University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany.
Med Phys. 2019 Apr;46(4):1863-1873. doi: 10.1002/mp.13417. Epub 2019 Feb 18.
The collimator monitoring fill factor (CM-FF) introduced by Stelljes et al. (2017) and the FWHM fill factor (FWHM-FF) introduced by Gago-Arias et al. (2012) were determined using the measured photon fluence response functions of various 2D-arrays. The error detection capabilities of 2D-arrays were studied by comparing detector signal changes and local gamma index passing rates in different field setups with introduced collimation errors.
The fill factor is defined as the ratio of the sensitive detector area and the cell area of a detector, defined by the detector arrangement on a 2D-array. Gago-Arias et al. calculated the FWHM-FF, using the FWHM² of a detector's fluence response function K (x) as the sensitive detector area. For the CM-FF a sensitive detector width w(Δ mm, d%) is calculated. The sensitive detector width is the lateral extent of K (x), lying inside the detector cell area, along which a collimator error of Δ mm yields a signal change exceeding a detection threshold of d%. The sensitive area for a single detector is calculated using w(Δ mm, d%)². The CM-FF is then calculated as the ratio of the sensitive area of a detector within its cell area and the detector cell area. The fluence response functions of the central detector of the OCTAVIUS 729, 1500, and 1000 SRS array (all PTW-Freiburg, Freiburg, Germany) and the MapCHECK 2 array (Sun Nuclear, Melbourne, US) were measured using a photon slit beam. The FWHM-FF and the CM-FF were calculated and compared for all 2D-arrays under investigation. The error detection capabilities of 2D-arrays in quadratic fields were studied by investigating the signal changes in the detectors adjacent to the collimator edge when changing the collimator position. The change in local gamma index passing rate with respect to the introduced collimator error was investigated for an ionization chamber and a diode array in quadratic and two intensity modulated fields.
Values for the CM-FF and FWHM-FF were 1.0 and 0.35, respectively for the area of the liquid-filled 1000 SRS ionization chamber array with a detector to detector distance of 5 mm and 0.32 and 0.04, respectively, for the MapCHECK 2 diode array. For the vented ionization chamber array OCTAVIUS 729 fill factors were calculated as CM-FF = 0.59 and FWHM-FF = 0.53, while the OCTAVIUS 1500 array yielded fill factors of CM-FF = 0.77 and FWHM-FF = 0.72. Signal changes in vented ionization chambers for collimator errors of 1 mm surpassed those of diodes by a factor of 2 in quadratic fields. The gamma index passing rates in quadratic fields reflect those findings. In intensity modulated fields, the decline of the gamma index passing rate is bigger for the ionization chamber array compared to the diode array when introducing collimator errors.
The calculated values of the CM-FF correlate with the signal changes in quadratic field setups with introduced collimator position errors of 1 mm, while the FWHM-FF underestimates the error detection capabilities of 2D-arrays. An increased error detection capability of the ionization chamber array compared to diode array was observed in quadratic and intensity modulated fields.
Stelljes 等人（2017 年）提出的准直器监测填充因子（CM-FF）和 Gago-Arias 等人（2012 年）提出的半峰全宽填充因子（FWHM-FF）是通过测量各种 2D 阵列的测量光子通量响应函数来确定的。通过比较不同场设置下探测器信号变化和局部伽马指数通过率与引入的准直器误差，研究了 2D 阵列的误差检测能力。
填充因子定义为探测器敏感区域与探测器排列在 2D 阵列上定义的探测器单元区域的比值。Gago-Arias 等人使用探测器的通量响应函数 K（x）的 FWHM²来计算 FWHM-FF，作为敏感探测器区域。对于 CM-FF，计算了敏感探测器宽度 w（Δmm，d%）。敏感探测器宽度是 K（x）的横向延伸，位于探测器单元区域内，沿着该区域，准直器误差为 Δmm 会导致信号变化超过检测阈值 d%。使用 w（Δmm，d%）²计算单个探测器的敏感区域。然后将 CM-FF 计算为探测器在其单元区域内的敏感区域与探测器单元区域的比值。使用光子狭缝光束测量了来自 OCTAVIUS 729、1500 和 1000 SRS 阵列（均来自德国 Freiburg 的 PTW-Freiburg）以及 MapCHECK 2 阵列（美国 Sun Nuclear）的中央探测器的通量响应函数。比较了所有研究的 2D 阵列的 FWHM-FF 和 CM-FF。通过研究改变准直器位置时准直器边缘相邻探测器的信号变化，研究了二次场中 2D 阵列的误差检测能力。研究了电离室和二极管阵列在二次和两个强度调制场中引入准直器误差时局部伽马指数通过率的变化。
液体填充的 1000 SRS 电离室阵列的探测器到探测器距离为 5mm 时，CM-FF 和 FWHM-FF 的值分别为 1.0 和 0.35，而 MapCHECK 2 二极管阵列的相应值分别为 0.32 和 0.04。对于通风电离室阵列 OCTAVIUS 729，计算的填充因子为 CM-FF=0.59 和 FWHM-FF=0.53，而 OCTAVIUS 1500 阵列的填充因子为 CM-FF=0.77 和 FWHM-FF=0.72。在二次场中，准直器误差为 1mm 时，通风电离室的信号变化是二极管的两倍。二次场中的伽马指数通过率反映了这些发现。在强度调制场中，当引入准直器误差时，与二极管阵列相比，电离室阵列的伽马指数通过率下降更大。
计算的 CM-FF 值与引入 1mm 准直器位置误差的二次场设置中的信号变化相关，而 FWHM-FF 低估了 2D 阵列的误差检测能力。在二次和强度调制场中，观察到电离室阵列的误差检测能力比二极管阵列更高。
