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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 Hampton University Proton Therapy Institute, Hampton, VA).


Associated correction factors for water calorimetry of both types of beams are being determined according to methods developed previously for photon beams. Room temperature water calorimetry measurements of Clinac 12 MeV, 16 MeV and 20 MeV electron beams at associated reference depths in a water phantom were interspersed with measurements of Clinac 6 MV x-ray beams (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 plane-parallel 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 may be a consequence of the value of kecal used for scaling the chamber results, which recent Monte Carlo studies suggest may be in error. More comparison measurements are planned for the coming year. In November 2011, NIST participated in an intercomparison involving representatives from 11 institutions (of which 9 were U.S. proton therapy facilities), each of whom brought thimble and plane-parallel chambers for testing in simulated brain and prostate portals at the MD Anderson Proton Therapy Center (Houston, TX). The study, commissioned by NCI through Massachusetts General Hospital (Boston, MA), also called for development of a national primary standard for proton beams, and NIST has embarked on this task by conducting initial measurements with the present water calorimeter at a nearby proton center (Hampton University Proton Therapy Institute). More visits are planned in the coming year, but the first visit, done in December 2012, yielded high-quality results from the calorimeter as well as four chambers (two thimble and two plane-parallel) used in the previous intercomparison.

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

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