Qutub Mohammad A Z, Hugtenburg Richard P, Al-Affan Ihsan A M
Swansea University, Swansea, SA2 8PP, UK.
Department of Physics, Umm Al-Qura University, Makkah, Saudi Arabia.
Med Phys. 2020 Sep;47(9):4522-4530. doi: 10.1002/mp.14304. Epub 2020 Jun 15.
The determination of x-ray spectra near the maze entrance of linear accelerator (LINAC) rooms is challenging due to the pulsed nature of the LINAC source. Mathematical methods to account for pulse pile-up have been examined. These methods utilize the highly periodic pulsing structure of the LINAC, differing from the effects of high-intensity radioactive sources.
Sodium iodide (NaI) and plastic scintillation detectors were used to determine the energy spectra at different points near the maze entrance of a medical LINAC. Monte Carlo calculations of the energy distribution of scattered photons were used to simulate the energy spectrum at the maze entrance. The proposed algorithm uses the Monte Carlo code, FLUKA, to calculate a response function for both detectors. To determine the effects of the pile-up in the spectra, the Poisson distribution was used, employing the average number of photons per pulse (μ) interacting with the detector. The quantity, μ, was obtained from the ratio of the number of events detected to the number of pulses delivered. The energy spectra at various distances from the maze entrance were measured using NaI and plastic scintillation detectors. From these measurements, the values of µ were calculated, and the pile-up probability was determined. The FLUKA Monte Carlo code was used to calculate the spectrum at the maze entrance and the response matrices of the NaI and plastic scintillation detectors. The algorithm based on the Poisson distribution was applied to calculate the spectrum.
The agreement between the calculated and measured spectra was within the first standard deviation of the variance expected in µ. This agreement confirms that photons at the maze entrance have energies between 30 and 240 keV for a maze with three turns, with an average energy of around 85 keV. After pile-up correction, the range of the pulse height distribution with the plastic scintillation detector, which has a low atomic number, was decreased (0 to 140 keV). In contrast, the range of the pulse height distribution with the NaI scintillation detector was closer to the photon spectrum (0 to 240 keV).
The corrected spectrum demonstrates that using a FLUKA Monte Carlo code and an algorithm based on the Poisson distribution are effective methods in removing the distortion due to the pile-up in LINAC spectra when measuring with NaI and plastic scintillation detectors. The agreement between the corrected and measured spectra indicates that Monte Carlo modeling can accurately determine the spectrum of a LINAC machine at the maze entrance.
由于直线加速器（LINAC）源的脉冲特性，测定LINAC机房迷宫入口附近的X射线能谱具有挑战性。已研究了考虑脉冲堆积的数学方法。这些方法利用了LINAC高度周期性的脉冲结构，这与高强度放射源的影响不同。
使用碘化钠（NaI）和塑料闪烁探测器来测定医用LINAC迷宫入口附近不同点的能谱。利用散射光子能量分布的蒙特卡罗计算来模拟迷宫入口处的能谱。所提出的算法使用蒙特卡罗代码FLUKA来计算两个探测器的响应函数。为了确定能谱中堆积的影响，使用泊松分布，采用与探测器相互作用的每个脉冲的平均光子数（μ）。量μ是从检测到的事件数与发出的脉冲数之比获得的。使用NaI和塑料闪烁探测器测量距迷宫入口不同距离处的能谱。从这些测量中，计算出μ的值，并确定堆积概率。使用FLUKA蒙特卡罗代码计算迷宫入口处的能谱以及NaI和塑料闪烁探测器的响应矩阵。应用基于泊松分布的算法来计算能谱。
计算能谱与测量能谱之间的一致性在μ预期方差的第一个标准差范围内。这种一致性证实，对于有三转弯的迷宫，迷宫入口处的光子能量在30至240keV之间，平均能量约为85keV。经过堆积校正后，原子序数较低的塑料闪烁探测器的脉冲高度分布范围减小（0至140keV）。相比之下，NaI闪烁探测器的脉冲高度分布范围更接近光子能谱（0至240keV）。
校正后的能谱表明，当使用NaI和塑料闪烁探测器进行测量时，使用FLUKA蒙特卡罗代码和基于泊松分布的算法是消除LINAC能谱中由于堆积而产生的畸变的有效方法。校正能谱与测量能谱之间的一致性表明，蒙特卡罗建模可以准确确定LINAC机器在迷宫入口处的能谱。
