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

摘要

Purpose 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 TrueBeam TM linear accelerator. Methods Neutron fluence spectra were measured using a Nested Neutron Spectrometer TM (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. Results 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 different locations within the treatment room. However, the FF‐ratio in neutron ambient dose equivalent was reduced significantly when the linac jaws were opened compared to closed, which implies that the jaws contribute more to the photoneutron spectrum of the 10?MV FFF beam than to the 10?MV beam. Finally, it was found that the 10?MV FFF beam produces more photoneutrons per electron striking the bremsstrahlung target than the 10?MV beam (FF‐ratio?=?2.56). Conclusions The photoneutron fluence per MU produced by the 10?MV FFF beam is 37% lower than the 10?MV beam of a Varian TrueBeam linac. Accordingly, a reduction in neutron dose received by patients is achieved through use of the unflattened beam, provided that treatment plans for each beam require approximately the same number of MU. It was found to be instructive to compare the photoneutron yield per source electron between the two beams as it helped provide an understanding of the physics underlying photoneutron production in both beams.