In vivo dosimetry Eirik Malinen Eva 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 Probalility

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 wedge Output, 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 accuracy Low precision 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: R: dosimeter reading N D : calibration factor C i : correction factor

Calibration R cal D cal beam d max water phantom ion chamber dosimeter 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 photons Entrance (in) or exit (out )

Supralinearity, TLD

Energy dependence, TLD

Correction factor for EPR/ alanine TLDDiode Dose rate 11<1 Linearity 1<11 Beam inclination 11>1 Temperature ≈11<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 –…

Clinical example

Methods Portal imageprofile

Measured dose / prescribed dose Action level: 2.5% measured dose dose after correction

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 prostate rectum