSIMETRY ❑When this energy is subsequently released in the form of ultraviolet, visible or infrared light, this phenomenon is called luminescence. ❑There are two types of luminescence: ❑ Fluorescence ❑ Phosphorescence ❑The difference depends on the time delay between the stimulation and the emission of light: ❑ Fluorescence has a time delay between 10−10 to 10−8 s ❑ Phosphorescence has a time delay exceeding 10−8 s ❑The process of luminescence 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. ❑If the exciting agent is light, the phenomenon is referred to as optically stimulated luminescence (OSL)  LUMINESCENCE DOSIMETRY Thermoluminescent dosimeter systems ❑TL dosimeters most commonly used in medical applications are (based on their tissue equivalence): ❑LiF:Mg,Ti ❑LiF:Mg,Cu,P ❑Li2B4O7:Mn ❑Other TLDs are (based on their high sensitivity): ❑CaSO4:Dy ❑Al2O3:C ❑CaF2:Mn ❑Before use, TLDs have to be annealed to erase any residual signal. M.Sc. Medical Physics Dr. Khalid I Hussein  LUMINESCENCE DOSIMETRY Optically stimulated luminescence systems ❑ Optically stimulated luminescence (OSL) is based on a principle similar to that of the TLD. Instead of heat, light (from a laser) is used to release the trapped energy in the form of luminescence. ❑ OSL is a novel technique offering a potential for in vivo dosimetry in radiotherapy. ❑ A further novel development is based on the excitation by a pulsed laser (POSL). ❑ The most promising material is Al2O3:C. ❑ To produce OSL, the chip is excited with a laser light through an optical fiber and the resulting luminescence (blue light) is carried back in the same fiber, reflected through a 90° by a beam-splitter and measured in a photomultiplier tube. M.Sc. Medical Physics Dr. Khalid I Hussein  LUMINESCENCE DOSIMETRY Optically stimulated luminescence systems Crystal: 0.4 mm x 3 mm Optical fiber read out  Class activity (Group 5 presentation) ❑ Basic Principle and applications of: ❑ Absolute dosimeter : Calorimeter ❑ Semiconductor dosimetry M.Sc. Medical Physics Dr. Khalid I Hussein  Phantom ❑Phantom is a common name for materials that are used to replace the patient in studies of radiation interactions in patients. ❑Phantom material should meet the following criteria: ❑Absorb photons in the same manner as tissue. ❑Scatter photons in the same manner as tissue. ❑Have the same density as tissue. ❑Contain the same number of electrons per gram as tissue. ❑Have the same effective atomic number as tissue. M.Sc. Medical Physics Dr. Khalid I Hussein  Phantom ❑Water is the standard and most universal phantom material for dosimetry measurements of photon and electron beams. ❑For photon beams, tissue equivalency or water equivalency implies a match in: ❑Mass-energy absorption coefficient ❑Mass stopping power ❑Mass scattering power ❑For electron beams, water equivalency implies a match in: ❑Linear stopping power ❑Linear scattering power M.Sc. Medical Physics Dr. Khalid I Hussein  Phantom ❑Water is recommended as the phantom material for the calibration of megavoltage photon and electron beams. ❑Depth of calibration is: ❑10 cm for megavoltage photon beams. ❑Reference depth zref for electron beams. ❑To provide adequate scattering conditions there must be: ❑A margin on the phantom around the nominal field size at least 5 cm of water in all directions. ❑At least 10 cm of water beyond the chamber.  Phantoms ❑This "solid water" has radiation characteristics very close volumetrically to those of water. When it is used as a dosimetry phantom for x- and gamma-ray beams in the radiotherapy range, phantom-to-water corrections and density corrections are eliminated..