Small Field Dosimetry: An Overview of Reference and Relative Dosimetry in Non-Equilibrium Condition

Transcription

1 Small Field Dosimetry: An Overview of Reference and Relative Dosimetry in Non-Equilibrium Condition Indra J. Das, PhD, FAAPM, FACR, FASTRO Department of Radiation Oncology Indiana University School of Medicine Indianapolis, IN IJDas [1]

2 Lecture Organization Overview of Reference and Relative Dosimetry Indra J Das SRS: Detector Selection Sonja Dieterich An Independent Audit and Quality Assurance David Followill IJDas [2]

3 IJDas [3] Outline of Presentation Definition of Small Field o Electron range o Electronic Equilibrium o Transient Equilibrium o Lateral Equilibrium Source Occlusion o Variation is shape, size o Machine specific, age, vintage o Source size and dose o Size of detector Detector Issues o Volume averaging o Perturbation IAEA/AAPM Framework o Nomenclature o Reference Dosimetry

4 IJDas [4] TG-155: Small Fields and Non-Equilibrium Condition Photon Beam Dosimetry Indra J. Das (Chair) Indiana University School of Medicine, Indianapolis, IN Paolo Francescon (Co-chair) Ospedale Di Vicenza, Viale Rodolfi, Vicenza 36100, Italy Anders Ahnesjö Uppsala University & Nucletron Scandinavia AB, Uppsala, Sweden Maria M. Aspradakis Department of Radiation Oncology, Kantonsspital Lucerne, Lucerne, Switzerland Chee-Wai Cheng University Hospitals Case Medical Center, Cleveland, OH, George X. Ding Vanderbilt University Medical Center, Nashville, TN John Fenwick Oxford University, Oxford, England Geoffrey S. Ibbott Radiological Physics Center, MD Anderson Cancer Center, Houston, TX Mark Oldham Duke University Medical Center, Durham, NC M. Saiful Huq University of Pittsburgh Cancer Institute, Pittsburgh, PA Chester S. Reft University of Chicago, Chicago, IL Otto A Sauer University of Wuerzburg, Wuerzburg, Germany

5 Magna-Fields 200x200 cm 2 Treatment Fields Traditional Fields 40x40 cm 2 4x4 cm 2 4x4 cm 2 Small Field 0.3x0.3 cm2 Advance Therapy Fields SRS/SRT Gamma Knife Cyber-Knife Tomotherapy IMRT IJDas [5]

6 Meaning of Small Field? Lack of charged particle o Dependent on the range of secondary electrons o Photon energy Collimator setting that obstructs the source size Detector is comparable to the field size IJDas [6]

7 CPE & Electron Range CPE, Charged Particle Equilibrium Electron range= d max in forward direction Electron range in lateral direction o Nearly energy independent o Nearly equal to penumbra (8-10 mm) o rlee (g/cm 2 )=5.973[TPR(20,10)] Field size needed for CPE o Lateral range o mm d max IJDas [7]

8 Electronic Equilibrium, CPE, TCPE CPE Dose = KERMA IJDas [8]

9 Bragg-Gray Cavity: Range and Cavity Size Range Dose is calculated only by crossers in the cavity & Cavity should be non-perturbing Cavity Dose =.S/ (number/cm 2 )(MeVcm 2 /g) = MeV/g= J/kg (Gy) IJDas [9]

10 Lateral Electronic Equilibrium, LEE Large Field Small Field IJDas [10]

11 Lateral Electron Range Small Range Electrons Large Range Electrons IJDas [11] LEE No LEE

12 Definition of Small Fields IJDas [12] Das et al, Med. Phys. 35, ,

13 Energy Fluence Penumbra/Output Source sizes cm Energy Fluence Scnormalized Large field x x1.0 cm 0.5x0.5 cm cm x cm x IJDas [13] Nyholm et al Radiother Oncol 78, 347, 2006 and Phys Med Biol 51,6245, 2006

14 Understanding Profiles Large Field Small Field IJDas [14]

15 Large Detector Small Detector Dose IJDas [15] Distance

16 Source Size IJDas [16] 90%, 70%, 50%, 30%, 10% iso-intensity line Jaffray et al, Med Phys 20, (1993)

17 Varian TrueBeam 6 MV T1 T2 T3 Dimensions x, y (mm) 0.9, MV FFF 0.8, MV 0.9, MV FFF 0.9, 0.8 IJDas [17] Sawkey et al, Med Phys, 40, 332, 2013

18 Dose and Penumbra with Spot Size IJDas [18] Scott et al, Med Phys, 36, 3132, 2009

19 Dosimetry Absolute odose Relative odepth Dose [D(r,d)/D(r,dm)] otmr oprofiles ooutput, S cp (total scatter factor) [D(r)/D(ref)] IJDas [19]

20 Cone Factor (St) Total scatter factor from different institutions 12% diff WEH U-Arizona Temple U U Penn Erlanger Serago et al. Fan et al. Zhu et al. Small Field Dosimetry Problem Institutional variability in 6 MV Radionics SRS dosimetry Cone Diameter (mm) IJDas [20] Das et al, J Radiosurgery, 3, , 2000

21 Dosimetric Variation with Detectors 1.00 Total scatter factor with various detectors Cone Factor (St) % Diamond CEA (Film) Kodak(Film) TLD Pinpoint(par) Pinpoint(per) 0.125ion(par) 0.125ion(per) MC(1mm) MC(5mm) Cone Diameter (mm) IJDas [21] Das et al, J Radiosurgery, 3, , 2000

22 Dose to Water For Small Fields: Volume Averaging IJDas [22] Capote et al, Med Phys, 31, , 2004

23 CyberKnife data Uncorrected output difference (%) with MC PinPoint Diamond(old) EBT2 MicroTLD IBA-SFD PTW Diode SN-Edge Moignier et al, Med Phys 41(7), , 2014 IJDas [23]

24 Ratio of Readings? h d( h) ( h) 0 dh ( ) ( ) ( ) max D E h d h D m QWS m e r m Dr () Qr () W S D ref Q ref e ref a m a r ref ab IJDas [24] Q(E,r) = Q r P ion P repl P wall P cec P pcf r m Dr () Qr () W S D ref Q ref e ref a r ref Q ( r ) Q ref CF CF 1 2

25 Radiation Measurements IJDas [25] Charged particle equilibrium o Range of secondary electrons o Medium (tissue, lung, bone) Photon energy and spectrum o Change in spectrum Field size Off axis points like beamlets in IMRT o Changes in ( en /) from reference field, f ref o Change in (L/) from reference field, f ref Detector o Volume o Density o Signal to noise ratio

26 IAEA/AAPM proposed pathway Alfonso, et al. Med Phys 35, (2008) IJDas [26]

27 k f Q clin f Relative Dosimetry D M w, Q k f Q clin clin f Q f Q f Q clin clin f Q N DW, Qo f fclin Dw, Q / M Q ( Output) clin w, clin rel f f D / M clin clin (Re ading rel, f, Q w, Q w, Q Q, Qo clin ) f Q clin clin, f, Q M M ( S ( S w, air w, air k f clin fclin Dw, Q M clin Qclin f f D / M ) ) w, Q fclin f k f Q, f, Q clin ref fclin / M Qclin fclin, f P P Q fclin M f Q k Q, Q IJDas [27]

28 IJDas [28] clin clin clin f wall cell stem grad fl w air f wall cell stem grad fl w air f f Q Q P P P P P L P P P P P L k,, Meaning of k Defined in IAEA/AAPM

29 k Q is not Constant in Small Field IJDas [29] Kawachi et al, Med Phys, 35, , 2008

30 Why So Much of Fuss? Reference (ref) conditions cannot be achieved for most SRS devices (cyberknife, gammaknife, tomotherapy etc) Machine Specific reference () needs to be linked to ref Ratio of reading (PDD, TMR, Output etc) is not the same as ratio of dose D 1 M1 D 1 M1 f clin, f k clin Q D2 M Q, 2 D2 M 2 IJDas [30]

31 Relative dose at dmax MV; Central Axis Scanditronix-SFD Scanditronix-PFD Exradin-A16 PTW-Pinpoint PTW-0.125cc PTW-0.3cc PTW-0.6cc PTW-Markus Wellhofer-IC Field Size (cm) 0.4 Relative dose at dmax Field Size Limit for Accurate Dose Measurements with Standard Detectors 15 MV; Central Axis Scanditronix-SFD Scanditronix-PFD Exradin-A16 PTW-Pinpoint PTW-0.125cc PTW-0.3cc PTW-0.6cc PTW-Markus Wellhofer-IC4 Das et al, TG-106, Med Phys, 35, 4186, Field Size (cm) IJDas [31]

32 IJDas [32] Relative Dose Validity of Proposed Method Field Size (cm)

33 IJDas [33]

34 Summary Small field is a condition where dosimetry is usually difficult due to electronic disequilibrium Use of IAEA/AAPM protocol for reference and relative dosimetry based on Alfonso et al (2008) is recommended Focal spot of a machine is a critical parameter in small field dosimetry Absolute dosimetry protocol for small field is forthcoming IJDas [34]

35 Small field is defined as a condition of? 4% 0% 2% 85% 9% 1. Lateral electronic disequilibrium 2. Range of electron beams in lateral direction 3. Collimator size that obstructs the primary radiation source 4. All of above 5. None of above, it is field 3x3 cm 2 IJDas [35]

36 Directions Answer: 4 - All of above Ref: Das et al, Small fields: Non-equilibrium radiation dosimetry," Med Phys 35, (2008) IJDas [36]

37 IAEA/AAPM have provided a frame work for small field dosimetry in fclin, f terms of the correction factor, k Q, Q which is dependent on? clin 2% 8% 1% 2% 88% 1. Beam energy 2. Type of detector (microchambers, Diode, Mosfet etc) 3. Type of machine (IMRT, CyberKnife, GammaKnife etc) 4. Focal spot size 5. All of above IJDas [37]

38 Answer: 5 all of above Ref: Alfonso et al, "A new formalism for referenece dosimetry of small and nonstandard fields," Med Phys 35, (2008) IJDas [38]

39 IJDas [39] Thanks