In vivo dosimetry

Eirik MalinenEva Stabell Bergstrand

Dag Rune Olsen

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

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)

Dose characteristics

Dose measurements

Patient curvature

beam

wedgeOutput, SSD

Wedge, curvature

Thickness, density

Entrance dose:

Exit dose:

Point detector

2D detector array

Desired in vivo dosimeter characteristics

• Accurate and precise

• Multiple readouts• Reusability

• No cables

• Non-destructive readout

High accuracyLow precision

Low accuracyHigh 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:

R: dosimeter reading

ND: calibration factor

Ci: correction factor

ii

D CRND

Calibration

Rcal

Dcal

beam

dmax

water phantom

ion chamber

dosimeter

cal

calD R

DN

• Under reference conditions:

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

• Regular calibration must be performed

Accumulated dose and sensitivity, diodes

Field size and sensitivity, diodes

8 or 18 MV photonsEntrance (in) or exit (out)

Supralinearity, TLD

Energy dependence, TLD

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

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

Clinical example

Methods

Portal image profile

Measured dose / prescribed dose

Action level: 2.5%

measured dose

dose after correction

%2.1

008.1r

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”

dose image

Backprojection of filtered dose image into patient image

→OK

→correction

target

prescribed isodose

Novel methods – ”dose guided radiotherapy”

Corrections

bladder

prosta

te

rectum