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Microdosimetry of 10–15 MeV bremsstrahlung x rays
Author(s) -
Amols H. I.,
Zellmer D. L.
Publication year - 1984
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.595499
Subject(s) - bremsstrahlung , dosimetry , physics , linear particle accelerator , spectral line , radiation , nuclear medicine , dose rate , nuclear physics , atomic physics , beam (structure) , optics , medical physics , electron , medicine , astronomy
Experimental techniques have been 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, have been produced at ultralow dose rates (50–200 μGy/h) which enables direct measurements of lineal energy distributions with a conventional Rossi‐type gas proportional counter. Extensive measurements have been 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 have been developed to correct measured lineal energy spectra for pileup caused by the low duty factor of the linear accelerator. The lineal energy spectra of these megavoltage beams differ significantly from Co‐60, with dose averaged lineal energies ( y ̄ D ) being 20%–30% lower than for Co‐60. 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.