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1 New Dosimetry Measurements - New Developments J. Seuntjens J. Gibbons I. Das WA-A-211A Continuing Education Session Much o what we will say Clinical Dosimetry Measurements in Radiotherapy (AAPM 2009 Summer School) D. W. O. Rogers & J. Cygler Editors ISBN: Published: ,128 pp Hardcover Price: US $ Session Outline Part I: Measurement dosimetry principles and application to nonstandard beams - J. Seuntjens Part II: Practical aspects o measurement dosimetry - J. Gibbons Part III: Small ield dosimetry - I. Das Outline - Part I Bacground Deinitions and nomenclature Fundamental issues in nonstandard beam dosimetry New ormalism or nonstandard beam reerence dosimetry AAPM, July 29,

2 Bacground Bacground Developments in RT techniques have substantially increased the use o small ields and larger uniorm or non-uniorm ields, composed o small ields Beam modiiers shaping small and composed ields such as standard-, mini- and micro- MLCs have become common equipment on conventional linacs Radiation therapy procedures are shiting rom traditional 3D conormal deliveries to: novel, dedicated treatment units speciically designed or stereotactic (GammaKnie, CyberKnie) or IMRT treatments (TomoTherapy) traditional units delivering dynamically composed ields (RapidArc, VMAT, IMRT, etc) AAPM, July 29, Modality In practice Typical calibration ield size (static) S&S or dynamic capabilities? IMRT, SRS 10 x 10 cm 2 Yes TomoTherapy 5 x 20 cm 2 Yes Cybernie 6 cm diameter Yes GammaKnie 1.6 cm / 1.8 cm diameter Yes AAPM, July 29,

3 Issues with nonstandard beams There are no primary standards that measure dose directly in nonstandard beams Delivery protocols in nonstandard beams are not standardized (and not easily standardizable ) The path between a static open 10x10 cm 2 ield calibration and delivery conditions introduces signiicant uncertainties Large dierences in Output Factors among users/machines Statistics o 45 Output Factors or 6 mm and 18 mm square ields Novalis, SSD = 100 cm, depth = 5 cm, various detectors) actor o 2 in dose determination! From Wolgang Ullrich, BrainLab 8 9 Small photon ields broad photon ield narrow photon ield Deinitions and nomenclature volume volume A small ield is deined as a ield with a size smaller than the lateral range o charged particles 11

4 Nonstandard ield: small and composite Non-standard ields are deined as: Single small ields CPE or TCPE is distorted Composition o small ields either in step-and-shoot, arcs, or dynamic multiple ields CPE or TCPE may be (but is not necessarily and not intentionally) distorted Examples: TomoTherapy, Cybernie, etc. Fundamental issues in nonstandard beam dosimetry Why are we worried about CPE or TCPE in measurement dosimetry? 12 Classiication o dosimeters and mechanisms Dosimeter Mechanism Gas-illed ionization chamber Ionization in gasses Liquid ionization chamber Semiconductors (diodes, diamond detectors, MOSFET) TLD Scintillation counters Film, gel, Frice Calorimetry Ionization in liquids Ionization in solids Luminescence Fluorescence Chemical reactions Heat 14 Measurement dosimetry in medium where: D med (r) M(r) c det med (r) D med (r) = c det M(r) med (r) dose to medium at the point raw signal, corrected or environmental conditions as given by detector detector cavity dose calibration coeicient (coupling constant) dose conversion coeicient converts average detector dose into dose to medium at point

5 Dosimeter dependent coeicients & coupling constants Factor or coeicient Dosimetry technique Calorimetry Frice dosimetry M T OD ρl c det C 1 ( 3 ε Fe + ) G med Unity ( D) med Frice Ion chamber dosimetry 1 Wgas mgas e s med,gas p Why do we worry about CPE or TCPE? restricted collision s.p. unperturbed medium luence (SA CPE) There is no theoretical reason why the SA evaluation cannot be perormed using disequilibrium luence. But,, in this case we don t expect s med,det to be a good representation o the ratio D med /D det 17 Stopping power ratios (SPRs) In regions o CPE and TCPE: SPR corrections accurately represent detector response (and are small or air-illed chambers in photon beams) In regions o non-cpe: SPR corrections DO NOT accurately relect changes in detector response and additional, sometimes large, corrections are needed small ields build-up regions in any ield, interace-proximal points in heterogeneous phantoms (build-up and build-down) sometimes: intensity modulated ields, etc 18 Narrow 1.5 mm ield - extreme example Ratio o avg. dose to water <D w > to dose to cavity air Model A14P chamber Collecting electrode diameter: 1.5 mm Separation: 1 mm Pasalev, Seuntjens, Podgorsa (2002) AAPM Proc. Series 13, Med. Phys. Publishing, Madison, Wi, <D w > /D air 1.70 averaged over cavity volume Stopping power 1.20 ratio w/air O-axis distance (mm) ICRU, September

6 Perturbation correction actors: traditional actorization or ionization chambers: P = P wall P gr P l P cel P wall : wall perturbation correction actor (p wall ) P gr : correction or gradient eect (eective p. o meas., p dis ) P l : luence perturbation correction actor (p cav ) P cel : central electrode perturbation correction actor (p cel ) Perturbation correction actors depth, ield size, and radiation quality dependent or reerence dosimetry using ionization chambers: evaluation based on Monte Carlo calculations, measurements and relatively well documented or relative dosimetry: their variation relative to the reerence point is traditionally ignored but can be very signiicant in non-cpe conditions. 21 IMRT is ar rom standard reerence conditions Data or IMRT dosimetry can be determined very accurately s w,air = s w,air = but what is the use, (Andreo, ESTRO 2008) From Art Boyer, Stanord ICRU, September ICRU, September

7 Exradin A14 Extreme nonstandard ields Exradin A12 Exradin A14 Bouchard et al, 2009 (Med Phys, in print) Bouchard et al, 2009 (Med Phys, in print) Reerence dose measurements in dynamic ields PinPoint IC10 NE2571 xne = s( xne ) = xpp = s( xpp ) = xic10 = s( xic10 ) = From Roberto Capote, IAEA ICRU, September Fraser, D. et al., (2009) 27

8 A new proposed ormalism or reerence dosimetry o nonstandard beams Reerence dosimetry protocol: the IAEA/AAPM proposal Two new reerence ields: Small static ield dosimetry: machine-speciicreerence ield (msr) or treatment machines that cannot establish a conventional reerence ield. Composite (dynamic) ield dosimetry: plan-class speciic reerence ield (pcsr). Represents a class o dynamic or step-and-shoot delivery ields, or a combination o ields, such that ull charged particle equilibrium (CPE) is achieved at the position o the detector Small Static Fields Reerence Calibration, re ield msr, msr msr re w, = msr msr D, w, 0, 0 msr, D M N 1. Small Static Fields Relative dosimetry, clinical ield clin r, Dw, = D w, Ω, clin ms clin msr clin msr clin msr, re msr, is a actor which corrects or the dierences between the conditions o ield size, geometry, phantom material and beam quality o the conventional reerence ield re and the machine-speciic reerence ield msr, re msr, = D D w, msr re w, M M msr re 30 Andreo, ESTRO 2008 clin, Ω is a ield actor which converts the absorbed dose to water or clin, msr the machine-speciic reerence ield, msr to the absorbed dose to water or the clinical ield clin. In relative dosimetry o single static ields this actor is conventionally called a ield output actor, which ought to be deined as a ratio o D w. It can be calculated directly as a ratio o D w using Monte Carlo alone or it can be measured as a ratio o detector readings multiplied by a Monte Carlo calculated correction actor. Ω clin M clin, msr clin clin, msr clin, = msr clin, msr Mmsr 31 Andreo, ESTRO 2008

9 Monte Carlo D-ratios vs experimental M-ratios. 6 MV, 5cm depth 1 REFERENCE DOSIMETRY D M N Broad beam reerence ield re N D, re msr,, w, 0, 0 Hypothetical reerence ield re BrainLAB micro MLC 10cmx10cm CyberKnie 0.6 cm RELATIVE DOSIMETRY, msr msr re clin clin, w, = msr msr D, w, 0, 0 msr, D w, = D clin w, Ω msr clin, msr Machine speciic reerence ield msr Radiosurgical collimators 18 mm Ω clin, clin, msr Clinical clin Data rom S.Doblado et al (2007) 32 msr Ionization chamber, re, GammaKnie 1.6/1.8 cm Tomotherapy 5cm x 20cm e.g. a GammaKnie clinical plan Andreo, ESTRO 2008 D 2. Composite Fields Reerence Calibration, re ield pcsr pcsr pcsr w, = M pcsr N pcsr D, w, 0, 0 pcsr, re pcsr, A plan-class speciic reerence ield, pcsr, is a reerence ield or a class o dynamic or step-and-shoot delivery ields, or a class o combinations o ields in a coniguration that is as close as possible to the inal clinical delivery scheme, but delivers a homogeneous absorbed dose to an extended and geometrically simple target volume clin, pcsr Ω clin, pcsr 2. Composite Fields Relative dosimetry, clinical ield clin pcsr clin, clin w, = w,, D D Ω clin pcsr clin pcsr pcsr has to be established or every treatment delivery, and can be determined using the same set-up and the same phantom as or calibration. E.g. or a patient speciic clinical plan, the phantom should be the same as or the dosimetry in the pcsr ield using a crosscalibrated chamber. 34 Andreo, ESTRO Andreo, ESTRO 2008

10 re 2 REFERENCE DOSIMETRY D pcsr pcsr w, = M pcsr N pcsr D, w, 0, 0 Broad beam reerence ield re (e.g. IMRT pcsr, Linac) pcsr, pcsr, re pcsr, Plan-class speciic reerence ield pcsr RELATIVE DOSIMETRY D w D Ω clin, pcsr clin, pcsr pcsr clin, clin pcsr, = clin w, Ω pcsr clin, pcsr 300º 340º Clinical clin 20º 60º Ionization Chamber Fully-rotated Delivery Collapsed Delivery Fully-rotated Delivery Exradin A ± ± NE ± ± Exradin A1SL ± ± Exradin A ± ± PinPoint ± ± N D, w, 0, 0 Hypothetical reerence ield, re msr, msr pcsr, pcsr, msr e.g. 9-ield prostate pcsr (e.g. Tomotherapy 5cm x 20cm) 260º 220º 180º 140º e.g. 9-ield prostate clinical plan Ionization chamber Andreo, ESTRO º _pcsr Farmer-type 0.6 cm 3 ionization chambers Exradin A12 NE2571 Exradin A1SL Exradin A14 PinPoint Ionization chamber smaller ionization chambers 7 beam pcsr Chung et al, 2009