Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
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DOI：
10.1088/0031-9155/60/10/3959
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发表时间：
2015-05-21
影响因子：
3.5
通讯作者：
Andreo, P.
中科院分区：
文献类型：
--
作者：
Benmakhlouf, H.;Johansson, J.;Andreo, P.
测量Leksell伽玛刀(R)(LGK)放射治疗设备产生的小光子束的输出因子(OF)对物理学家来说是一项挑战，因为商业上可用的绝大多数探测器对这些因素估计过低或过高。产出修正系数，在阿方索(2008地中海)出版的国际形式主义中引入。太棒了。35 5179-86)，通过修正探测器读数比以产生吸收剂量比来标准化小光子束的OFs的测定。在这项工作中，通过蒙特卡罗(MC)计算和测量，确定了LGK Perfexion(TM)Co-60伽马束的一些探测器的输出修正因子。计算是用MC系统Penelope进行的，计算了沉积在探测器活性体积和小体积水中的能量；模拟的探测器是两个硅二极管，一个液体电离室(LIC)，丙氨酸和TLD。计算的LIC输出修正系数在+/-0.4%以内，并被选为用于实验测定的参考探测器，其中测量了12个探测器的输出修正系数，将它们的读数归一化到LIC的读数。MC计算和测量的硅二极管输出修正系数对直径为4 mm的最小LGK准直器尺寸产生了高达5%的修正。空气电离室的测量导致了极大的输出修正因子，这是由众所周知的部分体积平均效应引起的。对于4 mm准直器中的天然钻石探测器，校正系数高达7%，这也是由于部分体积平均，而对于较小的合成钻石探测器，校正系数降低到约+/-0.6%。LIC显示了最小的修正，用于通过在四个具有不同剂量率的LGK单元中进行测量来调查机器对机器的输出因数差异。这使得4 mm和8 mm准直器的OFS分别在+/-0.6%和+/-0.2%以内，这为这些LGK光束使用普通OFS提供了证据。使用实验得出的输出校正系数，可以使用各种商业可用探测器来测量OFF。
The measurement of output factors (OF) for the small photon beams generated by Leksell Gamma Knife (R) (LGK) radiotherapy units is a challenge for the physicist due to the under or over estimation of these factors by a vast majority of the detectors commercially available. Output correction factors, introduced in the international formalism published by Alfonso (2008 Med. Phys. 35 5179-86), standardize the determination of OFs for small photon beams by correcting detector-reading ratios to yield OFs in terms of absorbed-dose ratios. In this work output correction factors for a number of detectors have been determined for LGK Perfexion (TM) Co-60 gamma-ray beams by Monte Carlo (MC) calculations and measurements. The calculations were made with the MC system PENELOPE, scoring the energy deposited in the active volume of the detectors and in a small volume of water; the detectors simulated were two silicon diodes, one liquid ionization chamber (LIC), alanine and TLD. The calculated LIC output correction factors were within +/- 0.4%, and this was selected as the reference detector for experimental determinations where output correction factors for twelve detectors were measured, normalizing their readings to those of the LIC. The MC-calculated and measured output correction factors for silicon diodes yielded corrections of up to 5% for the smallest LGK collimator size of 4 mm diameter. The air ionization chamber measurements led to extremely large output correction factors, caused by the well-known effect of partial volume averaging. The corrections were up to 7% for the natural diamond detector in the 4 mm collimator, also due to partial volume averaging, and decreased to within about +/- 0.6% for the smaller synthetic diamond detector. The LIC, showing the smallest corrections, was used to investigate machine-to-machine output factor differences by performing measurements in four LGK units with different dose rates. These resulted in OFs within +/- 0.6% and +/- 0.2% for the 4 mm and 8 mm collimators, respectively, providing evidence for the use of generic OFs for these LGK beams. Using the experimentally derived output correction factors, OFs can be measured using a wide range of commercially available detectors.