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Questions and Answers
What is the basic output for a clinical beam typically stated as?
What is the basic output for a clinical beam typically stated as?
In what scenario is the machine basic output usually measured in Gy/MU?
In what scenario is the machine basic output usually measured in Gy/MU?
Which statement best describes a relative dosimeter?
Which statement best describes a relative dosimeter?
What do dosimetry protocols or codes of practice provide guidelines for?
What do dosimetry protocols or codes of practice provide guidelines for?
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Which of the following factors is NOT typically part of the basic output calibration of a beam?
Which of the following factors is NOT typically part of the basic output calibration of a beam?
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What does radiation dosimetry primarily focus on measuring?
What does radiation dosimetry primarily focus on measuring?
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What is typically used to establish a patient's treatment setup on a treatment machine?
What is typically used to establish a patient's treatment setup on a treatment machine?
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Which of the following is a characteristic of basic output for kilovoltage x-ray generators?
Which of the following is a characteristic of basic output for kilovoltage x-ray generators?
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What is the reference point in an ionization chamber calibration?
What is the reference point in an ionization chamber calibration?
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Which of the following conditions must be fulfilled for the calibration factor to be applicable?
Which of the following conditions must be fulfilled for the calibration factor to be applicable?
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What must be done when influence quantities such as air pressure and humidity cannot be controlled?
What must be done when influence quantities such as air pressure and humidity cannot be controlled?
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What is the recommended depth of calibration for megavoltage photon beams?
What is the recommended depth of calibration for megavoltage photon beams?
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What is the significance of having at least 10 cm of water beyond the ionization chamber during calibration?
What is the significance of having at least 10 cm of water beyond the ionization chamber during calibration?
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Which of the following components is NOT an influence quantity in ionization chamber dosimetry?
Which of the following components is NOT an influence quantity in ionization chamber dosimetry?
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In the correction factor formula for air temperature and air pressure, what does the variable kT,P represent?
In the correction factor formula for air temperature and air pressure, what does the variable kT,P represent?
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What is the purpose of applying correction factors during ionization chamber calibration?
What is the purpose of applying correction factors during ionization chamber calibration?
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What are the three essentials provided by dosimetry protocols?
What are the three essentials provided by dosimetry protocols?
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Which type of calibration factors are used in dosimetry protocols?
Which type of calibration factors are used in dosimetry protocols?
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What is the primary measured quantity related to dose in an ionization chamber?
What is the primary measured quantity related to dose in an ionization chamber?
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In calibration, what does the symbol $N_{D,w,C0}$ represent?
In calibration, what does the symbol $N_{D,w,C0}$ represent?
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In the given calibration example, what is the beam quality used?
In the given calibration example, what is the beam quality used?
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What type of phantom is used in the calibration procedure described?
What type of phantom is used in the calibration procedure described?
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What should the position of the cylindrical chamber be in the water phantom during calibration?
What should the position of the cylindrical chamber be in the water phantom during calibration?
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What unit is used for the calibration factor $N_{D,w,Q0}$?
What unit is used for the calibration factor $N_{D,w,Q0}$?
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What does the calibration coefficient KQ correct for?
What does the calibration coefficient KQ correct for?
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Which formula is used to determine the absorbed dose to water in relation to the beam quality?
Which formula is used to determine the absorbed dose to water in relation to the beam quality?
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What is pivotal about the positioning of the ionization chamber in water?
What is pivotal about the positioning of the ionization chamber in water?
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What information can you derive from protocol tables for KQ?
What information can you derive from protocol tables for KQ?
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What is the phenomenon called when the use of potentials of opposite polarity in an ionization chamber yields different readings?
What is the phenomenon called when the use of potentials of opposite polarity in an ionization chamber yields different readings?
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What is the consequence of a chamber being positioned incorrectly?
What is the consequence of a chamber being positioned incorrectly?
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Which types of polarity effects are known?
Which types of polarity effects are known?
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What is necessary when measuring KQ for clinical beams?
What is necessary when measuring KQ for clinical beams?
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Under what circumstances are polarity effects negligible?
Under what circumstances are polarity effects negligible?
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Which aspect is NOT considered when determining KQ values?
Which aspect is NOT considered when determining KQ values?
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What does the polarity correction factor kpol represent?
What does the polarity correction factor kpol represent?
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What is an example of a measurement influenced by the calibration process?
What is an example of a measurement influenced by the calibration process?
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What should be done if the polarity correction factor kpol exceeds 3%?
What should be done if the polarity correction factor kpol exceeds 3%?
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If the user beam quality differs from the calibration quality, what is the first step that should be taken?
If the user beam quality differs from the calibration quality, what is the first step that should be taken?
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Which statement about charges produced in an ionization chamber is correct?
Which statement about charges produced in an ionization chamber is correct?
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What is the typical polarizing potential and polarity used during calibration?
What is the typical polarizing potential and polarity used during calibration?
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What primarily causes discrepancies in charge measurements in radiation chambers?
What primarily causes discrepancies in charge measurements in radiation chambers?
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What is the ideal ratio of V1 to V2 for deriving the correction factor KS for pulsed beams?
What is the ideal ratio of V1 to V2 for deriving the correction factor KS for pulsed beams?
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What is the significance of the TPR20,10 quality index in high energy photons?
What is the significance of the TPR20,10 quality index in high energy photons?
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Which coefficient is NOT part of the formula used to derive the correction factor KS?
Which coefficient is NOT part of the formula used to derive the correction factor KS?
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In continuous radiation, how is the correction factor kS typically estimated?
In continuous radiation, how is the correction factor kS typically estimated?
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Which type of radiation could be classified as low energy X-ray?
Which type of radiation could be classified as low energy X-ray?
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What is the purpose of measuring the charges M1 and M2 at different voltages V1 and V2?
What is the purpose of measuring the charges M1 and M2 at different voltages V1 and V2?
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Which of the following radiations is classified as a high energy photon?
Which of the following radiations is classified as a high energy photon?
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Study Notes
Photon Beam Calibration
- Accurate dose delivery with external photon or electron beams relies on a chain of processes
- Basic output calibration of the beam is a crucial link
- Relative dose data measurement procedures are essential
- Equipment commissioning and quality assurance are parts of the process
- Treatment planning is a necessary step
- Patient setup on the treatment machine is another essential aspect
Basic Output for a Clinical Beam
- Usually stated as dose rate for a point P
- Measured in Gy/min or Gy/MU
- Often at a reference depth Zref (the depth of dose maximum zmax)
- Within a water phantom
- Using a nominal source to surface distance (SSD) or source to axis distance (SAD)
- At a reference field size, typically 10x10 cm²
Machine Basic Output
- Usually given in Gy/min for kilovoltage X-ray generators and teletherapy units
- In Gy/MU for clinical linear accelerators
- For superficial and orthovoltage beams, sometimes given as air kerma rate (in Gy/min) at a given distance from the source and for a given collimator/applicator setting
Radiation Dosimetry
- Refers to the determination of absorbed dose to water under reference conditions in clinical beam of a radiation delivery unit
- Using calibration ionization chambers
Radiation Dosimeter
- Any device capable of providing a reading (M) that measures dose (D) deposited in its sensitive volume (V) by ionizing radiation
Dosimeter Types
- 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.
Calibration Procedure: Need for a Protocol
- Dosimetry protocols or codes of practice are needed for clinical photon or electron beam calibration procedures.
- Protocol selection depends on the individual department's needs.
Dosimetry Protocols
- National: UK's Institution of Physics and Engineering in Medicine and Biology (IPEMB), Germany's DIN 6800-2 and Deutsches Institut für Normung (DIN)
- Regional: AAPM (North America, TG-51), NCS (Netherlands & Belgium), and NACP (Scandinavia)
- International: IAEA (TRS 398)
Dosimetry Protocol Essentials
- Formalism (معالات): defines the method for calculations
- Procedure: outlines the step-by-step process
- Required data: tables assist in using calibrated ionization
Ionization Chamber
- Most practical and widely used for accurate machine output measurement in radiotherapy
- Can be used as an absolute or relative dosimeter
- Usually filled with ambient air
- Dose-related measured quantity: charge (Q)
- Dose rate-related measured quantity: current produced by radiation in the chamber sensitive volume
Principle of Calibration Procedure: Calibration and Calibration Coefficient
- Key calibration parameters are beam quality, field size, and SDD (source-to-detector distance) and phantom and depth
- For a given dose (Dw) at a 5 cm depth in a water phantom under specific calibration conditions there are factors to account for.
Calibration Chamber Positioning
- The chamber needs to follow precise positioning instructions based on the protocol
- Position of the chamber centre in the water phantom is crucial for accurate measurements
- Position should be referenced to a well-defined point inside the chamber.
- Chamber size and the beam range of charged particles must be considered to meet Bragg-Gray conditions.
Reference Conditions for Ionization Chambers
- Influences such as phantom material, phantom size, source-chamber distance (SCD), air temperature and pressure, the reference point of the ionization chamber, depth, field size, relative humidity, polarizing voltage, and polarity need to be accounted for in calibration
- Calibration coefficients and correction factors are needed for variations in influence quantities
- Total correction factors are the product of all individual correction factors
Ionization Chamber Based Dosimetry Systems
- Water is the standard phantom material for megavoltage photon and electron beam calibrations.
- Depth of calibration: typically 10 cm for megavoltage photons and reference depth (Zref) for electron beams
- To ensure adequate scattering, there should be a margin of at least 5 cm of water around the nominal field size and at least 10 cm beyond the chamber
Chamber Signal Corrections for Influence Quantities
- Influence quantities in ionization chamber dosimetry measurements include ambient air temperature, pressure, humidity, applied chamber voltage, polarity, chamber leakage currents, and chamber stem effects.
Chamber Signal Corrections: Polarity Effects
- Two types of polarity effects are voltage-dependent and voltage-independent.
- The effect is usually negligible for megavoltage photons, but may be significant with orthovoltage beams or in the buildup region of megavoltage photon beams, or in electron beams between the surface and the range Rp
- Use the mean of absolute values of measurements taken at opposite polarities to account for the polarity effect
Chamber Signal Corrections: Recombination Correction Factor
- In pulsed radiation (like linear accelerators), the dose rate is high, and recombination between ions can cause discrepancies between produced and collected charges.
- A recombination correction factor (Ks) is used to compensate for these effects.
- The factor Ks is determined using the two voltages method (measurements at different voltages).
- Values of coefficients for Ks are experimentally determined and tabulated in standards protocols
Chamber Voltage Effects: Recombination Correction Factor
- For continuous radiation (like Co-60), a different combination correction factor calculation may be necessary
Determination of Radiation Quality Q
- The TPR20,10 (Tissue Phantom Ratio) method and Alternative method using PDD20,10 are utilized to determine the quality index Q
- The method using TPR20,10 measures the ratio of the absorbed doses at depths of 20 and 10 cm
- The Alternative method with PDD20,10 determines dose ratios at 20 and 10 cm
- Methods used are important because they measure the beam quality in a way that is independent of the presence of any electron contamination (in the beam).
Summary: Beam Calibration of Photons Beams TRS 398
- The quality factor (kq) is given in tables within the protocol.
- For high energy photons, the beam quality (Q) is indicated by the TPR20,10, which can be calculated directly or determined from measured depth dose data.
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Description
Test your knowledge on the fundamental concepts of radiation dosimetry, including machine output, calibration, and dosimetry protocols. This quiz covers essential aspects of clinical beam measurement and patient treatment setup. Perfect for students and professionals in medical physics and radiation therapy.
