Photon Dosimetry concepts and Calculations

1
Photon Dosimetry concepts and Calculations

• Chapter 21
• W/L

2
Dose Calculation

• The treatment planning team has to quantify the
  overall prescribed dose of radiation and
  determine how much dose will be delivered over
  the time frame outlined.
• There are many parameters of photon beam
  calculation.

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Monitor Units

• Monitor Units (MU) a measure of output for
  linear accelerators.
• The dose rate varies slightly from one moment to
  the next
• Normally the dose rate for the linear accelerator
  is 1.0 cGy/MU for a 10 x 10 field size defined at
  the isocenter.

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Radiation Therapy Prescription

• Radiation Therapy Prescription
• Legal document defines the treatment volume
• Intended tumor dose (TD)
• Number of treatments
• Dose per treatment
• Frequency of treatment
• States the type and energy of radiation
• Beam-shaping devices such as wedges and
  compensators
• Any other appropriate factors.
• Clear, precise and complete

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Isodose Plan

• Isodose Plan part of the prescription
• May show field sizes
• Machine angels
• Doses
• Beam weighting
• Wedges compensators or blocks

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Treatment Time

• Treatment time length of time a unit is
  physically left on to deliver a measured dose.
• Factors considered
• Beam energy
• Distance from the source of radiation
• Field size

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Dose

• Dose (absorbed dose) measured at a specific
  point in a medium and refers to the energy
  deposited at that point.
• Measured in gray (Gy), which is defined so that 1
  Gy equals 1J/kg.

8
Depth

• Depth distance beneath the skin surface where
  the prescribed dose is to be delivered.
• Opposed fields patients midplane is used
• Multiple field arrangements isocenter used
• Depth affects measurements of dose attenuation.

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Separation

• Separation a measurement of the patients
  thickness from the point of beam entry to the
  point of beam exit.
• Normally measured along the beams central axis.
• Calipers
• ODI readings

10
Source Distance

• Source to Skin Distance (SSD) the distance from
  the source or target of the treatment machine to
  the surface of the patient.
• Source-axis Distance the distance from the
  source of photons to the isocenter.

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Setup

• SSD Isocenter established at the patients skin
  surface
• When the gantry rotates around the patient, the
  SSD will continually change.
• Dose calculations often at DMAX. (Given dose)
• SAD Isocenter established within the patient
• The SAD and the isocenter are at a fixed distance
  and therefore do not change.

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Isocenter

• Isocenter the intersection of the axis of
  rotation of the gantry and the axis of rotation
  of the collimator for the treatment unit.
• Cobalt 60 SAD of 80 cm
• Linear Accelerators SAD of 100 cm

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Field Size

• Field Size the physical size set on the
  collimator of the therapy unit that determines
  the size of the treatment field at a reference
  distance (defined at the machines isocenter)
• SAD the field size set inside the patient (size
  measured on patients skin will be smaller)
• SSD field size set on the collimator will be the
  same measured at the patients skin

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Scatter

• Backscatter radiation that is deflected back
  toward the patient
• Most of the absorbed dose received by the patient
  results from the collisions of the scattered
  electrons produced when the primary photon
  interacts with the collimator.

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DMAX

• DMAX the depth at which electronic equilibrium
  occurs for photon beams the point where dose
  reaches its maximum value.
• Mainly depends on the energy of the beam
• The depth of maximum ionization increases as the
  energy of the beam increases.
• Factors such as field size and distance may also
  influence the depth

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Photon Energy DMAX (cm)
Superficial 0.0
Orthovoltage 0.0
Cesium-137 0.1
Radium-226 0.1
Cobalt-60 0.5
4MV 1.0
6 MV 1.5
10 MV 2.5
15 MV 3.0
20 MV 3.5
25 MV 5.0
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DMAX

• DMAX dose occurs at the same depth for a given
  energy regardless of field size or distance from
  the source.
• The actual reading differs for different field
  sizes.

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Output

• Output the dose rate of the machine, the amount
  of radiation exposure produced by a treatment
  machine or source as specified at a reference
  field size and at a specified reference distance.
• Changing the field size, distance, or attenuating
  medium will change the dose rate.
• Increases with field size primary component the
  same, increased scatter adds to the output
• If the distance increases, dose rate decreases
  due to ISL

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Cobalt-60 Output

• Dose rate for Co-60 machine in cGy/min
• Dose rate due to the radioactive decay of the
  isotope Co-60
• Can be assumed constant over short periods of
  time
• The Time that a machine is left On is adjusted by
  1 increase every month
• TO2 TO1 x 1.01

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Linac Output

• Dose rate for Linac in cGy/MU
• Dose rate varies from one moment to the next
• Ionization chamber shuts down machine after
  predetermined dose has been given

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Output Factor

• Output Factor the ratio of the dose rate of a
  given field size to the dose rate of the
  reference field size
• Allows for the change in scatter as the
  collimator setting changes
• Relates the dose rate of a given collimator
  setting to the dose rate of the reference field
  size

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Dose Rate

• Commonly measured at the isocenter of the
  treatment machine.
• The dose rate of the beam is inversely
  proportional to the square of the distance
• Dose rate(Given f.s.) Dose rate(Reference f.s.)
  x Output factor(Given f.s.)
• Dose Rate
• Output (of machine)
• Output factor (C.S. field size)
• Scatter Ratio BSF or PSF (EFS/CS)
• PDD(EFS)/100
• Tray factor
• Inverse square correction

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Equivalent Squares

• Equivalent Squares rectangular field size that
  demonstrate the same measurable scattering and
  attenuation characteristics of a square field
  size.
• Used to find the output, output factor, and
  tissue absorption factors.
• 4 (A/P)

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Effective Field Size

• Effective Field Size (EFS) the equivalent
  rectangular field dimension of the open or
  treated area within the collimator field
  dimensions when blocks are used to customize the
  shape of the treatment area.
• The EFS equivalent square is normally used to
  determine the tissue absorption factors.

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Tissue Absorption Factors

• Tissue absorption factors different methods for
  measuring the attenuation of the beam as it
  travels through matter.
• Percent Depth Dose (PDD)
• works best with SSD setups
• Tissue Air Ratio (TAR)
• Tissue Phantom Ratio (TPR)
• Tissue Maximum Ratio (TMR)

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Percent Depth Dose (PDD)

• Percent Depth Dose (PDD) the ratio expressed as
  a percentage, of the absorbed dose at a given
  depth to the absorbed dose at a fixed reference
  depth usually DMAX.
• Calculated from two measurements at two different
  points in space.
• Requires SSD be constant.
• Written as PDD(d,s,SSD)
• Dependant on
• ? Energy- more penetrating- ? PDD
• ? Depth- ? PDD due to attenuation through matter
• ? Field size- more scatter- ? PDD
• ? SSD- ? PDD- ISL

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Percent Depth Dose (PDD)

• PDD Dose _at_ d .
• Dose _at_ Dmax

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Tissue Air Ratio (TAR)

• Tissue Air Ratio (TAR) the ratio of the absorbed
  dose at a given depth in tissue to the absorbed
  dose at the same point in air.
• Dependant of
• ?Energy- ?TAR
• ?Field size- ? TAR
• ?Depth- ? TAR
• Independent of SSD

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Tissue Air Ratio (TAR)

• Calculated using two measurements at the same
  point in space
• When the depth in tissue corresponds to the level
  of DMAX, the TAR is known as the backscatter
  factor.
• Build-up cap device made of acrylic or other
  phantom material that is placed over an
  ionization chamber to produce conditions of
  electronic equilibrium.

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Tissue Air Ratio (TAR)

• TAR Dose in tissue
• Dose in Air
• Normally used to perform calcs for SAD
  treatments of low energy machines.
• There is no patient backscatter in the in
  Air measurements.

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Backscatter Factor (BSF)

• When blocks are used.
• Backscatter Factor (BSF) the ratio of the dose
  rate with a scattering medium to the dose rate at
  the same point without a scattering medium at the
  level of maximum equilibrium. (TAR at the level
  of DMAX)
• PSF for megavoltage units.

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Backscatter Factor (BSF)

• The change in dose due to a change in field
  size.
• Readings are made at Dmax and compared to the
  dose in air.
• ?FS ? ?Backscatter ? ?BSF

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Dose to a Point

• The dose to any point in a medium is made up of
  two parts
• Primary radiation the photons that originate in,
  and fan out from the source.
• Scatter radiation other parts in the medium
  interact with the primary photons.

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Scatter Air Ratio (SAR)

• Scatter Air Ratio (SAR) the difference between
  the TAR for a field of definite area and that for
  a zero area.
• The primary part of the total absorbed dose is
  represented by the zero area TAR.
• A measure of the contribution from scattered
  radiation.

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Tissue Phantom Ratio (TPR)

• Tissue Phantom Ratio (TPR) the absorbed dose at
  a given depth in phantom to the absorbed dose at
  the same point at a reference depth in phantom.
• The deeper the reference depth the greater the
  TPR.

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Tissue Maximum Ratio (TMR)

• Tissue Maximum Ratio (TMR) TPR at DMAX
• TAR TMR x BSF

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Transmission Factors

• Transmission Factors the ratio of the radiation
  dose with the device to the radiation dose
  without the device and accounts for the material
  in the beams path.
• Tray transmission factor
• Wedge factor
• Compensator factor

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Other Factors

• Tray transmission factor defines how much of the
  radiation is transmitted through a block tray.
• Tray factors vary with beam energy- as energy
  increases, the effect of the material in the
  beams path is lessened because of the greater
  penetrating power of the higher energy.
• Wedge attenuates the radiation beam
  progressively across a field

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Given Dose

• Given dose the dose at DMAX
• AKA applied dose, entrance dose, peak absorbed
  dose, or DMAX dose
• Point where PDD is 100
• Given dose TD/ PDD x 100
• TD tumor dose, dose at depth
• Direct relationship
• DoseA/PDDA DoseB/PDDB

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Exit Dose

• Exit dose the dose absorbed by a point that is
  located at the depth of DMAX at the exit of the
  beam.

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Calculations

• Field size variations, energy changes, and
  modifiers in the beams path can alter the amount
  of radiation received by the patient.
• Time setting Dose at a point/Dose rate at that
  point
• Dose at a point the prescribed dose as
  determined by the doctor
• Dose rate at that point represents the dose rate
  of the treatment unit at the point of calculation

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Calculations

• When calculating dose to a given depth, all other
  points in the radiation field will be exposed to
  radiation for the same amount of time.
• The dose to these points is dependant on their
  depth.

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SSD Calculations

• Output or dose rate of the machine should be
  expressed at the depth of DMAX
• Field size defined at the skin surface
• Dose rate measured in tissue at the depth of
  DMAX
• Find equivalent square of the collimator setting
  (used for output factor)
• Find equivalent square of the EFS (used for PDD)
• Determine the PDD
• Determine prescribed dose
• Determine the treatment unit setting

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Extended Distance

• Extended Distance patient is set up at a
  distance beyond the isocenter or reference
  distance.
• PDD is used because its nonisocentric
• When MUs are calculated for setup at distances
  greater than the standard, ISL is used to account
  for the decrease in dose rates beyond the
  isocenter.

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Inverse Square Law

• Inverse Square Law describes the change in beam
  intensity caused by the divergence of the beam.
• I1 / I2 D22 / D12
• I1 original dose rate
• I2 new dose rate
• D22 original distance
• D12 new distance

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Mayneords Factor

• Mayneords Factor a special application of the
  inverse square law.
• Does not account for changes in scatter because
  of a change in beam divergence.
• New PDD PDD x (SSD1 d / SSD1 DMAX)2 x (SSD2
  DMAX / SSD1 d)2
• Inverse Square Factor
• (Reference source calibration distance
• Treatment SSD DMAX)2
• SSD reference point typically at DMAX
• SAD reference point at isocenter

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TMR/TPR SAD

• COF(C.S.) (collimator output factor) used to
  determine scatter, measured in air, from the
  collimators
• The increase in the collimator opening, the more
  collimator surface area the photons will have to
  interact with.
• PSF(EFS) (phantom scatter factor) used to
  determine scatter from the patient.