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Measuring neutron spectra in radiotherapy using the nested neutron
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resulting from photonuclear interactions in the head of a linear accelerator
pose an iatrogenic risk to patients and an occupational risk to personnel during
radiotherapy. To quantify neutron production, in-room measurements have
traditionally been carried out using Bonner sphere systems (BSS) with activation
foils and TLDs. In this work, a recently developed active detector, the nested
spectrometer (NNS), was tested in radiotherapy bunkers.
The NNS is designed for easy handling and is more practical than the traditional
BSS. Operated in current-mode, the problem of pulse pileup due to high
dose-rates is overcome by measuring current,
similar to an ionization chamber. In a bunker housing a Varian Clinac 21EX, the
performance of the NNS was evaluated in terms of reproducibility, linearity, and
dose-rate effects. Using a custom maximum-likelihood
expectation–maximization algorithm, measured
spectra at various locations inside the bunker were then compared to
simulations of an identical setup. In terms of dose,
ambient dose equivalents were calculated from the measured spectra and
compared to bubble detector neutron
The NNS-measured spectra for neutrons at various locations in a treatment room
were found to be consistent with expectations for both relative shape and absolute
magnitude. Neutron fluence-rate decreased with distance from the
and the shape of the spectrum changed from a dominant fast neutron peak near the
head to a dominant thermal neutron peak in the moderating conditions of the maze.
Carlo data and NNS-measured spectra agreed within 30% at all
locations except in the maze where the deviation was a maximum of 40%.
ambient dose equivalents calculated from the authors’ measured spectra were
consistent (one standard deviation) with bubble detector measurements in the
The NNS may be used to reliably measure the neutron spectrum of a
radiotherapy beam in less than 1 h, including setup and data unfolding. This work
thus represents a new, fast, and practical method for neutron spectral
measurements in radiotherapy.
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