10 - Electron Beam Therapy

10 Electron Beam - Lecture 10(Ch 14 Electron Beam Therapy Tarun K Podder PhD DABR Department of Radiation Oncology Brody School of Medicine Leo

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1 Electron Beam Therapy Tarun K. Podder, PhD, DABR Department of Radiation Oncology Brody School of Medicine Leo Jenkins Cancer Center East Carolina University February 10, 2011 Lecture # 10 (Ch. 14)
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2 Clinical Applications of Electron Beams Sharp drop-off dose beyond tumor - minimizing dose to deeper tissue Dose uniformity in the tumor/target Treatment of superficial tumors Skin and lip cancers Chest wall irradiation for breast cancer Boost dose to nodes Head and neck cancers Example clinical sites: Clinical Characteristics:
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3 Clinical Applications of Electron Beams Boost dose Portal Image
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4 Electron Interactions 1) Inelastic collisions with atomic electrons – ionization and excitation 2) Inelastic collisions with nuclei – bremsstrahlung 3) Elastic collisions with atomic electrons 4) Elastic collisions with nuclei Inelastic collisions a part of kinetic energy is lost in producing ionization or converted to other forms of energy (photon and excitation) Elastic collision kinetic energy is not lost; redistributed among the particles emerging from the collision
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5 Electron Interactions Excitation : During interaction, if the gain in energy of the orbital electron is equal to the difference in energy between its own energy level and a higher energy level, then the electron is excited to the higher energy level. Ionization : If the gain in energy of the orbital electron is greater than the binding energy for the electron, then an electron is removed from its orbital. The atom is “ionized”.
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6 Electron Interactions Low atomic number (Z) media (water, tissue) electron lose energy through ionization High Z materials (lead) bremsstrahlung production Delta-ray (δ-ray) or secondary electron if the KE of the stripped electron is large enough for it to cause further ionization Energy of an electron beam traveling through medium is degraded continuously – until the electrons reach thermal energies and are absorbed by the surrounding atoms
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7 Rate of Energy Loss Collisional Loss (ionization and excitation) – rate of energy loss per gram per cm 2 ( mass stopping power ) is higher for low Z materials; about 2 MeV/ cm for MV beams Radiation Loss (breamsstrahlung) – rate of energy loss per cm is proportional to Z 2 and increases with the electron KE; producing photon beams.
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8 Rate of Energy Loss
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9 Stopping Power ICRU defines the total mass stopping power ( S/ρ ) tot of a material as- dE/ρdl = ( S/ρ ) tot = ( S/ρ ) col + ( S/ρ ) rad dE = total energy loss dl = traveling path length ρ = density of the material
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10 Absorbed Dose Electron fluence Restricted collision stopping power, i.e. linear energy transfer (LET) Absorbed dose depends on-
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11 Absorbed Dose = cutoff energy, i.e. below which the secondary electron dissipate energy near the site of release Collision mass stopping power is - Absorbed dose: is the differential distribution of fluence with respect to energy
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This note was uploaded on 01/21/2012 for the course PHYS 6720 taught by Professor Hu during the Spring '10 term at East Carolina University .

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10 Electron Beam - Lecture 10(Ch 14 Electron Beam Therapy Tarun K Podder PhD DABR Department of Radiation Oncology Brody School of Medicine Leo

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