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Pencil-Beam Redefinition Algorithm Robert Boyd, Ph.D.

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Pencil Beam Algorithms central axis of broad beam (Z) Y Z X X-Y plane normal to beam axis (Z) pixel bounding pencil beams (2x2 mm 2 at isocenter)

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Pencil Beam Redefinition XX Z Z+ Z X-Y planes are spaced 5 mm apart on Z axis X-Y planes are spaced 5 mm apart on Z axis

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PBRA Physics Primary electron transport only Primary electron transport only delta-rays not modeled delta-rays not modeled Multiple Coulomb scattering approximated with a Gaussian distribution Multiple Coulomb scattering approximated with a Gaussian distribution large-angle scattering not modeled large-angle scattering not modeled Mean collisional energy loss only Mean collisional energy loss only catastrophic energy losses not modeled catastrophic energy losses not modeled

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PBRA Physics Approximations PBRA requires measured central-axis depth dose curve PBRA requires measured central-axis depth dose curve PBRA uses an energy- dependent correction factor C(E) to match calculated and measured central axis depth dose curve PBRA uses an energy- dependent correction factor C(E) to match calculated and measured central axis depth dose curve

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Polyenergetic Spectrum

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PBRA Correction Factor C(E) Solid Line: Monoenergetic PBRA C(E) Dashed Line: Polyenergetic PBRA C(E)

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Polyenergetic PBRA

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20-MeV Horizontal Bone Slab Varian Clinac 2100, 15x15-cm 2 open applicator, 100 cm SSD

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20-MeV Horizontal Bone Slab Varian Clinac 2100, 15x15-cm 2 open applicator, 100 cm SSD

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20-MeV Horizontal Air Slab Varian Clinac 2100, 15x15-cm 2 open applicator, 100 cm SSD

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20-MeV Vertical Air Slab Varian Clinac 2100, 15x15-cm 2 open applicator, 100 cm SSD

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20-MeV Vertical Air Slab Off-axis profile at 4.5 cm depth Varian Clinac 2100, 15x15-cm 2 open applicator, 102 cm SSD

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20-MeV Nose Surface Varian Clinac 2100, 15x15-cm 2 open applicator, 100 cm SSD

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9-MeV Nose Surface Off-axis profile at 1 cm depth Varian Clinac 2100, 15x15-cm 2 open applicator, 100 cm SSD

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PBRA Evaluation with Measured Data Set - Results PBRA was not able to achieve 4% or 2 mm dose calculation accuracy for all data points PBRA was not able to achieve 4% or 2 mm dose calculation accuracy for all data points

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Beam Modeling isocenter patient custom beam collimation L0L0 electron source SAD vir

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Dual-Source Beam Modeling isocenter patient custom beam collimation primary electron source secondary electron source

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Dual-Source Model - 100 cm SSD Varian Clinac 1800, 9 MeV, 6x6-cm 2 open applicator

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Dual-Source Model - 110 cm SSD Varian Clinac 1800, 9 MeV, 6x6-cm 2 open applicator

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IMC - Transverse Plane Varian 2100, 16 MeV, 15x15-cm 2 applicator, 105 cm SSD

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IMC - Transverse Plane Varian 2100, 16 MeV, 15x15-cm 2 applicator, 105 cm SSD

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IMC - Sagittal Plane Varian 2100, 16 MeV, 15x15-cm 2 applicator, 105 cm SSD

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IMC - Sagittal Plane Varian 2100, 16 MeV, 15x15-cm 2 applicator, 105 cm SSD

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Parotid Gland - Transverse View Varian 2100, 16 MeV, 15x15-cm 2 applicator, 100 cm SSD

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Parotid Gland - Transverse View Varian 2100, 16 MeV, 15x15-cm 2 applicator, 100 cm SSD

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Ethmoid Sinuses - Transverse Plane Varian 2100, 16 MeV, 10x10-cm 2 applicator, 100 cm SSD

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Ethmoid Sinus - Transverse Plane Varian 2100, 16 MeV, 10x10-cm 2 applicator, 100 cm SSD

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Ethmoid Sinus - Profile at Y = 13.0 cm Varian 2100, 16 MeV, 10x10-cm 2 applicator, 100 cm SSD

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Clinical Evaluation - Results Accuracy criteria was not achieved for entire irradiated volume, albeit only a small volume (< 3.5%) had dose differences greater than 4% and greater than 2 mm DTA. Accuracy criteria was not achieved for entire irradiated volume, albeit only a small volume (< 3.5%) had dose differences greater than 4% and greater than 2 mm DTA. PBRA showed good agreement with Monte Carlo in matching isodose lines. PBRA showed good agreement with Monte Carlo in matching isodose lines. Better modeling of physics will improve the accuracy of PBRA- calculated dose. Better modeling of physics will improve the accuracy of PBRA- calculated dose.

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Custom Bolus / Skin Collimation

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Electron Arc Therapy

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Skin Collimation

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Arc Therapy with Skin Collimation

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Pencil-Beam Divergence Current PBRA virtual source distance is equal to distance to broad beam virtual source virtual source distance is equal to distance to broad beam virtual source mathematics assume “parallel”point beams mathematics assume “parallel”point beams integration performed over projected area integration performed over projected area zz s vir xx

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Pencil-Beam Divergence divPBRA virtual source distance is a pencil beam-specific parameter virtual source distance is a pencil beam-specific parameter mathematics assume divergent point beams mathematics assume divergent point beams integration performed over normal pixel width integration performed over normal pixel width zz s vir xx

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Local Pencil-Beam Divergence

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20-MeV Horizontal Air Slab Varian Clinac 2100, 15x15-cm 2 open applicator, 100 cm SSD

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Pencil-Beam Divergence Results divPBRA was more accurate than PBRA for most data points divPBRA was more accurate than PBRA for most data points divPBRA was not able to achieve 4% or 2 mm accuracy for all data points divPBRA was not able to achieve 4% or 2 mm accuracy for all data points Calculation times were approximately 30% longer Calculation times were approximately 30% longer

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Arc Beam Modeling

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Future Work Dosimetry studies using PBRA Dosimetry studies using PBRA Tomotherapy vs. conventional electron therapy Tomotherapy vs. conventional electron therapy Field matching for chest wall treatments Field matching for chest wall treatments Electron arc therapy planning using divPBRA Electron arc therapy planning using divPBRA Realistic dose deposition kernels using Monte Carlo Realistic dose deposition kernels using Monte Carlo Automated custom bolus/skin collimation planning using PBRA Automated custom bolus/skin collimation planning using PBRA Translating PBRA to commercial system Translating PBRA to commercial system

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Acknowledgements Kenneth Hogstrom, Ph.D. Kenneth Hogstrom, Ph.D. Almon Shiu, Ph.D. Almon Shiu, Ph.D. Dennis Leavitt, Ph.D. Dennis Leavitt, Ph.D. Mitch Price, M.S. Mitch Price, M.S. Melinda Chi, M.S Melinda Chi, M.S Paul Alderson, B.S. Paul Alderson, B.S.

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