in vivo dosimetry eirik malinen eva stabell bergstrand dag rune olsen
TRANSCRIPT
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In vivo dosimetry
Eirik MalinenEva 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
error
Prescribed dose
Pro
bali
lity
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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
Patient curvature
beam
wedgeOutput, SSD
Wedge, curvature
Thickness, density
Entrance dose:
Exit dose:
Point detector
2D detector array
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Desired in vivo dosimeter characteristics
• Accurate and precise
• Multiple readouts• Reusability
• No cables
• Non-destructive readout
High accuracyLow precision
Low accuracyHigh precision
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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
ii
D CRND
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Calibration
Rcal
Dcal
beam
dmax
water phantom
ion chamber
dosimeter
cal
calD R
DN
• Under reference conditions:
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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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• Regular calibration must be performed
Accumulated dose and sensitivity, diodes
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Field size and sensitivity, diodes
8 or 18 MV photonsEntrance (in) or exit (out)
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Supralinearity, TLD
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Energy dependence, TLD
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Correction factor forEPR/
alanineTLD Diode
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
Total uncertainty(following corrections)
3-4 %(~1 Gy)
2-3 % 2 %
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– …
prescribed
measured
D
Dr 1
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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
%2.1
008.1r
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Frequency distribution of relative dose
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2D dose maps
Treatment planning algorithm Portal image
Collapsed cone algorithm Location of normalization point
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Novel methods – ”dose guided radiotherapy”
dose image
Backprojection of filtered dose image into patient image
→OK
→correction
target
prescribed isodose
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Novel methods – ”dose guided radiotherapy”
Corrections
bladder
prosta
te
rectum