05 - Dose Distribution and Scatter Analysis

05 - Dose Distribution and Scatter Analysis - Lecture # 5...

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1 Dose Distribution and Scatter Analysis Tarun K. Podder, PhD, DABR Department of Radiation Oncology Brody School of Medicine Leo Jenkins Cancer Center East Carolina University January 25, 2011 Lecture # 5 (Ch. 9)
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X-ray Beam Quality Changes in spectral distribution of 200-kVp x-ray beam with various filters. Curve A is for Al, curve B is for Sn + Al, and curve C is for Sn + Cu + Al. 2 Beam energy is defined by the peak/max energy. Average energy = ?
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X-ray Beam Quality HVL 1 < HVL 2 < HVL 3 (why?) Transmission of an x-ray beam with a spectrum of photon energies through an aluminum absorber. First half-value layer (HVL) = 0.99 mm Al, second HVL = 1.9 mm Al, and third HVL = 2.0 mm Al. 3
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4 Phantom In radiation therapy, it is hardly possible to measure dose distribution directly in patients. Radiation absorption and scattering – approximately same Universally available Reproducible Measuring instruments (chamber) need to be waterproof Water/tissue equivalent solid phantoms - effective atomic number - number of electron per gram - mass density Dose distribution data are usually measured in water phantom. Water instead of Tissue Need to be same
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5 Phantom Water/tissue equivalent solid phantoms Predominating mode of interaction for MV photon beams in clinical range – Compton Necessary condition for water equivalence for such beams – same electron density, i.e. No. of electron/cm 3 ρ e = electron density ρ m = mass density Z = atomic number A = atomic weighty N A = Avogadro’s number (6.0228 x 10 23 ) a i = fraction by wt. of i -th element Z i = atomic No. of i -tl element A i = atomic wt. of i -th element (Khan ch. 5)
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6 Phantoms – physical properties
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7 Phantoms – physical properties Mass attenuation coefficient Mass energy absorption coefficient Electron mass stopping Angular scattering power ratio Fillers to epoxy resins for tissue substitute require to have the following important radiation properties.
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8 Anthropomorphic Phantoms Anthropomorphic phantom of woman Rando phantom Spine Skull
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9 Depth Dose Distribution
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05 - Dose Distribution and Scatter Analysis - Lecture # 5...

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