The effect of the flattening filter on photoneutron production at 10 MV in the Varian TrueBeam linear accelerator

Medical Physics
Logan MontgomeryJohn Kildea

Abstract

Neutrons are an unavoidable by-product of high-energy radiation therapy treatments that deliver unwanted nontarget dose to patients. Use of flattening-filter-free (FFF) photon beams has been shown to significantly reduce photoneutron production per monitor unit (MU) of dose delivered. The purpose of this investigation was to characterize the photoneutron production of the 10 MV and 10 MV FFF beams of the Varian TrueBeamTM linear accelerator. Neutron fluence spectra were measured using a Nested Neutron SpectrometerTM (NNS, Detec Inc., Gatineau, Canada). The ratios of neutron fluence and ambient dose equivalent for the 10 MV FFF beam relative to the 10 MV beam, dubbed FF-ratios (FFF/FF), were used to characterize the difference between the two beams. FF-ratios were compared under the following three conditions (a) per MU, at various locations in the treatment room, (b) per MU, with the linac jaws opened and closed, and (c) per electron striking the bremsstrahlung target, as opposed to per MU, at one location with the jaws closed. On average, the neutron fluence for the 10 MV FFF beam was 37% lower per MU than the 10 MV beam (FF-ratio = 0.63). The FF-ratio in neutron fluence and ambient dose equivalent did not vary by much between...Continue Reading

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