Microdosimetry of 10-15 MeV bremsstrahlung x rays

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Abstract
Experimental techniques were developed for obtaining microdosimetric spectra on a hospital-based linear accelerator. Teletherapy beams of 10 and 15 MeV bremsstrahlung X-rays from a Varian Clinac-18 and Clinac-20, respectively, were produced at ultralow dose rates (50-200 .mu.Gy[gray]/h) which enables direct measurements of lineal energy distributions with a conventional Rossi-type gas proportional counter. Extensive measurements were made to insure that the dosimetric properties of these low dose rate beams are nearly identical to those produced under high dose rate clinical conditions. Analytical procedures were developed to correct measured lineal energy spectra of these megavoltage beams differ significantly from Co with dose averaged lineal energies (.hivin.yD) being 20-30% lower than for 60Co. Although such differences may not be important at clinical doses, the theory of dual radiation action does predict a lower biological effectiveness for these beams at very low dose levels.
Funding Information
  • U.S. Public Health Service (CA32414)

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