Lect 08 Photon & Electron Dosimetry PDF

M.Sc. MEDICAL PHYSICS Calibration of Photon Beams 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 ❑ Procedu...

M.Sc. MEDICAL PHYSICS Calibration of Photon Beams 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.  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).  INTRODUCTION ❑ Machine basic output is usually given in: ❑Gy/min for kilovoltage x-ray generators and teletherapy units. ❑Gy/MU for clinical 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.  INTRODUCTION Radiation dosimetry Radiation dosimetry refers to a determination by measurement and/or calculation of Absorbed dose to water under reference conditions in clinical beam of a radiation delivery unit (accelerator), using a calibration ionization chambers.  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 produces a signal from which the dose in its sensitive volume can be determined without requiring calibration in a known radiation field. ◦ Relative dosimeter requires calibration of its signal in a known radiation field. Principles of the calibration procedure: Need for a Protocol ❑ Dosimetry protocols or code of practice state procedure to be followed when calibrating a clinical photon or electron beam. ❑ The selection of protocol is the choice of individual department. Dosimetry Protocols  Dosimetry Protocol ❑ Dosimetry protocols provides three essentials: ❑ The formalism ❑ The procedure ❑ The required data, tables for the use of calibrated ionization ❑ Two types of protocol are using now a day based on: ❑ Calibration factors in air kerma ❑ Calibration factors in absorbed dose to water.  Dosimetry Protocols ❑ 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.  Dosimetry Protocols Examples of dosimetry protocols International: International Atomic Energy Agency (IAEA) Principle of calibration Procedure: Calibration and Calibration Coefficient For a given dose Dw at 5cm depth of water phantom under a specific calibration conditions Beam quality 60Co gamma radiation Field size: 10cm x10cm SDD: 100cm Phantom: Water Phantom Depth 5cm Position of a cyl. chamber: Central electrode at measuring depth Position of plane-parallel Centre of front surface of inner air cavity Principle of calibration Procedure: Calibration and Calibration Coefficient ❑ The cylindrical user chamber is then place with center at a depth of cm in a water phantom. ❑ The calibration factor Nd,w is obtained by 𝐷𝑊 𝑁𝐷,𝑤,𝐶𝑜 = 𝑀 ❑ Where M is the dosimeter reading. Unit: Gray per reading (Gray/Coulomb) Principle of calibration Procedure: Calibration and Calibration Coefficient ❑ The absorbed dose to water at a reference depth zref in water for reference beam of quality Q0 (Co) can obtain by. 𝐷𝑤,𝐶𝑜 = 𝑀𝑄0 × 𝑁𝐷,𝑊,𝑄0 ❑ Where 𝑀𝑄0 is the reading of the dosimeter corrected for influence quantities to the reference conditions as used at calibration. ❑ 𝑁𝐷,𝑊,𝑄0 is the calibration factor in terms of absorbed dose to water of the dosimeter obtained from the standard laboratory. Principle of calibration Procedure: Calibration and Calibration Coefficient ❑ Example of Calibration Certificate Principle of calibration Procedure: Calibration and Calibration Coefficient ❑ The chamber can also be used for different beam quality Q such as ❑ High photon energy ❑ High electron energy. ❑ Which is differs from standard quality 60Co ❑ The formula is changed From 𝐷𝑤,𝐶𝑜 = 𝑀𝑄0 × 𝑁𝐷,𝑊,𝑄0 To 𝐷𝑤,𝐶𝑜 = 𝑀𝑄0 × 𝑁𝐷,𝑊,𝑄0 × 𝐾𝑄,𝑄0 Where 𝐾𝑄,𝑄0 is a factor correcting for the differences between the reference beam quality Q0 and the actual user quality Q Principle of calibration Procedure: Calibration and Calibration Coefficient ❑ How to get the beam quality factor KQ ❑ An experimentally obtained Kq is available from standard laboratory. ❑ If the measured KQ is difficult to obtain for realistic clinical beams, calculated correction factors can be used, which is normally provided in dosimetry protocols.. Principle of calibration Procedure: Calibration and Calibration Coefficient ❑ Values for KQ are dependent on the quality of radiation (type, energy, machine). ❑ Type of ionization chamber needs a particular KQ ❑ Values for KQ are given in protocol tables for a large variety of beam qualities and chambers (e,g in TRS 398) Principle of calibration Procedure: Calibration and Calibration Coefficient ❑ The absorbed dose to water is to be determine in a point P in water at the reference depth zref. ❑ Using the chamber, the dose is given by 𝐷𝑤,𝑄 (𝑃) = 𝑀𝑄 × 𝑁𝐷,𝑊,𝑄0 × 𝑘𝑄 ❑ How the chamber must be positioned? Principle of calibration Procedure: Position of the ionization chamber in water ❑ According to the Bragg-Gray condition 1: The cavity must be small when compared with the range of charged particles, so the presence does not perturb the fluence of charged particles in the water. Principle of calibration Procedure: Position of the ionization chamber in water ❑ Which positioning is correct? ❑ One may think that the correct way is the positioning of the chamber at the effective point of measurement. Principle of calibration Procedure: Position of the ionization chamber in water ❑ It does not matter as long as the positioning is well defined, and any deviation of the correct position is taken into account in the calibration factor 𝑁𝐷,𝑊,𝑄0 , or in the quality correction factor kQ. ❑ Positioning of the chamber must be referred to a well-defined point within the chamber, which is known as reference point of the chamber. Principle of calibration Procedure: Position of the ionization chamber in water ❑ Measurement of charge under reference condition. ❑ The calibration factor and the quality correction factor are applicable if the reference conditions are fulfilled. ❑ The reference conditions are described by a set of values influence these quantities. Principle of calibration Procedure: Reference conditions for the calibration of ionization chambers Principle of calibration Procedure: Reference conditions for the calibration of ionization chambers ❑ Some influence quantities can not be controlled, for example air pressure and humidity. ❑ So, the change of the influence quantities required to apply appropriate correction factors. ❑ The total correction factors is product of all correction factors ❑ 𝑀𝑞 = 𝑀𝑄𝑟𝑎𝑤 × ς 𝑘𝑖 IONIZATION CHAMBER BASED DOSIMETRY SYSTEMS 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. CHAMBER SIGNAL CORRECTIONS FOR INFLUENCE QUANTITIES Examples of influence quantities in ionization chamber dosimetry measurements are: ◦ Ambient air temperature ◦ Ambient air pressure ◦ Ambient air humidity ◦ Applied chamber voltage ◦ Applied chamber polarity ◦ Chamber leakage currents ◦ Chamber stem effects  CHAMBER SIGNAL CORRECTIONS ❑ In the user’s beam, the correction factor for air temperature and air pressure kT,P is: 273.16 + T Pn kT,P =  273.16 + Tn P ❑ This correction factor is applied to convert the measured signal to the reference (normal) conditions used for the chamber calibration at the standards laboratory: ❑T and P are chamber air temperature (oC) and pressure at the time of measurement. ❑Tn and Pn are chamber air temperature (oC) and pressure for the normal conditions at the standards laboratory.  CHAMBER SIGNAL CORRECTIONS ❑Under identical irradiation conditions the use of potentials of opposite polarity in an ionization chamber may yield different readings. This phenomenon is called the polarity effect. ❑When user quality is the same as the calibration quality (normally Co-60) ❑The chamber is used at the same polarizing potential and polarity as used during the calibration. ❑When a chamber is used in a beam that produces a measurable polarity effect, the true reading is taken to be the mean of the absolute values of readings taken at the two polarities.  CHAMBER SIGNAL CORRECTIONS ❑Two types of polarity effect are known: ❑Voltage dependent ❑Voltage independent ❑Basic characteristics of the polarity effects: ❑They are negligible for megavoltage photon beams at depths beyond the depth of dose maximum; i.e., at z > zmax. ❑They can be significant for orthovoltage beams and in the buildup region of megavoltage photon beams. ❑They are present in electron beams at all depths between the surface and the practical range RP.  CHAMBER SIGNAL CORRECTIONS Polarity correction factor kpol is defined as: M+ (V ) + M− (V ) kpol (V ) = 2M ◦ M+ is the chamber signal obtained at positive chamber polarity ◦ M- is the chamber signal obtained at negative chamber polarity ◦M is the chamber signal obtained at the polarity used routinely (either positive or negative). If the polarity correction factor kpol for a particular chamber exceeds 3 %, the chamber should not be used for output calibration.  CHAMBER SIGNAL CORRECTIONS If the user beam quality is not the same as the calibration quality, then user must first estimate the polarity correction factor kpol that was not applied at time of calibration using the same polarizing potential and polarity as was used at the calibration laboratory ( using C0-60 or 6MV). The correction factor then calculated using: In the same way, the polarity effect at the user beam quality 𝑘𝑝𝑜𝑙 can be determined. The correct polarity correction then is given by:  CHAMBER SIGNAL CORRECTIONS ❑Charges produced in an ionization chamber by radiation may differ from the charges that are actually collected in the measuring electrode. ❑These discrepancies (charge loss caused by charge recombination or excess charge caused by charge multiplication and electrical breakdown) occur as a result of: ▪ Constraints imposed by the physics of ion transport in the chamber sensitive volume. ▪ Chamber mechanical and electrical design.  Chamber voltage effects: recombination correction factor In pulsed radiation as in linear accelerator, the dose rate during a pulse is relatively high and general recombination is often significant. According to the IAEA Code of practice the correction factor KS for pulsed beams be derived using the two voltages method: 𝑀 𝑀1 2 𝑘𝑆 = 𝑎0 + 𝑎1 1 + 𝑎2 𝑀2 𝑀2 where the values of the collected charges M1 and M2 are measured at the polarizing voltage V1 and V2, respectively. a0, a1, and a3 are the fitted coefficients V1 is the normal operating voltage and V2 a lower voltage. The ratio V1/V2 should ideally be equal to or larger than 3. Chamber voltage effects: Recombination correction factor Chamber voltage effects: recombination correction factor ❑ In continuous radiation such as Co-60 the combination correction factor can be estimated using: 𝑉1 /𝑉2 2 −1 𝑘𝑆 = 𝑉1 /𝑉2 2 −𝑀1 /𝑀2 Chamber voltage effects: recombination correction factor Radiation quality may refer to: ❖ Low energy X-ray (um energy p to 100kV). ❖Medium energy X-rays (above 80KV) ❖60Co gamma radiation ❖High energy photons(1-50MV) ❖Electrons (3-50MeV) ❖Protons (50-250MeV ❖Heavy ions. Chamber voltage effects: recombination correction factor Definition of the quality index Q foe HE photons ❖ For high energy photons produced by clinical accelerators the beam quality Q is specified by the tissue phantom ratio TPR20,10 ❖This is the ratio of the absorbed doses at depths of 20 and 10 cm in a water phantom, measured with a constant SCD of 100cm and a field size of 10cm x 10cm at the plane of the chamber. ❖The most important characteristic of the beam quality index TPR20,10 is its independence of the electron contamination in the incident beam. Determination of radiation quality Q Method using TPR20,10 Determination of radiation quality Q Alternative Method using PDD20,10  Summary: Beam Calibration of Photons Beams TRS 398 1. Calibration formula: 𝐷𝑤,𝑄 (𝑃) = 𝑀𝑄 × 𝑁𝐷,𝑊,𝑄0 × 𝑘𝑄 2. Positioning of chamber: following instruction of the protocol: 1. For depth dose measurement: position the effective point of the chamber at the measuring depth. 2. For beam calibration measurements: Position the reference point of the chamber at measurement depth. 3. The most important correction factors required to meet the reference conditions are: 1. kT,P for air density. 2. kpol for polarity effects. 3. kS for recombination effect. Summary: Beam Calibration of Photons Beams TRS 398 1. The quality factor kQ is given in tables provided in the protocol (TRS 398(: 2. For high energy photons produced by clinical accelerators, the beam quality Q is specified by the TPR20,10, which can be measured directly or determined by the depth dose methods.