How accurately can we measure dose clinically? Accurate clinical dosimetry. Dosimetric accuracy requirement

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1 Accurate clinical dosimetry Part I: Fundamentals and New developments in reference dosimetry Jan Seuntjens McGill University Montreal, Canada, H3G 1A4 Dosimetric accuracy requirement effect of dose uncertainty on complication-free tumour control ( utility function, Schultheiss & Boyer, 1988) ICRU Report 24 (1976) Mijnheer et al (1987) Brahme (1988) 1% higher accuracy 2% increase of cure 5% 3.5% 3% The structures these numbers are referring to are targets volumes 1 Dosimetric and positional accuracy of TPA: acceptability criteria Homogeneous water slab - simple fields Stack of tissue slabs simple fields antropomorphic phantom and/or complex beams central axis 2% 3% high dose region low dose gradient 3% 3% 4% large dose gradient 4 mm 4 mm 4 mm low dose region low dose gradient 3% (50% loc.dose) 3% (50% loc.dose) 3% (50% loc.dose) Van Dyk (1993) How accurately can we measure dose clinically? Which accuracy do we expect for a given clinical situation? How should we measure dose accurately? when is a situation well-behaved? when do we have non-equilibrium conditions? What is a suitable detector? How do we handle complication areas? build-up region narrow stereotactic beams modulated fields and dynamic time-dependent measurement 2 3 1

2 Outline Principles of measurement dosimetry Detectors and phantoms Reference dosimetry in TG51 compliant beams Issues in non-equilibrium dosimetry Relative dosimetry in non-equilibrium situations Reference dosimetry in TG-51 non-compliant beams Classification of dosimeters Dosimeter Mechanism Gas-filled ionization chamber Ionization in gasses Liquid ionization chamber Semiconductors (diodes, diamond detectors, MOSFET) TLD Scintillation counters Film, gel, Fricke Calorimetry Ionization in liquids Ionization in solids Luminescence Fluorescence Chemical reactions Heat 4 5 Measurement dosimetry in medium where: D med (r) M(r) c det f med (r) D med (r) = c det M(r)f med (r) dose to medium at the point raw signal, corrected for environmental conditions as given by detector detector cavity dose calibration coefficient (coupling constant) dose conversion coefficient converts average detector dose into dose to medium at point 6 Dosimeter dependent coefficients & coupling constants Factor or coefficient Dosimetry technique Calorimetry Fricke dosimetry M T OD ρl c det C 1 ( 3 ε Fe + ) G f med Unity ( D) med Fricke Ion chamber dosimetry Q 1 Wgas mgas e s med,gas p Q 7 2

3 Interpretation of a dosimeter measurement (measurement at point r) D med (r) = c det M(r)s med,det (r)p Q (r) D det f med c det is the detector dose calibration factor and ties the clinical dose measurement to the chamber/detector calibration; for ionization chamber: cavity gas calibration coefficient M is the result of a measurement corrected for technical influence quantities to ensure that what we actually measure a quantity proportional to the dose to the detector material s med,det is the stopping power ratio medium to cavity material p Q is an overall perturbation correction factor that accounts for everything a cavity theory does not account for. f med is the ratio of dose to medium to dose to cavity; factorization of f med is questionable in regions of strong disequilibrium 8 Reference dosimetry: TG-51 calibration protocol Cavity gas calibration coefficient derived from an absorbed dose to water calibration at 60 Co Co N a.k.a c det = D,w N D,air = N gas ) s w,air p Q ( ) Co ( = Co N D w = D,w Ms ( s w,air p Q ) w,air p Q Co s Co = N D,wMkQ k Q = w,air p Q with s w,air p Q ( ) Co = f w 9 Q ( ) Co Calibration chain for ionization chambers calibration beam air kerma calibration coefficient absorbed dose calibration coefficient TG-51 or TRS 398 N Co Dw, TG-21 or TRS 277 NK or NX cavity gas calibration coefficient ' R * 50 ecal * gr or k k P ND, air or Ngas clinical beam absorbed dose calibration coefficient 10 k Q N D, w water L P air wallprepl ( / ρ) 11 3

4 Reference dosimetry: TG-51 calibration protocol (photons) Reference dosimetry: TG-51 calibration protocol (electrons) Co k Dw = MND,w ecal k'r50 Pgr Co k Dw = MND,w Q - Measurement at dref in water - 10 x 10 cm2 or 20 x 20 cm2 field - beam quality specifier: R50 - Measurement at 10 cm depth in water - 10 x 10 cm2 field - beam quality specifier: %ddx(10 cm) Almond et al, Reference Dosimetry New developments - WGTG51 13 Accuracy of clinical dosimetry A number of more recent chamber types are not listed in TG-51 Extensive experimental data on kq factors is available that allows to estimate uncertainties on kq as well as the overall reference dosimetry calibration process. Stability, linearity, reproducibility of the detector and the measurement setup Energy dependence - what is the accuracy with which one can convert detector dose to medium dose? CPE or TCPE and detector is small compared to the range of secondary electrons: fmed can be factorized as a stopping power ratio + correction factors (and the corrections are small or constant) If these conditions are not fulfilled: fmed cannot be factorized or very large correction and situation dependent correction factors are needed (unpractical) Monte Carlo calculations (assumed accurate within the constraints of the algorithm and basic cross-section datasets) Choose a more suitable detector (perturbation free, medium equivalent, etc) AAPM Work Group (WGTG51, chaired by M. McEwen) to supply this information (See important presentation: McEwen and Rogers, TH-D-352-2)

5 Why do we worry about CPE or TCPE? For the measurement of a dose distribution or for measurement of output: In regions of CPE and TCPE: SPR corrections accurately represent detector response (and are small for air-filled chambers in photon beams) In regions of non-cpe: SPR corrections DO NOT accurately reflect changes in detector response and additional, sometimes large, corrections are needed build-up regions in any field, interface-proximal points in heterogeneous phantoms (build-up and build-down) narrow stereotactic fields intensity modulated fields 16 Different measurement issues in open photon fields in water in-field, CPE or TCPE standard measurement, SPR is nearly constant and represents detector response well; detector perturbations are small (point-ofmeasurement shift is 0.6*r inner upstream) in-field, non-cpe (build-up) non standard measurement, SPR varies modestly but does not represent variation in detector response well; detector perturbations are large, detector dependent. penumbra (non-cpe) detector size must be small relative to field-size and penumbra width, SPR does not represent detector response well, lateral fluence perturbations, detector dependent. out-of-field (CPE or TCPE) detector size must be sufficiently large to collect sufficiently large signal. SPR represents detector response well. 17 Stopping power ratios (SPR s) SPR s are depth, field size, and radiation quality dependent for reference dosimetry: values have been calculated with Monte Carlo techniques and are well documented in standard dosimetry protocols for relative dosimetry in TCPE: their variation relative to the reference point can be calculated using Monte Carlo techniques for relative dosimetry in non-equilibrium situations: their variation relative to the reference point can still be calculated but no longer represents an accurate dose conversion. s w,air (z) for plane parallel bremsstrahlung beams - no e - contamination, central axis 60 Co 10 MV 20 MV 30 MV 18 Andreo and Brahme, Phys. Med. Biol., 31, 839 (1986) 5

6 Narrow 1.5 mm field Ratio of dose to water to dose to air Small fields 1.70 averaged over cavity volume Dw/Dair Collecting electrode diameter: 1.5 mm Separation: 1 mm 1.30 stopping power ratio w/air Paskalev, Seuntjens, Podgorsak (2002) AAPM Proc. Series 13, Med. Phys. Publishing, Madison, Wi, SanchezDoblado et al Phys. Med. Biol (2003) 20 Perturbation correction factors: traditional factorization for ionization chambers: PQ = Pwall Pgr Pfl Pcel Off-axis distance (mm) 8 21 Perturbation correction factors depth, field size, and radiation quality dependent for reference dosimetry using ionization chambers: based on Monte Carlo calculations, measurements and relatively well documented for relative dosimetry: their variation relative to the reference point is traditionally ignored but can be very significant in non-cpe conditions. Pwall: wall perturbation correction factor (pwall) Pgr: correction for gradient effect (effective p. of meas., pdis) Pfl: fluence perturbation correction factor (pcav) Pcel: central electrode perturbation correction factor (pcel)

7 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 (2006) Verhaegen et al, Phys. Med. Biol (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

8 NACP chamber response in buildup region Q W Dcav = mair e air Dcav ( d) = Q( d) Dcav ( dmax ) Q( dmax ) accurate DOSRZnrc cavity simulations NACP measurement Field size (cm 2 ) 10x10 40x40 Calculated relative surface cavity dose 45 ± 1 % 67 ± 2 % Measured relative surface ionization 47 ± 2 % 69.0 ± 0.3 % Perturbation correction factors 10x10 cm 2 water ( ) ( ) L( d) D water water d = Dcav d ρ Φ ( d) air air Depth (cm) Perturbation correction 0.78 ± ±

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

9 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 Kawrakow (2006) Med. Phys. 33, 1829 McEwen et al (Med. Phys. 35, 950 (2008) ) 32 Inflection point variability 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. Ververs, Siebers and Kawrakow TU-C-AUD B

Reference dose measurements in dynamic fields Fraser, D. et al., (2007) (D C /D meas ) IMRT 1.10 1.05 1.00 0.95 0.90 0.85 PinPoint IC10 NE2571 xpp = 0.944 s( xpp ) = 0.035 xic10 = 0.

10 Reference dose measurements in dynamic fields Fraser, D. et al., (2007) (D C /D meas ) IMRT PinPoint IC10 NE2571 xpp = s( xpp ) = xic10 = s( xic10 ) = xne = s( xne ) = Plan No. 36 Correction to chamber measurements in IMRT fields Field name Static #1 Static #2 Static #3 Static #4 Static #5 Dynamic #1 Dynamic #2 Dynamic #3 Dynamic #4 Dynamic #5 Dynamic #6 Dynamic #7 Dynamic #8 Bouchard and Seuntjens, Med Phys 31, (2004) C IMRT,6 MV measured ± 4.0% ± 6.1% ± 3.2% ± 6.0% ± 2.5% ± 3.0% ± 2.5% ± 3.9% ± 3.2% ± 5.4% ± 5.9% ± 5.7% ± 5.9% C IMRT,6 MV calculated Difference 6.8% 1.9% 2.7% 3.2% 2.7% -0.3% 0.7% 7.8% -2.4% 3.8% -1.4% 5.9% 6.9% 37 Reference dosimetry protocol IAEA - AAPM joint initiative IAEA has had two consultancy meetings to develop the protocol (Dec 07, May 08) AAPM Workgroup on non-compliant beam dosimetry IPEM endorsement expected Other national organizations to follow An international protocol that forms a supplement to existing protocols Reference dosimetry protocol Two new reference fields: Small static field dosimetry: machine-specificreference field (msr) for treatment machines that cannot establish a conventional reference field. Composite (dynamic) field dosimetry: plan-class specific reference field (pcsr). Represents a class of dynamic or step-and-shoot delivery fields, or a combination of fields, such that full charged particle equilibrium (CPE) is achieved at the position of the detector

1.8 cm 6 cm 40 41 Physics behind the conversion factors Example: Tomotherapy calibration from measurements

11 1.8 cm 6 cm Physics behind the conversion factors Example: Tomotherapy calibration from measurements using a primary absorbed dose standard f msr (route 1) Jeraj et al, 2005 f pcsr (route 2) Duane et al, cm x 5 cm Helical, 5 cm cm x 2 cm Helical, 2.5 cm from measurements using a suitable detector from Monte Carlo calculations or other model 2 cm x 2 cm Helical, 1 cm

12 Discussion: defining a pcsr field minimize disequilibrium effects near detector delivery of pcsr must resemble delivery of actual plans corresponding to the same class measurements in solid phantoms Design treatment plan for pcsr : must preserve the essence of the modulation conversion factors can be accurately measured or calculated Patient-specific QA procedure is much closer to pcsr than to ref field. 44 Summary Reviewed basics of dosimetry TG-51 and current extensions applicability of cavity theory to non-reference conditions illustrated via small fields, build-up dose and non-compliant beams accurate measurements require accurate understanding of the detector response conventional cavity theory for detector response is not accurate in non-equilibrium conditions this has, in some cases, significant clinical impact on a dose measurement suitable detectors or corrections are needed 45 12

How accurately can we measure dose clinically? Dosimetric accuracy requirement. Accurate Clinical dosimetry. Part I: Fundamentals and Issues

How accurately can we measure dose clinically? Dosimetric accuracy requirement. Accurate Clinical dosimetry. Part I: Fundamentals and Issues Accurate Clinical dosimetry Part I: Fundamentals and Issues Jan Seuntjens McGill University Montreal, Canada, H3G 1A4 jseuntjens@medphys.mcgill.ca Dosimetric accuracy requirement effect of dose uncertainty