Accurate clinical dosimetry Dosimetric accuracy requirement Part I: • effect of dose 1% higher accuracy Fundamentals and uncertainty on New developments in reference dosimetry complication -free → 2% increase of cure tumour control ( “utility function ”, Schultheiss & Boyer, 1988) Jan Seuntjens 5% McGill University • ICRU Report 24 (1976) Montreal, Canada, H3G 1A4 • Mijnheer et al (1987) 3.5% [email protected] • Brahme (1988) 3% The structures these numbers are referring to are targets volume s
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Dosimetric and positional accuracy of TPA: acceptability criteria How accurately can we measure dose clinically? high dose large dose low dose central region gradient region • Which accuracy do we expect for a given clinical axis low dose low dose situation? gradient gradient • How should we measure dose accurately? Homogeneous 3% (50% – when is a situation “well -behaved ”? water slab - 2% 3% 4 mm loc.dose) – when do we have non -equilibrium conditions? simple fields Stack of tissue 3% (50% • What is a suitable detector? 3% 3% 4 mm slabs simple fields loc.dose) • How do we handle complication areas? antropomorphic – build -up region 3% (50% phantom and/or 4% 4 mm loc.dose) – narrow stereotactic beams complex beams – modulated fields and dynamic time -dependent measurement Van Dyk (1993)
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1 Outline Classification of dosimeters Dosi meter Mechanis m • Principles of measurement dosimetry Gas -filled ionizatio n c hamber Ioni zat ion in gas ses • Detectors and phantoms Li quid i oni zat ion cha mber Ioni zat ion in li quids • Reference dosimetry in TG51 compliant Se micon ductors (d iod es, Ioni zat ion in solids diamon d d etec tors, beams MOSFE T) • Issues in non -equilibrium dosimetry TLD Lu minesc ence – Relative dosimetry in non -equilibrium situations Scintillation count ers Fluorescen ce – Reference dosimetry in TG -51 non -compliant Fil m, g el, Fricke Che mical reacti ons beams Calori metr y Heat
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Dosimeter dependent coefficients Measurement dosimetry in medium & coupling constants D (r) = c M(r)f (r) med det med Factor or Dosimetry technique where: coefficient Calorimetry Fricke Ion chamber dosimetry dosimetry Dmed (r) dose to medium at the point M ∆T ∆OD Q M(r) raw signal, corrected for environmental ρL conditions as given by detector cdet C 1 1 Wgas 3+ cdet detector cavity dose calibration coefficient ε()Fe G megas (coupling constant) f Unity med smed,gaspQ med ()D Fricke fmed (r) dose conversion coefficient converts average detector dose into dose to medium at point Accurate clinical dosimetry, Part I, 2008 6 Accurate clinical dosimetry, Part I, 2008 7
2 Interpretation of a dosimeter measurement (measurement at point r) Reference dosimetry: = TG-51 calibration protocol Dmed (r) cdet M(r)smed ,det (r)pQ(r) • Cavity gas calibration coefficient derived from D f det med an absorbed dose to water calibration at 60 Co • cdet is the detector dose calibration factor and ties the clinical do se measurement to the chamber/detector calibration; for ionization chamber: NCo a.k.a “cavity gas calibration coefficient ” c = D,w ( = N = N ) det ()s p D,air gas • M is the result of a measurement corrected for “technical ” influence w,air Q Co quantities to ensure that what we actually measure a quantity pr oportional to the dose to the detector material NCo • smed ,det is the stopping power ratio medium to cavity material = D,w Dw Ms w,air pQ • pQ is an overall perturbation correction factor that accounts for e verything a ()s p cavity theory does not account for. w,air Q Co s p Co w,air Q Q = N Mk with kQ = = ()fw fmed is the ratio of dose to medium to dose to cavity; factorization of fmed is D,w Q ()s p Co questionable in regions of strong disequilibrium w,air Q Co
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Calibration chain for ionization chambers TG -51 or TRS 398 TG -21 or TRS 277 air kerma absorbed NCo NorNKX calibration beam calibration dose calibration Dw, coefficient coefficient
NorND,air gas cavity gas calibration kkP' ** coefficient Recalgr50 or k absorbed Q water ()LPP/ ρ wall repl clinical beam dose calibration air coefficient ND,w Accurate clinical dosimetry, Part I, 2008 10 Accurate clinical dosimetry, Part I, 2008 11
3 Reference dosimetry: Reference dosimetry: TG-51 calibration protocol (photons) TG-51 calibration protocol (electrons) Co - Measurement at d in water D = MN Co k D = MN k k' P ref w D,w Q w D,w ecal R50 gr - 10 x 10 cm 2 or 20 x 20 cm 2 field
- beam quality specifier : R50 - Measurement at 10 cm depth in water - 10 x 10 cm 2 field - beam quality specifier :
%dd x(10 cm)
Almond et al, 1999 Accurate clinical dosimetry, Part I, 2008 12 Accurate clinical dosimetry, Part I, 2008 13
Reference Dosimetry Accuracy of clinical dosimetry New developments - WGTG51 • Stability, linearity, reproducibility of the detector and the • A number of more recent chamber types are measurement setup not listed in TG -51 • Energy dependence - what is the accuracy with which one can • Extensive experimental data on k factors is convert detector dose to medium dose? Q – CPE or TCPE and detector is small compared to the range of secon dary available that allows to estimate uncertainties electrons: on kQ as well as the overall reference • fmed can be factorized as a stopping power ratio + correction factors (and the dosimetry calibration process. corrections are small or constant) – If these conditions are not fulfilled:
• fmed cannot be factorized or very large correction and situation depe ndent → correction factors are needed (unpractical) AAPM Work Group (WGTG51, chaired by M. • Monte Carlo calculations (assumed accurate within the constraint s of the McEwen) to supply this information (See important algorithm and basic cross -section datasets) presentation: McEwen and Rogers, TH -D-352 -2) • Choose a more suitable detector (perturbation free, medium equiv alent, etc)
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4 Different measurement issues in Why do we worry about CPE or TCPE? open photon fields in water
For the measurement of a dose distribution or for • in -field, CPE or TCPE measurement of output: – standard measurement, SPR is nearly constant and represents detector response well; detector perturbations are small (point -of - • In regions of CPE and TCPE: SPR corrections measurement shift is 0.6* rinner upstream) accurately represent detector response (and are • in -field, non -CPE (build -up) small for air -filled chambers in photon beams) – non standard measurement, SPR varies modestly but does not represent variation in detector response well; detector perturba tions • In regions of non -CPE: SPR corrections DO NOT are large, detector dependent. accurately reflect changes in detector response and • penumbra (non -CPE) additional, sometimes large, corrections are needed – detector size must be small relative to field -size and penumbra – build -up regions in any field, interface -proximal points in width, SPR does not represent detector response well, lateral heterogeneous phantoms (build -up and build -down) fluence perturbations, detector dependent. – narrow stereotactic fields • out -of -field (CPE or TCPE) – intensity modulated fields – detector size must be sufficiently large to collect sufficiently large signal. SPR represents detector response well.
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sw,air (z) for plane parallel bremsstrahlung beams Stopping power ratios (SPR’s) - no e - contamination, central axis
• SPR ’s are depth, field size, and radiation quality 60 dependent Co – for reference dosimetry : values have been calculated with Monte Carlo techniques and are well documented in 10 MV standard dosimetry protocols – for relative dosimetry in TCPE: their variation relative to the reference point can be calculated using Monte Carlo techniques 20 MV – for relative dosimetry in non -equilibrium situations: their variation relative to the reference point can still be calculated 30 MV but no longer represents an accurate dose conversion.
Andreo and Brahme , Phys. Med. Biol . , 31, 839 (1986) Accurate clinical dosimetry, Part I, 2008 18
5 Small Narrow 1.5 mm field fields Ratio of dose to water to dose to air 1.70 averaged over cavity volume 1.60 1.50 Collecting electrode diameter: 1.5 mm 1.40
Separation: 1 mm r i
a 1.30 D / stopping power w 1.20 D ratio w/air 1.10 1.00 Paskalev , Seuntjens , Sanchez - Podgorsak (2002) AAPM 0.90 Doblado et al Proc. Series 13, Med. Phys. Med. Phys. Publishing, 0.80 Biol . 48 2081 - Madison, Wi , 298 – 318. 02468 2099 (2003) Off -axis distance (mm)
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Perturbation correction factors: traditional factorization for ionization Perturbation correction factors chambers: • depth, field size, and radiation quality dependent = PQ Pwall Pgr Pfl Pcel – for reference dosimetry using ionization chambers: based on Monte Carlo calculations, measurements and relatively well documented • Pwall : wall perturbation correction factor ( pwall ) • P : correction for gradient effect (effective p. of – for relative dosimetry : their variation relative to the gr reference point is traditionally ignored but can be meas ., p ) dis very significant in non -CPE conditions . • Pfl : fluence perturbation correction factor ( pcav )
• Pcel : central electrode perturbation correction factor (pcel )
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6 NACP chamber in 6 MeV and 20 MeV electrons NACP -02
Partial compensatio n of wall and fluence perturbation effects
Buckley and Rogers, Med. Phys. 33 1788 - 1796 (2006) Verhaegen et al, Phys. Med. Biol . 51 1221 -1235 (2006)
Build-up dose measurements
• Measurements are needed to ensure a treatment planning system accurately calculates dose in the build -up region. • The behaviour of ionization chambers in the build -up region is questionable.
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7 NACP chamber response in buildup region
QW = D(d ) Q(d ) Dcav cav = mair e air Dcav (dmax)Q(d max )
NACP accurate DOSRZnrc cavity measurement simulations
Field size Calculated relative Measured relative surface (cm 2) surface cavity dose ionization 10x10 45 ± 1 % 47 ± 2 % 40x40 67 ± 2 % 69.0 ± 0.3 %
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Perturbation correction factors – 10x10 cm2
water =L(d ) Φ water Dwater (d)Dcav (d)ρ (d ) air air
Depth (cm) Perturbation correction 0.0 0.78 ± 0.02 1.5 0.99 ± 0.008
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8 Effective point of measurement in build-up region is not the same as in CPE conditions Experimental shifts of point of measurement
Reconciling measurements and calculations in the build -up region by adjusting EPOM
→simple if it works; but is chamber dependent!
18 MV
McEwen et al (Med. Phys. 35, 950 (2008) ) Kawrakow (2006) Med. Phys. 33 , 1829
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Inflection point variability Match @ inflection point Reference dosimetry in TG- 51/TRS-398 non-compliant beams
• Many radiation treatment machines cannot produce reference field sizes or SSDs according to the recommendations of TG -51 or TRS -398 protocols (e.g., GammaKnife, CyberKnife, and TomoTherapy). • IMRT/IMRS and SRS/SRT/SBRT treatment plans are designed to treat from multiple beam directions. • Delivery of these plans use small fields that are not very similar to the reference conditions used for beam calibration. → Reference conditions for beam calibration should be as close as possible to the patient irradiation conditions.
Accurate clinical dosimetry, Part I, 2008 35 Ververs , Siebers and Kawrakow 2008 - TU -C-AUD B -1
9 Correction to chamber measurements in Reference dose measurements in dynamic fields IMRT fields 1.10 PinPoint Field name CIMRT,6 MV CIMRT,6 MV Difference IC10 measured calculated NE2571 Static #1 1.019 ± 4.0% 0.950 6.8% x = 0.973 1.05 NE Static #2 1.173 ± 6.1% 1.150 1.9% s(x )= 0.022 NE Static #3 1.124 ± 3.2% 1.094 2.7%
T Static #4 1.274 ± 6.0% 1.233 3.2% R M I 1.00 ± ) Static #5 1.172 2.5% 1.141 2.7% s a
e Dynamic #1 1.139 ± 3.0% 1.143 -0.3% m D
/ Dynamic #2 1.169 ± 2.5% 1.161 0.7% C 0.95
D Dynamic #3 1.089 ± 3.9% 1.004 7.8% ( Dynamic #4 1.007 ± 3.2% 1.031 -2.4% = Dynamic #5 0.920 ± 5.4% 0.885 3.8% 0.90 xIC10 0.963 = Dynamic #6 1.583 ± 5.9% 1.605 -1.4% = s(x IC10 )0.024 xPP 0.944 = Dynamic #7 1.077 ± 5.7% 1.014 5.9% s(x PP )0.035 Dynamic #8 1.079 ± 5.9% 1.005 6.9% Fraser, D. 0.85 et al., 0 10 20 30 40 50 (2007) Plan No. Bouchard and Seuntjens , Med Phys 31 , 2453 -2464 (2004)
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Reference dosimetry protocol Reference dosimetry protocol
• IAEA - AAPM joint initiative • Two new reference fields: – IAEA has had two consultancy meetings to – Small static field dosimetry : machine -specific - develop the protocol (Dec 07, May 08) reference field (msr ) for treatment machines that – AAPM Workgroup on non -compliant beam cannot establish a conventional reference field. dosimetry – Composite (dynamic) field dosimetry : plan -class – IPEM endorsement expected specific reference field (pcsr ). Represents a class – Other national organizations to follow of dynamic or step -and -shoot delivery fields, or a combination of fields, such that full charged • An international protocol that forms a particle equilibrium (CPE) is achieved at the supplement to existing protocols position of the detector.
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10 1.8 cm
6 cm
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Physics behind the conversion factors Example: Tomotherapy calibration
from measurements using a f (route 1) f (route 2) primary absorbed dose standard msr pcsr Jeraj et al, 2005 Duane et al, 2006
10 cm x 5 cm 0.997 Helical, 5 cm 1.000
10 cm x 2 cm 0.993 Helical, 2.5 cm 1.000
from measurements from Monte Carlo 2 cm x 2 cm 0.990 Helical, 1 cm 0.997 using a “suitable ” calculations or other detector model
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11 Discussion: defining a pcsr field Summary • Reviewed • minimize disequilibrium effects near detector – basics of dosimetry • delivery of pcsr must resemble delivery of actual plans – TG -51 and current extensions corresponding to the same class – applicability of cavity theory to non -reference conditions illustrated via small fields, build -up dose and non -compliant • measurements in solid phantoms beams • accurate measurements require accurate → Design treatment plan for pcsr : understanding of the detector response → must preserve the essence of the modulation • conventional cavity theory for detector response is → conversion factors can be accurately measured or calculated not accurate in non -equilibrium conditions → Patient -specific QA procedure is much closer to pcsr than to ref field. • this has, in some cases, significant clinical impact on a dose measurement •“suitable detectors ” or “corrections ” are needed
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