Effects of tertiary MLC configuration on secondary neutron spectra from 18 MV x-ray beams for the Varian 21EX linear accelerator
Schematic Monte Carlo model of the Varian 2100 linac showing neutron tally location at a source-to-axis distance of 100 cm and at a point 40 cm away from the gantry isocenter (in the inferior direction along the treatment couch). Measurements were made in the corresponding location.
Unfolded neutron spectra measured in configurations 1 and 3.
Neutron spectra measured in configurations 2 and 4.
Neutron spectra measured in configurations 1 and 2.
Measured spectrum and Monte Carlo calculated spectrum for configuration 1.
Measured spectrum and Monte Carlo calculated spectrum for configuration 3.
Measured photoneutron spectra for 18 MV photon beams for the Varian Clinac 21EX, Elekta Precise, and Siemens ONCOR linacs. Secondary neutron spectra for 18 MV photon beams for Siemens and Elekta spectra with the MLC and jaws closed and those for the Varian linac under two different conditions: Jaws closed and MLC closed (configuration 1) and jaws closed and MLC removed (configuration 3).
Collimator configurations (i.e., position of jaws and MLC) for the neutron spectrum measurements in this study.
Parameters calculated from the measured spectra for the four collimator configurations. Total uncertainties are shown to the right as percentages.
Quantitative comparison of the measured and calculated data for the four collimator configurations is presented in Table III, which shows the measured neutron fluence per MU, the average energy of the measured spectrum, and the ambient dose equivalent for each configuration.
Absolute neutron production ( neutrons per source particle) for beamline components in each collimator configuration. Values in parentheses provide neutron production as a percentage of the net neutron production.
Our current measured photoneutron data and the measured and Monte Carlo calculated data of Kase et al. for a Varian linac operated at 18 MV beam energy (Refs. 3 and 11).
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