Saße Katrin, Albers Karina, Eulenstein Daniela, Weidlich Georg, Poppe Björn, Looe Hui Khee
University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany.
Zap Surgical, Medical Physics Department, PTW-Freiburg, Freiburg, Germany.
J Appl Clin Med Phys. 2025 Feb;26(2):e14610. doi: 10.1002/acm2.14610. Epub 2025 Jan 7.
The self-shielding radiosurgery system ZAP-X consists of a 3 MV linear accelerator and eight round collimators. For this system, it is a common practice to perform the reference dosimetry using the largest 25 mm diameter collimator at a source-to-axis distance (SAD) of 45 cm with the PTW Semiflex3D chamber placed at a measurement depth of 7 mm in water. Existing dosimetry protocols do not provide correction for these measurement conditions. Therefore, Monte Carlo simulations were performed to quantify the associated beam quality correction factor .
The of the Semiflex3D chamber was computed from the ratio of the absorbed doses in a water voxel and in the sensitive air volume of the chamber simulated using a Co spectrum as the calibration beam quality (Q) and the spectrum of the ZAP-X 3 MV photon beam (Q). was computed as a function of measurement depth from 4 to 50 mm. Furthermore, detailed simulations were performed to determine the individual chamber's perturbation correction factors by modifying the chamber's model step-wise.
All perturbation correction factors, except S ⋅P, show depth-dependent behavior up to a depth of 15 mm. In particular, the volume-averaging P and density P perturbation correction factors and, consequently, the resulting gradient perturbation correction factor P= P∙P increase with decreasing measurement depth. Therefore, is larger than unity, amounting to at 7 mm measurement depth. At larger depths (> 15 mm), the can be considered as constant.
At small measurement depths, was found to be depth-dependent with values larger than unity due to the gradient-related perturbation factors. Therefore, the uncertainty related to the chamber's positioning can be reduced by performing the reference dosimetry at ZAP-X at depths larger than 15 mm, where can be regarded as depth independent.
自屏蔽放射外科系统ZAP-X由一台3兆伏直线加速器和八个圆形准直器组成。对于该系统，常见的做法是使用直径最大为25毫米的准直器，在源轴距（SAD）为45厘米的条件下进行参考剂量测定，将PTW Semiflex3D电离室置于水中7毫米的测量深度处。现有的剂量测定方案未对这些测量条件进行校正。因此，进行了蒙特卡罗模拟以量化相关的射束质量校正因子。
Semiflex3D电离室的 由水体素中的吸收剂量与使用钴谱作为校准射束质量（Q）和ZAP-X 3兆伏光子束谱（Q）模拟的电离室灵敏空气体积中的吸收剂量之比计算得出。 作为测量深度从4到50毫米的函数进行计算。此外，通过逐步修改电离室模型进行了详细模拟，以确定各个电离室的扰动校正因子。
除S·P外，所有扰动校正因子在深度达15毫米时均呈现深度依赖性。特别是，体积平均P和密度P扰动校正因子，以及由此产生的梯度扰动校正因子P = P∙P随测量深度减小而增加。因此， 大于1，在7毫米测量深度时达到 。在较大深度（> 15毫米）， 可视为常数。
在小测量深度时，由于与梯度相关的扰动因子，发现 与深度有关且值大于1。因此，通过在ZAP-X大于15毫米的深度进行参考剂量测定，可以降低与电离室定位相关的不确定性，在该深度 可视为与深度无关。
