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Spectroscopy of Low Dose Rate (LDR) Brachytherapy Seeds

Spectroscopy of Low Dose Rate (LDR) Brachytherapy Seeds . Stephen M. Beach, M.S. University of Wisconsin Radiation Calibration Laboratory Department of Medical Physics. Description of Ge Spectrometer. Outer Al ‘ can, ’ common to most spectrometers . 500 micron Be entrance window.

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Spectroscopy of Low Dose Rate (LDR) Brachytherapy Seeds

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  1. Spectroscopy of Low Dose Rate (LDR) Brachytherapy Seeds Stephen M. Beach, M.S. University of Wisconsin Radiation Calibration Laboratory Department of Medical Physics

  2. Description of Ge Spectrometer • Outer Al ‘can,’commontomostspectrometers. • 500 micron Be entrance window. • Thicker N+ contact on inside. • Thin P+ contact on the outside. • Useful energy range from 2 MeV to 3 keV.

  3. Physical Measurement Setup • Ge detector in the “in air” configuration. Background shield and beam defining aperture. Linear optic bench for reproducible radial positioning of sources. For various source strengths there are three choices of apertures: 2 cm, 1 cm, and 2 mm.

  4. Calibration of Spectrometer • For meaningful results both energy and efficiency calibrations must be carried out. Two sources are used that have NIST traceable activities. A 55Fe with a 5.9 keV line, and a multi-nuclide source with 14 lines from 22.1 keV up to 1.836 MeV.

  5. Spectral Characteristics of LDR Nuclides • Bare 103Pd spectra from NIST. Measured 103Pd seed (TheraSeed). Measured 103Pd seed (IBt). Bare 125I spectra from NIST. Measured 125I seed with no Ag (IoGold). Measured 125I seed with Ag present (6711).

  6. Spectral Characteristics of LDR Nuclides Cont. • Activity is based on the emissions of bare nuclides. LDR seeds are not bare nuclides, therefore apparent activity is used. The differing spectra between seeds, as shown, is good evidence of why the use of apparent activity, and its associated “dummy” variables (exposure or air-kerma rate constants), is a bad idea. • ‘Take home point’: Air Kerma Strength is the way to go!

  7. Transmission and Attenuation Measurements • Setup is same as “in air” with the addition of an attenuator stand with a primary aperture.

  8. Transmission and Attenuation Measurements Cont. Preliminary experiments have been carried out with C552 and Al. Calculations of the attenuation coeff. (m) were made using our data in this well known equation: Results for m from Al are within 2% of published NIST data. Point to note: Most brachytherapy dosimetry is carried out with an average global m, my approach is to separate the lines and use discrete m’s.

  9. Future Uses of Spectrometer • Water measurements performed in a liquid, not a solid, water medium. Prototype water tank has been constructed, measurements are pending.

  10. Future Uses of Spectrometer Cont. • Depth dose measurements in liquid water, in conjunction with film. • Measurement verification and comparison on a spectral line by line basis of TG-43’s parameters against the published ‘integrated’ measurements. • Spectral determination and cataloging of the absolute emission spectra (on a photon by photon basis) from each LDR seed in clinical use.

  11. Acknowledgements • Professor Larry DeWerd for squeezing all of his brains into this project

  12. Thank you for your attention, and for not snoring.

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