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Questions and Answers
What type of photon beam is characterized by having all photons with the same energy?
What type of photon beam is characterized by having all photons with the same energy?
- Heterogeneous photon beam
- Monoenergetic photon beam (correct)
- Isotropic photon beam
- Non-isotropic photon beam
Which type of photon source is typically classified as non-isotropic?
Which type of photon source is typically classified as non-isotropic?
- Gamma ray sources
- Monoenergetic sources
- Isotropic sources
- X-ray sources (correct)
In the megavoltage energy region, in which direction does the x-ray emission predominantly occur?
In the megavoltage energy region, in which direction does the x-ray emission predominantly occur?
- Perpendicular to the electron beam
- Forward direction towards the electron beam (correct)
- In all directions equally
- Backward direction away from the target
What law governs the propagation of a photon beam through air or vacuum?
What law governs the propagation of a photon beam through air or vacuum?
What complicates the determination of dose deposition in a patient during radiotherapy?
What complicates the determination of dose deposition in a patient during radiotherapy?
Which statement is true about gamma ray sources?
Which statement is true about gamma ray sources?
How is the photon spectrum defined in relation to a photon source?
How is the photon spectrum defined in relation to a photon source?
Which of the following best describes a heterogeneous photon source?
Which of the following best describes a heterogeneous photon source?
What is necessary for a successful outcome in patient radiation treatment?
What is necessary for a successful outcome in patient radiation treatment?
Which of the following factors does NOT affect the determination of dose at the reference point?
Which of the following factors does NOT affect the determination of dose at the reference point?
What occurs to the dose distribution of an external photon beam as it penetrates deeper into the tissue?
What occurs to the dose distribution of an external photon beam as it penetrates deeper into the tissue?
For megavoltage x-ray beams, how does the surface dose compare to the maximum dose at depth zmax?
For megavoltage x-ray beams, how does the surface dose compare to the maximum dose at depth zmax?
What type of ionization chambers are used to measure surface dose?
What type of ionization chambers are used to measure surface dose?
Which photon interaction contributes to the surface dose in addition to those from collimators?
Which photon interaction contributes to the surface dose in addition to those from collimators?
What defines the surface dose for superficial and orthovoltage beams?
What defines the surface dose for superficial and orthovoltage beams?
What is a characteristic value of surface dose for cobalt-60 gamma rays?
What is a characteristic value of surface dose for cobalt-60 gamma rays?
What is the primary component of exposure in air and dose to small mass of medium in air?
What is the primary component of exposure in air and dose to small mass of medium in air?
What collimator factor value is expected for a field size of 10×10 cm²?
What collimator factor value is expected for a field size of 10×10 cm²?
How does the collimator factor change as field size A exceeds 10×10 cm²?
How does the collimator factor change as field size A exceeds 10×10 cm²?
What defines the relative dose factor (RDF) for a given field size?
What defines the relative dose factor (RDF) for a given field size?
What statement is true regarding dose at point P under varying field sizes?
What statement is true regarding dose at point P under varying field sizes?
What is the expected value of RDF when A equals 10×10 cm²?
What is the expected value of RDF when A equals 10×10 cm²?
What does the scatter component depend on?
What does the scatter component depend on?
What is normalized to 1 for the nominal field of 10×10 cm²?
What is normalized to 1 for the nominal field of 10×10 cm²?
What is the characteristic of an equivalent field in radiation treatment?
What is the characteristic of an equivalent field in radiation treatment?
Which field shape is NOT used in radiotherapy?
Which field shape is NOT used in radiotherapy?
How is the dosimetric field size defined?
How is the dosimetric field size defined?
What is represented by the equation $a_{eq} = \frac{2ab}{a+b}$?
What is represented by the equation $a_{eq} = \frac{2ab}{a+b}$?
Which option best describes the geometric field size?
Which option best describes the geometric field size?
For a rectangular field, what does the equivalent square field represent?
For a rectangular field, what does the equivalent square field represent?
What shape is produced with special collimators during radiation therapy?
What shape is produced with special collimators during radiation therapy?
When calculating the equivalent circle for a square field, which formula is used?
When calculating the equivalent circle for a square field, which formula is used?
What are the two components of PDD?
What are the two components of PDD?
How does field size affect PDD?
How does field size affect PDD?
What remains independent of field size in PDD?
What remains independent of field size in PDD?
What does the Mayneord F factor allow for?
What does the Mayneord F factor allow for?
What type of relationship does the Mayneord F factor rely on?
What type of relationship does the Mayneord F factor rely on?
What is the characteristic of PDD behavior with increasing depth in water?
What is the characteristic of PDD behavior with increasing depth in water?
How do field dimensions and shape impact PDD?
How do field dimensions and shape impact PDD?
What happens to PDD when the field size is less than the range of laterally scattered secondary electrons?
What happens to PDD when the field size is less than the range of laterally scattered secondary electrons?
What happens to the dose at point P as the field size A increases?
What happens to the dose at point P as the field size A increases?
How is the relative dose factor RDF or scatter factor Sc,p expressed mathematically?
How is the relative dose factor RDF or scatter factor Sc,p expressed mathematically?
In the context of central axis depth doses, what is the percentage depth dose (PDD) normalized to?
In the context of central axis depth doses, what is the percentage depth dose (PDD) normalized to?
What is the relationship of PDD to depth in a phantom?
What is the relationship of PDD to depth in a phantom?
What happens to the depth of maximum dose as beam energy increases?
What happens to the depth of maximum dose as beam energy increases?
Which of the following parameters does NOT affect the percentage depth dose (PDD)?
Which of the following parameters does NOT affect the percentage depth dose (PDD)?
What is the effect of using extra shielding on an accessory tray or multileaf collimator (MLC) according to the given approximation?
What is the effect of using extra shielding on an accessory tray or multileaf collimator (MLC) according to the given approximation?
What does the term ‘photon beam energy’ refer to in the context of depth dose?
What does the term ‘photon beam energy’ refer to in the context of depth dose?
Flashcards
Photon Spectrum
Photon Spectrum
A graph that shows the number of photons at different energy levels within a beam.
Monoenergetic Photon Beam
Monoenergetic Photon Beam
All photons within the beam possess the same energy level.
Heterogeneous Photon Beam
Heterogeneous Photon Beam
Photons in the beam have various energy levels.
Isotropic Photon Source
Isotropic Photon Source
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Non-isotropic Photon Source
Non-isotropic Photon Source
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Inverse Square Law
Inverse Square Law
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Photon Beam Attenuation
Photon Beam Attenuation
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Photon Beam Scattering
Photon Beam Scattering
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Beam Reference Point
Beam Reference Point
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Surface Dose (Ds)
Surface Dose (Ds)
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Maximum Dose (zmax)
Maximum Dose (zmax)
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Exit Dose (Dex)
Exit Dose (Dex)
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Skin Sparing Effect
Skin Sparing Effect
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Depth of Maximum Dose (zmax)
Depth of Maximum Dose (zmax)
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Photon Beam Dose Distribution
Photon Beam Dose Distribution
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Scattered Photons
Scattered Photons
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Geometric Field Size
Geometric Field Size
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Dosimetric Field Size
Dosimetric Field Size
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Equivalent Square Field
Equivalent Square Field
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Equivalent Circular Field
Equivalent Circular Field
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Calculating Equivalent Square Side
Calculating Equivalent Square Side
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Calculating Equivalent Circle Radius
Calculating Equivalent Circle Radius
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Irregular Fields
Irregular Fields
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Field Size Categories
Field Size Categories
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Relative Dose Factor (RDF)
Relative Dose Factor (RDF)
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Collimator Factor (CF)
Collimator Factor (CF)
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Field Size Influence on Dose
Field Size Influence on Dose
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Primary Component of Dose
Primary Component of Dose
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Scatter Component of Dose
Scatter Component of Dose
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Percentage Depth Dose (PDD)
Percentage Depth Dose (PDD)
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Depth Dose Curve
Depth Dose Curve
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Dose Distribution
Dose Distribution
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Mayneord F Factor
Mayneord F Factor
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Study Notes
Photon Beams: Physical Aspects Part 1
- Photon sources for external beam therapy come in various types depending on the photon: gamma ray sources or X-ray sources
- Photon energy sources can be monoenergetic or heterogeneous
- Intensity distribution of photon sources can be isotropic or non-isotropic
- A plot of photon number per energy interval versus photon energy for a given source is called a photon spectrum
- Gamma ray sources are typically isotropic and yield monoenergetic photon beams
- X-ray targets produce heterogeneous photon spectra and are non-isotropic
- In superficial and orthovoltage energy areas, X-ray emission occurs predominantly at 90° to the electron beam
- In megavoltage energy areas, X-ray emission is primarily in the direction of the electron beam
- In external beam radiotherapy, photon sources are often modeled as point sources, and the resulting beams are considered divergent
- A photon beam propagating through air or a vacuum follows the inverse square law
- A photon beam passing through a phantom or patient is influenced by inverse square law, attenuation, and scattering within the medium
Penetration of Photon Beams into Patient
- Dose distribution in external photon beams follows a pattern:
- The beam enters the patient at the surface, delivering a surface dose (Ds)
- Beneath the surface, the dose rises rapidly to a maximum (Dmax) at depth (Zmax)
- It then decreases exponentially until it reaches a value (Dex) at the exit point
- The dose maximum depth (Zmax) is influenced by photon beam energy and field size.
- Surface dose (Ds) for megavoltage X-rays is lower than the maximum dose (Dmax) due to the skin-sparing effect
- For superficial and orthovoltage beams, Zmax = 0 and Ds = Dmax
- Surface dose measurements are made using parallel-plate ionization chambers
- Contributors to surface dose include scattering from collimators, flattening filters, air, and backscatter within the patient
- Typical surface dose values: Cobalt-60 (100%), superficial or orthovoltage X-rays (100%), 6 MV X-rays (15%), and 18 MV X-rays (10%)
Buildup Dose Region
- The region between the surface and Zmax is called the buildup region
- Buildup dose results from the relatively long range of secondary charged particles released by photon interactions within the patient, depositing their energy through Coulomb interactions.
Depth of Dose Maximum
- Zmax is influenced by photon beam energy: higher energy results in a deeper Zmax
- Zmax also varies based on field size: smaller fields have shallower Zmax, while larger fields result in deeper Zmax
Exit Dose
- Exit dose is the dose delivered at the beam exit point
- Near the exit point, the dose distribution slightly curves downward compared to an infinitely thick phantom, due to missing scatter contribution beyond the exit point. This effect is usually negligible
Radiation Treatment Parameters
- Key parameters in external beam photon therapy include treatment depth (z), field size (A), source-skin distance (SSD), source-axis distance (SAD), photon beam energy, number of beams, treatment time, and monitor units (MUs)
- Different dosimetric functions are used in different photon energy ranges, like PDD, RDF, PSF, CF, SF, S, TAR, SAR, TMR, TPR, and SMR
Radiation Beam Field Size
- Field size is classified as geometric and dosimetric
- The geometric field size is the projection of the collimator's distal end onto a plane perpendicular to the central axis
- The dosimetric field size is defined by the intercept between an isodose curve (often 50%) and a plane perpendicular to the central axis
- Equivalent square field from rectangular field, and an equivalent circle for a square field can be determined using area formulas
Collimator Factor
- Collimator factor (CF) is the ratio of exposure (or dose) rate to a point in air from a given field/size to a 10x10 cm² field. CF is normalized to 1 at the reference field size
- CF is >1 for larger fields and is <1 for smaller fields
Relative Dose Factor (RDF)
- RDF is the ratio of dose at point P for a certain field size (A cm²) to the dose at the same point using a reference field of 10x10 cm²
- RDF also depends on photon energy (hν)
- RDF considers the total scatter of the larger field size
- RDF is normalized to 1 for a 10x10 cm² field.
Central Axis Depth Dose (PDD)
- PDD represents dose distribution along the central axis in water (or a tissue equivalent phantom) for a given photon beam energy, field size, SSD/SAD
- PDD values are normalized to 100% at the maximum dose depth (Zmax)
- PDD values decrease exponentially with increasing depth from Zmax
- PDD is dependent on four parameters: depth (z), field size (A), SSD, photon beam energy (hv).
Mayneord F Factor
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The Mayneord F factor is an approximate correction factor that adjusts PDD values from one SSD to another. It is based on the inverse square law, ignoring scatter corrections. The formula for it is provided.
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Additional items like dosimetry equipment were mentioned, however, these are not sufficient for study notes, so they are omitted.
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Description
Test your knowledge on the characteristics of photon beams and their behavior in radiation therapy. This quiz covers topics like the types of photon sources, dose deposition complications, and the definitions of photon spectra. Perfect for students and professionals in the field of radiology and medical physics.
