In vivo dosimetry

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In vivo dosimetry

• Eirik Malinen
• Eva Stabell Bergstrand
• Dag Rune Olsen

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In vivo dosimetry

• In vivo In the living
• Dosimetry Estimates of radiation dose by theory
  and measurement
• Verification of delivered
• dose to individual patients
• Radiotherapy requires
• accurate dose delivery

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Errors in patient dose

• Patient contour / planning basis (CT images)
• Patient motion
• Organ motion
• Dose calculations (inhomogeneities, scatter)
• Patient positioning
• Transfer of treatment data from simulator to
  linac
• Linac settings (energy, monitor units, field
  size) and calibration
• Beam modifiers (blocks, wedges)

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Dose characteristics
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Dose measurements
Point detector
beam
Entrance dose
wedge
2D detector array
Output, SSD
Wedge, curvature
Patient curvature
Exit dose
Thickness, density
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Desired in vivo dosimeter characteristics

• Accurate and precise
• Multiple readouts
• Reusability
• No cables
• Non-destructive readout

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In vivo dosimetry principles

• Point detector
• Semiconductors (diodes)
• Thermoluminescent crystals
• EPR (electron paramagnetic resonance) sensitive
  materials
• .
• 2D detector, (electronic) portal imaging device
  EPID
• Film
• Arrays (ion chambers, semiconductors)

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Dosimeter reading ? absorbed dose

• Absorbed dose, D
• R dosimeter reading
• ND calibration factor
• Ci correction factor

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Calibration

• Under reference conditions

beam
dosimeter
Rcal
dmax
ion chamber
Dcal
water phantom
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Example diodes
spherical
droplet
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Buildup cap
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Correction factors

• Dosimeter reading may depend on
• Temperature
• (Accumulated) Dose
• Dose rate
• Beam energy
• Field size
• ...
• Accuracy may be reduced if dependence is not
  corrected

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Temperature and sensitivity, diodes
Detector temperature after placing on patient
Sensitivity dependence
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Accumulated dose and sensitivity, diodes

• Regular calibration must be performed

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Field size and sensitivity, diodes
8 or 18 MV photons Entrance (in) or exit (out)
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Supralinearity, TLD
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Energy dependence, TLD
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Comparison
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Action level

• Relative dose difference
• At what dose difference level should the
  treatment be revised? 1 ? 2.5 ? 5 ?
• Depends on
• dosimetric accuracy and precision
• non-systematic errors

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Clinical example
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Methods
Portal image
profile
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Measured dose / prescribed dose
Action level 2.5
measured dose
dose after correction
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Frequency distribution of relative dose
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2D dose maps
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Novel methods dose guided radiotherapy
prescribed isodose
target
Backprojection of filtered dose image into
patient image
?OK
?correction
dose image
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Novel methods dose guided radiotherapy
bladder
prostate
rectum