04 - Treatment Machines and Generators II

04 - Treatment Machines and Generators II - Lecture # 4...

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1 Lecture # 4 Treatment Machines and Generators - 2 Tarun K. Podder, PhD, DABR Department of Radiation Oncology Brody School of Medicine Leo Jenkins Cancer Center East Carolina University January 20, 2011
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2 Penumbra – transmission, geometric Dosimetrically physical penumbra – transmission, geometric and side scattering Physical penumbra - lateral distance between 90% (or 80%) and 20% isodose lines at the depth of D max Penumbra Source Collimator Transmission penumbra Geometric penumbra (1) (3) (2) Penumbra Field size
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3 Penumbra increases with increased source size (s), field size, energy, SSD, SCD (SSD+d) and decreased SDD Penumbra Geometric penumbra (P d ) Proton Why is the penumbra for Cobalt so large?
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4 MLCs and Penumbra SDD
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5 Penumbra (geometric) Source size (s): • Co-60 is 1-2 cm • Linac (4-8 MV) is about 3 mm How does the penumbra vary with SDD?
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6 Penumbra (physical) Podder et al. 2010 (Eur. J. MedPhys)
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7 Penumbra (physical) Podder et al. 2010 (Eur. J. MedPhys)
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Cobalt-60: Teletherapy Machine • Low photon energy (γ-ray) • β particles – absorbed in steel capsule Half-life 5.26 yr • Source off-on positioning - rotating wheel - sliding drawer - shut-off by Hg layer - shut-off by a shutter • Large penumbra
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9 Microtron B. Electron orbits and accelerating cavities, racetrack type. A. Circular microtron unit Microwave power source - Klystron or magnetron Oscillating electric field in microwave cavity Cavity voltage, freq. ,and magnetic field are adjusted suitably for correct phase 10-22 MeV energy Simplicity, small beam energy spread Circular microtron - bulky
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10 Betatron
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11 Once were competitive with linear accelerators Consists of a strong magnetic field
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04 - Treatment Machines and Generators II - Lecture # 4...

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