total skin electron therapy treatment...
TRANSCRIPT
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Total Skin Electron Therapy Treatment Optimization:
MONTE CARLO SIMULATION OF LARGE SCATTERED ELECTRON
BEAMS AT EXTENDED DISTANCES
Parvaneh Shokrani, PhD.Isfahan University of Medical Sciences
Isfahan, [email protected]
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Total Skin Electron Therapy
• Total skin electron therapy (TSET) (AAPM 1987) is an external beam therapy indicated for the Treatment of mycosis fungoides (t-cell lymphoma) of the skin and lymph nodes
• It is a complex technique for which special irradiation and dosimetry conditions have been studied based on the particular methods implemented (AAPM 1987, Arrans et al 1999, Chen et al 1998)
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Irradiation Techniques
• Beam scatterers are used • Placed in the treatment head or
externally:– near treatment plane, or– near the treatment unit head located
several meters from the patient
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Problem
• Broad treatment field are required for TSET
• In a confined treatment room little divergence of electron beam occurs (Lamba, 1988)
• Therefore an efficient scatterer is required to be placed at the treatment head
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Problem
Selecting properties of the optimal TSET scatterer in order to achieve the optimal treatment field characteristics
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Parameters
Treatment unit parameters influencing treatment field characteristics for a given electron beam and angle:
Type and thickness ofTSET scatterer
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Treatment Field Characteristics
• Depth dose curves• Dose rate• Field uniformity• Bremsstrahlung contamination
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Solution
• Experimental methods• Monte CARLO methods
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Materials and Methods
• Phillips SL-20 linear accelerator• 0.025 cm copper scatterer• PRM parallel plate ionization
chamber (Ngas= 2.33x1010)• Victoreen 500 electrometer• Polystyrene phantom
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Materials and Methods
• EGS4 Monte Carlo code system (nelson and Hirayama et al., 1985), on a VAX 11/780
• ECUT=560 KeV, PCUT=100 keV
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Variance Reduction Techniques
DNEAR variable• PRESTA algorithm• Sectioned problems
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GEOMETRY for a Single Field TSET Set up SSD=300
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GEOMETRY for a Single Field TSET Set up SSD=300
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HOWFAR Subprogram
Consisted of subroutines:• Cylinder• Cone• Plane2p• Plane1
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SL-20 Treatment Head:Components Used In The
Simulations
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Central Axis PDD IN Water:6 MeV Open Beam, SSD=95
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CENTRAL AXIS PDD:MEASURED 6 MeV and CALCULATED 7
MeVSSD=95
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Simulation Of Air Gap Between VACCUM Window And The Flattening Scattering Foil:
Effect On CA PDD
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PHYLLIPS SL-20 Flattening Scattering Foil: Dimensions
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Geometry Used In HOWFAR To Simulate The Treatment Machine
Head
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Field Uniformity (dmax)Scattering Foil Simulated As a Flat Base and
One Conic Section, SSD=95
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Field Uniformity at dmax:Scattering Foil As a Flat Base and
Two Conic Sections, SSD=95
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Energy Distribution of Electron Beam Before Entering the TSET
Scatterer
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Energy Distribution of Electron Beam Before Entering the TSET
Scatterer
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Energy Distribution Of Bremsstrahlung Contamination At
The Level Of Scatterer
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Radial Distribution of Electrons Reaching TSET Scatterer
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Radial Distribution of Bremsstrahlung Radiation Reaching TSET Scatterer
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Geometry Used To Simulate Scattered Electron Field At
Extended SSD
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Geometry Used To Calculate Depth Dose Distributions
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Depth Dose Curve At 300 SSDScatterer: 0.025 Cm OF COPPER
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Values Used To Benchmark Monte Carlo Simulations
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BREMSS CONTAMINATIONCOPPER SCATTERER
RELATIVE TO DMAX, TREATMENT PLANE AT 300 SSD, D=10cm
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Uniformity At DMAXCopper SCATERER
300 SSD
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Conclusions
• EGS4 Monte Carlo code together with the user code developed in this research, simulated a single field TSET technique accurately
• Problem areas: 1- central axis surface dose2- distance dependency of x-ray
contamination
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Thank you for your time and attention
P. Shokrani, Ph.D.Chief Physicist,Sayed-al-shohada Hospital,Khayam Ave, Isfahan, Iran.
++98 913 315 8618