Maglieri Robert, Licea Angel, Evans Michael, Seuntjens Jan, Kildea John
Medical Physics Unit, McGill University, Montreal, Quebec H4A 3J1, Canada.
Canadian Nuclear Safety Commission, Ottawa, Ontario K1P 5S9, Canada.
Med Phys. 2015 Nov;42(11):6162-9. doi: 10.1118/1.4931963.
Out-of-field neutron doses 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 neutron 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 neutron spectra at various locations inside the bunker were then compared to Monte Carlo simulations of an identical setup. In terms of dose, neutron ambient dose equivalents were calculated from the measured spectra and compared to bubble detector neutron dose equivalent measurements.
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 source and the shape of the spectrum changed from a dominant fast neutron peak near the Linac head to a dominant thermal neutron peak in the moderating conditions of the maze. Monte Carlo data and NNS-measured spectra agreed within 30% at all locations except in the maze where the deviation was a maximum of 40%. Neutron ambient dose equivalents calculated from the authors' measured spectra were consistent (one standard deviation) with bubble detector measurements in the treatment room.
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.
直线加速器头部的光核相互作用产生的野外中子剂量，在放射治疗期间对患者构成医源性风险，对工作人员构成职业风险。为了量化中子产生量，传统上使用带有活化箔和热释光剂量计（TLD）的邦纳球系统（BSS）在室内进行测量。在这项工作中，一种最近开发的有源探测器——嵌套中子谱仪（NNS），在放射治疗掩体中进行了测试。
NNS设计得易于操作，比传统的BSS更实用。在电流模式下运行时，通过测量电流克服了高剂量率导致的脉冲堆积问题，类似于电离室。在一个装有瓦里安Clinac 21EX的掩体中，从再现性、线性和剂量率效应方面评估了NNS的性能。然后使用定制的最大似然期望最大化算法，将掩体内部不同位置测得的中子谱与相同设置的蒙特卡罗模拟进行比较。在剂量方面，根据测得的谱计算中子周围剂量当量，并与气泡探测器中子剂量当量测量结果进行比较。
发现治疗室中不同位置的中子经NNS测量得到的谱在相对形状和绝对量级方面均与预期一致。中子注量率随离源距离的增加而降低，谱的形状从直线加速器头部附近占主导的快中子峰，变为迷宫缓和条件下占主导的热中子峰。除了在迷宫中偏差最大为40%外，蒙特卡罗数据和NNS测量的谱在所有位置的偏差均在30%以内。根据作者测得的谱计算出的中子周围剂量当量与治疗室中气泡探测器的测量结果一致（一个标准差）。
NNS可用于在不到1小时的时间内可靠地测量放射治疗束的中子谱，包括设置和数据展开。因此，这项工作代表了一种用于放射治疗中中子谱测量的新的、快速且实用的方法。
