Benmakhlouf H, Johansson J, Paddick I, Andreo P
Department of Medical Physics, Karolinska University Hospital, 171 76 Solna, Stockholm. Department of Physics, Medical Radiation Physics, Stockholm University, 114 18 Stockholm, Sweden.
Phys Med Biol. 2015 May 21;60(10):3959-73. doi: 10.1088/0031-9155/60/10/3959. Epub 2015 Apr 24.
The measurement of output factors (OF) for the small photon beams generated by Leksell Gamma Knife® (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™ (60)Co γ-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.
对于由医科达伽玛刀（LGK）放疗设备产生的小光子束，输出因子（OF）的测量对物理学家来说是一项挑战，因为市场上绝大多数探测器对这些因子的估计都存在偏低或偏高的情况。阿方索在其发表的国际形式主义（2008年，《医学物理》35卷，5179 - 5186页）中引入的输出校正因子，通过校正探测器读数比，以吸收剂量比的形式得出OF，从而规范了小光子束OF的测定。在这项工作中，通过蒙特卡罗（MC）计算和测量，确定了医科达Perfexion™（60）Coγ射线束的多种探测器的输出校正因子。计算使用的是MC系统PENELOPE，对探测器活性体积和一小部分水体积中沉积的能量进行计分；模拟的探测器有两个硅二极管、一个液体电离室（LIC）、丙氨酸和热释光剂量计（TLD）。计算得出的LIC输出校正因子在±0.4%以内，在实验测定中，这被选作参考探测器，测量了12种探测器的输出校正因子，并将它们的读数与LIC的读数进行归一化处理。对于直径4毫米的最小LGK准直器尺寸，MC计算和测量得出的硅二极管输出校正因子高达5%。空气电离室测量导致输出校正因子极大，这是由众所周知的部分体积平均效应引起的。对于4毫米准直器中的天然金刚石探测器，校正因子高达7%，同样是由于部分体积平均效应，而对于较小的合成金刚石探测器，校正因子降至约±0.6%以内。LIC的校正最小，通过在四个不同剂量率的LGK设备中进行测量，用它来研究不同设备之间的输出因子差异。对于4毫米和8毫米准直器，结果分别得到的OF在±0.6%和±0.2%以内，这为这些LGK束使用通用OF提供了证据。利用实验得出的输出校正因子，可以使用多种市售探测器来测量OF。
