Cyriac Swapna Lilly, Liu Jian, Calugaru Emel, Chang Jenghwa
Department of Radiation Oncology, KIMS Cancer Care and Research Center Pvt Ltd, Thiruvananthapuram, Kerala, India.
Department of Radiation Medicine, Northwell Health, Lake Success, NY, USA.
J Appl Clin Med Phys. 2020 Oct;21(10):80-88. doi: 10.1002/acm2.13011. Epub 2020 Sep 6.
The objective of this work was to identify the exact location of the effective point of measurement (EPM) of four different active detectors to compare the relative collimator output factors (ROF) of Leksell Gamma Knife (LGK) according to IAEA TRS-483 recommendations. ROF was measured at the center of the spherical LGK-Solid Water (LGK-SW) Phantom for three (4-, 8-, and 16-mm in diameter) collimators using four (PTW-TN60008, PTW-TN60016, PTW-TN60017, and PTW-60019 diode/diamond) detectors. Since diode detectors have a much smaller sensitive volume than the PTW-31010 ion chamber used for reference dosimetry, its EPM might not be at the center of the phantom, or (100, 100, 100) of the Leksell Coordinate System, particularly in the z-direction. Hence for each diode detector, a CBCT image was acquired after it was inserted into the phantom, from which the z-Leksell coordinate of EPM was determined. Relative collimator output factors was then measured by focusing GK beams on the determined EPM of each diode. Measured ROFs were compared with the vendor-provided values in GK treatment planning system. For validation, a plan was generated to measure the output of 4-mm collimator for PTW-TN60017 at various couch locations along the z-axis. For PTW-TN60008, the percentage variations were 0.6 ± 0.4%, and -0.8 ± 0.2% for 4 and 8-mm collimators, respectively. For PTW-TN60016, the percentage variations were 0.8 ± 0.0%, and 0.2 ± 0.1%, respectively. The percentage variations were -3.3 ± 0.0% and -0.9 ± 0.1%, respectively, for PTW-TN60017, and -0.5 ± 0.0% and -0.8 ± 0.2%, respectively, for PTW-TN60019. Center of the measured profile for PTW-TN60017 was only 0.1 mm different from that identified using the CBCT. In conclusion, we have developed a simple and effective method to determine the EPMs of diode detectors when inserted into the existing LGK-SW phantom. With the acquired positional information and using TRS-483 protocol, good agreements were obtained between the measured ROFs and manufacturer recommended values.
这项工作的目的是确定四种不同有源探测器的有效测量点（EPM）的确切位置，以便根据国际原子能机构（IAEA）TRS - 483建议比较Leksell伽马刀（LGK）的相对准直器输出因子（ROF）。使用四个探测器（PTW - TN60008、PTW - TN60016、PTW - TN60017和PTW - 60019二极管/金刚石探测器），在球形LGK - 固体水（LGK - SW）模体的中心，针对三种（直径分别为4、8和16毫米）准直器测量ROF。由于二极管探测器的灵敏体积比用于参考剂量测定的PTW - 31010电离室小得多，其EPM可能不在模体中心，也不在Leksell坐标系的（100, 100, 100）处，特别是在z方向上。因此，对于每个二极管探测器，在将其插入模体后采集CBCT图像，从中确定EPM的z - Leksell坐标。然后通过将伽马刀束聚焦在每个二极管确定的EPM上来测量相对准直器输出因子。将测量的ROF与GK治疗计划系统中供应商提供的值进行比较。为了进行验证，生成了一个计划，用于测量PTW - TN60017在z轴上不同治疗床位置处4毫米准直器的输出。对于PTW - TN60008，4毫米和8毫米准直器的百分比变化分别为0.6 ± 0.4%和 - 0.8 ± 0.2%。对于PTW - TN60016，百分比变化分别为0.8 ± 0.0%和0.2 ± 0.1%。对于PTW - TN60017，百分比变化分别为 - 3.3 ± 0.0%和 - 0.9 ± 0.1%，对于PTW - TN60019，百分比变化分别为 - 0.5 ± 0.0%和 - 0.8 ± 0.2%。PTW - TN60017测量轮廓的中心与使用CBCT确定的中心仅相差0.1毫米。总之，我们开发了一种简单有效的方法，用于确定插入现有LGK - SW模体中的二极管探测器的EPM。利用获取的位置信息并使用TRS - 483协议，测量的ROF与制造商推荐值之间取得了良好的一致性。
