SmallSmall--Field DosimetryField Dosimetry

Indra J. DasIndiana University School of Medicine, Indianapolis, IN, USA

Maria M. Aspradakis

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Maria M. AspradakisCantonal Hospital of Lucerne, Lucerne Switzerland

Otto A. SauerUniversity of Wurzburg, Germany

Anders AhnesjöUppsala University & Nucletron Scandinavia AB, 751 47 ,Uppsala, Sweden

Challenges in Small Field MV Photon Dosimetry

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Maria Mania Aspradakis

Cantonal Hospital of Lucerne Switzerland

Klinik und Poliklinik für StrahlentherapieDirektor: Prof. Dr. Dr. M. Flentje

Small Field DosimetryRelative dosimetry: detector choice and corrections

Otto A SauerOtto A Sauer

Modelling of small fields in Treatment Planning Systems

Anders AhnesjöAdj. Prof. Uppsala University, Sweden

Senior Research Scientist, Nucletron, Uppsala

Treatment Fields

Magna-FieldsTraditional Fields

40x40 cm2 4x4 cm2200x200 cm2

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Advance Therapy FieldsSRS/SRTGamma KnifeCyber-KnifeTomotherapyIMRT

4x4 cm2 0.3x0.3 cm2

Small Field

What is a Small Field?

� Lack of charged particle■ Dependent on the range of secondary

electrons

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■ Photon energy

� Collimator setting that obstructs the source size

� Detector size comparable to the field size

[ Detail discussion by Maria in this talk ]

CPE & Electron Range

� Electron range= dmax in forward direction

� Electron range in lateral direction

■ Nearly energy independent

� CPE, Charged Particle Equilibrium

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■ Nearly energy independent

■ Nearly equal to penumbra (8-10 mm)

� Field size needed for CPE■ Lateral range

■ 16-20 mm

dmax

[ Detail discussion by Anders on treatment planning in this talk ]

Definition of Small Fields

An elegant presentation of this issue by Anders in this talk

IJD/AAPM-10Das et al, Med. Phys. 35, 206-215, 2008

�� Ion ChambersIon Chambers■■ Diameter, volume, and window thicknessDiameter, volume, and window thickness■■ W/e, S/W/e, S/ρρ, , µµen/en/ρρ■■ PerturbationPerturbation

�� FilmsFilms■■ Silver bromide filmsSilver bromide films

●● high Z, sensitivity, thicknesshigh Z, sensitivity, thickness■■ GAFchromicGAFchromic polymer filmspolymer films

Dosimeters & Associated ProblemsDosimeters & Associated Problems

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■■ GAFchromicGAFchromic polymer filmspolymer films●● Sensitivity, thickness, polymerization at low energySensitivity, thickness, polymerization at low energy

�� TLD/OSL (sensitivity at low energy, F center)TLD/OSL (sensitivity at low energy, F center)■■ Size 10Size 10--15 15 µµm, difficulty of handlingm, difficulty of handling

�� Fricke Dosimeter (G value)Fricke Dosimeter (G value)■■ (Size and dose)(Size and dose)

�� MOSFET (thickness, dose rate, burnout, life, cost)MOSFET (thickness, dose rate, burnout, life, cost)�� Monte Carlo (benchmarking, expertise, reliability)Monte Carlo (benchmarking, expertise, reliability)

[ Detail discussion by Otto in this talk ]

6 MV; Central Axis

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Scanditronix-SFDScanditronix-PFDExradin-A16PTW-PinpointPTW-0.125ccPTW-0.3ccPTW-0.6ccPTW-MarkusWellhofer-IC4

15 MV; Central Axis

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Field Size Limit for Accurate Dose Measurements with Available Detectors

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Field Size (cm)

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Scanditronix-SFD

Scanditronix-PFDExradin-A16

PTW-PinpointPTW-0.125cc

PTW-0.3ccPTW-0.6cc

PTW-MarkusWellhofer-IC4

Das et al, TG-106, Med Phys, 35, 4186, 2008

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ThanksThanks