In vivo dosimetry
Eirik Malinen
Eva Stabell Bergstrand
Dag Rune Olsen
In vivo dosimetry
error
Probalility
• 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
Prescribed dose
Errors in patient dose
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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)
Dose characteristics
Dose measurements
Point detector
beam
2D detector array
wedge
Entrance dose:
Output, SSD
Patient curvature
Wedge, curvature
Exit dose:
Thickness, density
Desired in vivo dosimeter characteristics
• Accurate and precise
High accuracy
Low precision
• Multiple readouts
• Reusability
• No cables
• Non-destructive readout
Low accuracy
High precision
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)
Dosimeter reading → absorbed dose
• Absorbed dose, D:
D  RN D  Ci
i
R: dosimeter reading
ND: calibration factor
Ci: correction factor
Calibration
• Under reference conditions:
beam
dosimeter
Rcal
dmax
ion chamber
Dcal
water phantom
Dcal
ND 
Rcal
Example – diodes
spherical
droplet
Buildup cap
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
Temperature and sensitivity, diodes
Detector temperature
after placing on patient
Sensitivity dependence
Accumulated dose and sensitivity, diodes
• Regular calibration must be performed
Field size and sensitivity, diodes
8 or 18 MV photons
Entrance (in) or exit (out)
Supralinearity, TLD
Energy dependence, TLD
Comparison
EPR/
alanine
TLD
Dose rate
1
1
<1
Linearity
1
<1
1
Beam inclination
1
1>
1
Temperature
≈1
1
<1
Energy
≈1
≈1
≈1
Stability
≈1
≈1
Immediate
readout
2%
Correction factor for
Total uncertainty
(following corrections)
3-4 %
(~1 Gy)
2-3 %
Diode
Action level
• Relative dose difference:
Dmeasured
r  1
Dprescribed
• At what dose difference level should the treatment be
revised? 1% ? 2.5 % ? 5 %?
• Depends on:
– dosimetric accuracy and precision
– non-systematic errors
–…
Clinical example
Methods
Portal image
profile
Measured dose / prescribed dose
Action level: 2.5%
measured dose
dose after correction
r  1.008
  1.2%
Frequency distribution of relative dose
2D dose maps
Treatment planning algorithm
Portal image
Collapsed cone algorithm
Location of normalization point
Novel methods – ”dose guided radiotherapy”
prescribed isodose
target
→OK
Backprojection of filtered
dose image into patient image
→correction
dose image
Novel methods – ”dose guided radiotherapy”
bladder
rectum
Corrections