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

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1 Accurate Clinical dosimetry Part I: Fundamentals and Issues 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 N a.k.a c det = D,w N D,air = N gas ) s w,air p Q ( ) ( = N D w = D,w Ms ( s w,air p Q ) w,air p Q s = N D,wMkQ k Q = w,air p Q with s w,air p Q ( ) = f w 9 Q ( ) Calibration chain for ionization chambers calibration beam air kerma calibration coefficient absorbed dose calibration coefficient TG-51 or TRS 398 N 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) k Dw = MND,w ecal k'r50 Pgr 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 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) New AAPM Work Group (WGTG51, chaired by M. McEwen) to supply this information (addendum to TG-51)

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 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 llecting 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 usually 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.) 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 Discrepancies have been reported between measured and calculated build-up doses at high photon energies. Benchmark measurements are needed to ensure a treatment planning system accurately calculates dose in the build-up region. Benchmark measurements are prone to uncertainties ranging up to 50% of the local dose

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 ± ±

9 Effective point of measurement in build-up region is not the same as in CPE conditions 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, 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. 33 Mackie and Huq Possible Irradiation Protocol Reference Geometry Issues with non-compliant beams Calibration Lab Irradiation With a -60 Unit User Delivering Vendor s Protocol (Protocols will Vary from Vendor to Vendor) 34 Mackie and Huq There are no primary standards that measure dose directly in non-compliant beams Delivery protocols in non-compliant beams are not standardized (and not standardizable ) Fundamental studies are required to understand detector response in noncompliant beams - goal is to make an accurate connection between non-compliant beams and compliant beams 35 9

10 Proposed correction factor, C Q non-e How can this factor be determined? The calibration coefficient for any composed (possibly, non-equilibrium) field can be assessed as follows: non-e non-e Q N D,w = C Q ND,w Calibration coefficient applicable to composed field rrection factor Calibration coefficient in TG-51 reference conditions MLC photon transmission map Primary photon dose kernels Photon radial fluence Bouchard, H. and J. Seuntjens, Med. Phys. 31(9), 2004 Kernel acquisition 6 MV dose kernel to air cavity Chamber longitudinal kernel Longitudinal air-cavity kernels for the Exradin A12 Farmer chamber, z = 3 cm, 1e6 histories SSD = 100 cm 1.60E E E E cm 8.00E-26 on-axis (y = 0) off-axis (y = 0,14 cm) 6.00E-26 H 2 O Exradin A12 Farmer model (0.653 cm 3 ) 4.00E E Tantot et al E x/cm 10

mparison with measurements mparison with measurements 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

11 mparison with measurements mparison with measurements 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 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% 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 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% Bouchard and Seuntjens, Med Phys 31, (2004) 40 Bouchard and Seuntjens, Med Phys 31, (2004) Summary Reviewed basics of dosimetry TG-51 and current extensions applicability of cavity theory to non-reference conditions Accurate measurements require accurate understanding of the detector response nventional 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 Calibration of TG51/TRS-398 non-compliant beams require more investigations 42 Acknowledgments Students: Emily Heath, Kristin Stewart, Khalid Al-Yahya, Keyvan Jabbari, Andrew Alexander, Adam Elliott, Arman Sarfehnia, Monica Margeanu, Edwin Sham Post-doc: Gabriela Stroian Staff: Frank Verhaegen, Wamied Abdel-Rahman, Slobodan Devic, Ervin Podgorsak, Michael Evans, William Parker Funding (operating): Canadian Inst. Health Research (CIHR), National Cancer Inst. Canada (NCIC), Natural Sciences and Engineering Research uncil (NSERC) 43 11

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

How accurately can we measure dose clinically? Accurate clinical dosimetry. Dosimetric accuracy requirement Accurate clinical dosimetry Part I: Fundamentals and New developments in reference dosimetry Jan Seuntjens McGill University Montreal, Canada, H3G 1A4 jseuntjens@medphys.mcgill.ca Dosimetric accuracy requirement