Small-Field Dosimetry: Its Importance In Clinic

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1 Indra J. Das/ Small Fields (Page-1) Small-Field Dosimetry: Its Importance In Clinic isadministration edia Coverage IJDas (1) Indra J. Das, PhD, FAAP, FACR, FASTRO Department of Radiation Oncology Indiana University School of edicine & IU Health Proton Therapy Center Indianapolis, IN, USA IJDas (2) Wrong detector used for BrainLab cone calibration IJDas (3) isadministration edia Coverage TG-155: Small Fields and Non-Equilibrium Condition Photon Beam Dosimetry Indra J. Das (Chair) Indiana University School of edicine, Indianapolis, IN 4622 Paolo Francescon (Co-chair) Ospedale Di Vicenza, Viale Rodolfi, Vicenza 361, Italy Anders Ahnesjö Uppsala University & Nucletron Scandinavia AB, Uppsala, Sweden aria. Aspradakis Department of Radiation Oncology, Kantonsspital Lucerne, Lucerne Switzerland Chee-Wai Cheng idwest Proton Radiotherapy Institute, Bloomington, IN, George X. Ding Vanderbilt University edical Center, Nashville, TN Geoffrey S. Ibbott Radiological Physics Center, D Anderson Cancer Center, Houston, TX 773 ark Oldham Duke University edical Center, Durham, NC Saiful Huq University of Pittsburgh Cancer Institute, Pittsburgh, PA Chester S. Reft University of Chicago, Chicago, IL 6637 IJDas (4) Otto A Sauer University of Wuerzburg, Wuerzburg, Germany agna-fields 2x2 cm 2 IJDas (5) Treatment Fields Traditional Fields 4x4 cm 2 4x4 cm 2 Small Field 4x4 cm 2.3x.3 cm 2 Advance Therapy Fields SRS/SRT Gamma Knife Cyber-Knife Tomotherapy IRT IJDas (6) What is a Small Field? Lack of charged particle Dependent on the range of secondary electrons Photon energy Collimator setting that obstructs the source size Detector is comparable to the field size

2 Indra J. Das/ Small Fields (Page-2) Source Size Calculation of Fluence ap Elements Components Critical for Small Fields Direct beam source size Open beam off axis distribution Fluence modulation Step&shot Dynamic IJDas (7) 9%, 7%, 5%, 3%, 1% iso-intensity line Jaffray et al, ed Phys 2, (1993) IJDas (8) Courtesy Anders Ahnesjö Wedges Head scatter sources flattening filter collimators wedges onitor back scatter Collimator leakage, including LC interleaf leakage shape of LC leaf ends LC positioning Beam spectra Spectral changes Electron contamination Processes to include Definition of Small Fields Calculation of Fluence ap Elements Raytrace to calculate attenuation or use approximate penumbra shape IJDas (9) Das et al, ed. Phys. 35, , 28 IJDas (1) POI-eye-view of the source! For each element, find the contributions from the relevant sources Courtesy Anders Ahnesjö Energy Fluence Penumbra/Output Profile and Leaf Tip Positions & Location Energy FluenceSc normalized Large field cm x x2. 1.x1. cm.5x.5 cm cm x Source sizes cm Source plane isocenter plane x HVL -x w cm x x HVL x w Potential inconsistency dose calculations delivery Delivery: uses of lookup table or parameterized correction TPS: - - or direct calculation of 5% transmission x cm IJDas (11) Nyholm et al Radiother Oncol 78, 347, 26 and Phys ed Biol 51,6245, 26 IJDas (12) Courtesy Anders Ahnesjö

3 Indra J. Das/ Small Fields (Page-3) Dose and Penumbra with Spot Size Dosimetry Absolute Dose Relative Depth Dose o [D(r,d)/D(r,dm)] TR Profiles Output, S cp (total scatter factor) o [D(r)/D()] IJDas (13) Scott et al, ed Phys, 36, 3132, 29 IJDas (14) Small Field Dosimetry Problem Dosimetric Variation with Detectors 1.2 Total scatter factor from different institutions Total scatter factor with various detectors Cone Factor (St) % diff WEH U-Arizona Temple U U Penn Erlanger Serago et al. Fan et al. Zhu et al. Institutional variability in 6 V Radionics SRS dosimetry Cone Factor (St) % Diamond CEA (Film) Kodak(Film) TLD Pinpoint(par) Pinpoint(per).125ion(par).125ion(per) C(1mm) C(5mm) IJDas (15) Cone Diameter (mm) Das et al, J Radiosurgery, 3, , 2 IJDas (16) Cone Diameter (mm) Das et al, J Radiosurgery, 3, , Dose to Water For Small Fields: Volume Averaging Profiles with different detectors Output Factor IJDas (17) onte Carlo Diamond Diode LAC+film Film PTW 3114 (Pinpoint) PTW 311 (Semiflex) PTW 36 (Farmer) Side of Square Field (cm) From Roberto Capote, IAEA Normalized Dose (%) Dose profile of a 12.5 mm cone Distance (mm) Film Diamond Ion (Pinpoint) Ion (Parallel Plate) Hedrian, Hoban & Beddoe, PB, 41, 93-11, 1996 IJDas (18) Das et al, J Radiosurgery, 3, , Normalized Dose (%) Dose profiles for a 4 mm cone with different detectors Distance (mm) Film Diamomd Ion (Pinpoint) Ion (parallel-plate) Ion (.125 cc)

4 Indra J. Das/ Small Fields (Page-4) D en IJDas (19) Ratio of Readings? d( h) ( h) dh ( ) h m ax D E ab h d h W S D m m e a r m Dr () r () W S D e a (E,r) = r P ion P repl P wall P cec P pcf, m r r ( ) ( ) r m Dr () r () W S ( r ) C F1 C F2 D e a IJDas (2) Radiation easurements Charged particle equilibrium or electronic equilibrium Range of secondary electrons edium (tissue, lung, bone) Photon energy and spectrum Change in spectrum Field size Off axis points like beamlets in IRT Changes en / and S/ Detector size Volume Signal to noise ratio CPE & Electron Range IAEA/AAP proposed pathway CPE, Charged Particle Equilibrium IJDas (21) Electron range= d max in forward direction Electron range in lateral direction Nearly energy independent Nearly equal to penumbra (8-1 mm) Field size needed for CPE Lateral range 16-2 mm d max IJDas (22) Alfonso, et al. ed Phys 35, (28) IJDas (23) f Dw, Relative Dosimetry f Dw, / ( Output) clin w, clin rel Dw, / w, (Re ading rel, f clin, ) k k f clin, f clin, clin f ( S ( S f w, air N DW, o k Dw, clin clin f f D / ) w, air w, ) f, o k f, f, / clin, f k f clin, P P IJDas (24) Why So uch of Fuss? Reference () conditions cannot be achieved for most SRS devices (cyberknife, gammaknife, tomotherapy etc) achine Specific erence () needs to be linked to Ratio of reading (PDD, TR, Output etc) is not the same as ratio of dose D1 D2 D 1 1, f k clin, D

5 Indra J. Das/ Small Fields (Page-5) Relative dose at dmax 6 V; Central Axis Scanditronix-SFD Scanditronix-PFD.5 Exradin-A16.4 PTW-Pinpoint PTW-.125cc.3 PTW-.3cc PTW-.6cc.2 PTW-arkus Wellhofer-IC Field Size (cm) Das et al, TG-16, ed Phys, 35, 4186, 28 IJDas (25) Relative dose at dmax Field Size Limit for Accurate Dose easurements with Available Detectors 15 V; Central Axis Scanditronix-SFD Scanditronix-PFD Exradin-A16 PTW-Pinpoint PTW-.125cc PTW-.3cc PTW-.6cc PTW-arkus Wellhofer-IC Field Size (cm) IJDas (26) Detectors Detector anufacturer Type volume SFD Scanditronix Photon diode 1.7x1-5 cm 3 PFD Scanditronix Photon diode 1.9x1-4 cm 3 Exradin A-16 Standard Imaging Ion chamber.7cm 3 Wellhofer-IC4 Scanditronix Ion chamber.4 cm 3 Pinpoint PTW Ion chamber.15cm 3.125cc PTW Ion chamber.125cm 3.3cc PTW Ion chamber.3 cm 3.6cc PTW Ion chamber.6 cm 3 Diamond PTW Diamond.3cm 3 arkus PTW Parallel plate.55cm 3 Edge Detector Sun Nuclear Diode 1-5 cm 3 Other Radiation Detectors PTW Choice for small Fields IJDas (27) IJDas (28) Relative sensitivity Sensitivity vs Volume of Detectors PFD cc SFD.3cc cc 1 A-16 PinPoint arkus IC4 1.E-4 1.E-3 1.E-2 1.E-1 1.E+ Volume (cm 3 ) Understand Detector, Connector & Cable Srivastava et al, SU-GG-T-27, 21 IJDas (29) IJDas (3)

6 Indra J. Das/ Small Fields (Page-6) IJDas (31) Current Ion Chamber Issues in Small Fields Volume averaging Poor signal: signal to noise ratio Pion: Ion recombination 3 volts may not be needed ay be in proportional counter mode or gas multiplication Two voltage method may not be applicable Voltage Small volume Ion chamber Large volume Ion chamber Photon Fluence Photon Fluence 1.E+ 1.E-1 1.E-2 1.E-3 1.E-4 1.E+1 1.E+ 1.E-1 1.E-2 1.E-3 1.E-4 1.E-5.5 cm AIR, at exit E (ev) WATER, at dmax.5 cm, primary 5 cm, scatter 5 cm.5 cm, scatter 5 cm, primary IJDas (32) E (ev) (c) (a) Average Energy (ev) Spectra & Effective Energy from SRS Cones (.5-5 cm) V Diameter of Cone (cm) Verhaegen, Das, Palmans, PB, 43, , 1998 Radiological Parameters IJDas (33) IJDas (34) Sanchez-Deblado et al, Phy. ed. Biol., 48, 281, 23 Cyber Knife Dosimetry Francescon et al, ed. Phys. 25(4), 53, % C C C C C C IJDas (35) F. Araki, ed. Phys. 33, (26). IJDas (36)

7 Indra J. Das/ Small Fields (Page-7) Correction Factors Errors in easured Reading Correction Factor depends on: Field size Source size (FWH) Detector type IJDas (37) Francescon, et al ed Phys 35, 54, 28 IJDas (38) Kawachi et al, ed Phys, 35, , 28 Correction factor Published data on 1.16 Siemens; PTW diode Elekta; PTW diode Siemens; Exradin A16 Elekta; Exradin A Siemens; Sun Nuclear Dedge 1.8 "Elekta; Sun Nuclear Dedge" Siemens; PTW Pinpoint Elekta; PTW Pinpoint Siemens; PTW microlion Elekta; PTW microlion IJDas (39) Francescon et al ed Phys,38, 6513, 211 Field size (mm) IJDas (4) Detector Density Effect Scott et al, Phys ed Biol , 212 Correction Factor vs Ion Chambers of Linear Accelerators (Varian) IJDas (41) Chung et al, ed Phys, 37, , 21 IJDas (42) ed. Phys. 38 (12), 6992, 211

8 Indra J. Das/ Small Fields (Page-8) of Linear Accelerators Cyber Knife ed. Phys. 38 (12), , 211 Radiotherapy and Oncology 1 (211) IJDas (43) IJDas (44) Pantelis et al, ed Phy. 37, , 21 Tomotherapy k Reference Dosimetry k is not Constant in Small Field Reference: 5x1 cm 2, 85 cm SSD, 1 cm IJDas (45) Sterpin et al, ed Phys, 39, 466, 212 IJDas (46) Kawachi et al, ed Phys, 35, , 28 Depth Dose & Source Size Profile & Source Size IJDas (47) Sham et al, ed Phys, 35, , 28 IJDas (48) Sham et al, ed Phys, 35, , 28

9 Indra J. Das/ Small Fields (Page-9) Effect of Inhomogeneity Electron range & inhomogeneity IJDas (49) Range of secondary electrons Simple scaling based on density. K. Woo, and J. R. Cunningham, "The valididty of density scaling method in primary electron transport for photon and electron beams," ed. Phys. 17, (199). Perturbations of the detector T. auceri, and K. R. Kase, "Effects of ionization chamber construction on dose measurements in heterogeneity," edical Physics 14, (1987). R. K. Rice, J. L. Hansen, L.. Chin, B. J. ijnheer, and B. E. Bjarngard, "The influence of ionization chamber and phantom design on the measurement of lung dose in photon beam," edical Physics 15, (1988). IJDas (5) Relative Dose Inhomogeneity Corrections cm,.25 cm3 lung Homogeneous onte Carlo EPL.9 TR Ratio Power Law TR.8 Convolution CCC Water Depth (cm) IJDas (51) Jones & Das, ed. Phys. 3, 296, 23 Jones & Das, ed. Phys. 32, 766, 25 IJDas (52) TG-155 Recommendation Dosimetric measurements should be carried out with more than one detector system. Small volume detector should be used that has minimum energy, dose and dose rate dependence as discussed in TG-12 and Report No13 should be used. Stereotactic diodes or electron diodes are recommended for field sizes < 1x1cm2 icro chambers are best suited for dosimetric measurements for field sizes > 1x1 cm2 however, signal to noise as well as polarity effect should be evaluated. The quality of electrometer and triaxial cable as well as any connector and cables need to be of high quality. Stereotactic diode with micron size sensitive volume should be the detector of choice for measurements in beams in radiosurgery. The energy spectrum does vary in small fields such as SRS, and IRT but these changes result in insignificant variations in stopping power ratios when compared to those of the erence field used in dosimetry codes of practice. The treatment planning system performance should be caully validated when used for the treatment planning incorporating small fields. Although pencil beam and convolution/superposition dose engines are expected to perform well in small field treatment geometries and in almost homogeneous media, dose engines based on the onte Carlo method are the most accurate method for modelling dose from small fields in heterogeneous media. The calculation grid size should be significantly smaller (~1/1) compared to the field size. Small field dosimetry should have an independent audit by a different physicist either internal or external like Radiological Physics Center verification. IJDas (53) Summary Small volume detector should be used that has minimum energy, dose and dose rate dependence. icro-ion chambers are best suited for small field dosimetry; however, signal to noise should be evaluated. Stereotactic diode are ideally suited for radiosurgery beams. If field size is small compared to detector measurements should be performed at a greater source to surface distance with proper correction. IJDas (54) -Summary Energy spectrum does vary in small fields such as SRS, and IRT, however, its impact is not significant. Stopping power ratio in small fields for most ion chambers is relatively same as the erence field. Spot check and verification of smaller fields should be carried out with at least another independent method (TLD, film, C, etc). Stay tuned to newer data and IAEA and AAP TG guidelines.

10 Indra J. Das/ Small Fields (Page-1) Thanks IJDas (55)