Zoros E, Moutsatsos A, Pappas E P, Georgiou E, Kollias G, Karaiskos P, Pantelis E
Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, 75 Mikras Asias, 115 27 Athens, Greece.
Phys Med Biol. 2017 Sep 5;62(18):7532-7555. doi: 10.1088/1361-6560/aa8590.
Detector-, field size- and machine-specific correction factors are required for precise dosimetry measurements in small and non-standard photon fields. In this work, Monte Carlo (MC) simulation techniques were used to calculate the [Formula: see text] and [Formula: see text] correction factors for a series of ionization chambers, a synthetic microDiamond and diode dosimeters, used for reference and/or output factor (OF) measurements in the Gamma Knife Perfexion photon fields. Calculations were performed for the solid water (SW) and ABS plastic phantoms, as well as for a water phantom of the same geometry. MC calculations for the [Formula: see text] correction factors in SW were compared against corresponding experimental results for a subset of ionization chambers and diode detectors. Reference experimental OF data were obtained through the weighted average of corresponding measurements using TLDs, EBT-2 films and alanine pellets. [Formula: see text] values close to unity (within 1%) were calculated for most of ionization chambers in water. Greater corrections of up to 6.0% were observed for chambers with relatively large air-cavity dimensions and steel central electrode. A phantom correction of 1.006 and 1.024 (breaking down to 1.014 from the ABS sphere and 1.010 from the accompanying ABS phantom adapter) were calculated for the SW and ABS phantoms, respectively, adding up to [Formula: see text] corrections in water. Both measurements and MC calculations for the diode and microDiamond detectors resulted in lower than unit [Formula: see text] correction factors, due to their denser sensitive volume and encapsulation materials. In comparison, higher than unit [Formula: see text] results for the ionization chambers suggested field size depended dose underestimations (being significant for the 4 mm field), with magnitude depending on the combination of contradicting phenomena associated with volume averaging and electron fluence perturbations. Finally, the presence of 0.5 mm air-gap between the diodes' frontal surface and their phantom-inserts may considerably influence OF measurements, reaching 4.6% for the Razor diode.
在小尺寸和非标准光子场中进行精确的剂量测量时，需要探测器、射野大小和机器特定的校正因子。在这项工作中，使用蒙特卡罗（MC）模拟技术来计算一系列电离室、合成微金刚石和二极管剂量计的[公式：见正文]和[公式：见正文]校正因子，这些剂量计用于伽玛刀Perfexion光子场中的参考和/或输出因子（OF）测量。对固体水（SW）和ABS塑料模体以及相同几何形状的水体模进行了计算。将SW中[公式：见正文]校正因子的MC计算结果与一部分电离室和二极管探测器的相应实验结果进行了比较。参考实验OF数据是通过使用热释光剂量计（TLD）、EBT - 2胶片和丙氨酸颗粒的相应测量值的加权平均值获得的。水中大多数电离室的[公式：见正文]值接近1（在1%以内）。对于具有相对较大气腔尺寸和钢质中心电极的电离室，观察到高达6.0%的更大校正。分别计算出SW模体和ABS模体的模体校正值为1.006和1.024（从ABS球体为1.014，从附带的ABS模体适配器为1.010分解而来），在水中总计为[公式：见正文]校正。由于二极管和微金刚石探测器的敏感体积和封装材料更致密，它们的测量和MC计算结果导致[公式：见正文]校正因子低于1。相比之下，电离室的[公式：见正文]结果高于1，表明射野大小相关的剂量低估（对于4毫米射野很显著），其幅度取决于与体积平均和电子注量扰动相关的相互矛盾现象的组合。最后，二极管前表面与其模体插入物之间存在0.5毫米的气隙可能会对OF测量产生相当大的影响，对于Razor二极管达到4.6%。
