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Electron and Proton Absorbed-Dose-to-Water Primary Standards


Work on absorbed-dose standards for particle beams is in progress, with preliminary results obtained for both high-energy electrons (at the NIST Clinac 2100C) and double-scattered protons (at the Proton Therapy Center, MD Anderson Cancer Center, in Houston,TX; and the Hampton University Proton Therapy Institute, in Hampton, VA).


The NIST water calorimeter, originally designed and tested in photon beams (viz. Co-60 and subsequently in 6 MV and 18 MV x-rays from the NIST Clinac 2100C), is being put to use in particle beams as well. In 2011, measurements conducted in the NIST Clinac 2100C with high-energy electron beams under standard reference conditions at 12 MeV, 16 MeV and 20 MeV in a water phantom were carried out along with similar sets of measurements obtained in 6 MV photons. (The latter were done as a consistency check on the calorimeter, as comparisons could be made with a depth-dose profile obtained from a Farmer chamber that had been calibrated both in Co-60 and Clinac 6 MV beams.) These were followed by measurements with a parallel-plate chamber that had been calibrated previously in Co-60. While calorimeter results were within 0.5 % of the depth-dose profile at each depth, they were ~1 % higher than the chamber results at all depths. The discrepancy is believed to be a consequence of an error in the value of kecal, the so-called photon-electron conversion factor specified by TG-51. Additional measurements and Monte Carlo simulations are expected to resolve the issue. The calorimeter has also been used in a vertical, double-scattered proton beam at the Hampton University Proton Therapy Institute (HUPTI), in what had amounted to a first off-site measurement. The first set of measurements, done in December 2012, were conducted with the NIST calorimeter and four ionization chambers – two each from NIST and HUPTI, all four of which had been used in a 2011 proton intercomparison hosted by the Proton Therapy Center (PTC), at the MD Anderson Cancer Center. Results from the calorimeter, averaged over 22 exposures, showed agreement within experimental uncertainties for all four chambers, using the IAEA TRS-398 CoP for converting charge to proton dose. NIST looks forward to future visits both to HUPTI and to the PTC, in order to provide calorimetry results to support the results of the chamber intercomparison.

Electron and Photon Absrobed-Dose-to-Water
Photograph by: Ronald Tosh

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Ronald Tosh