Godson Henry Finlay, Manickam Ravikumar, Ponmalar Y Retna, Ganesh K M, Saminathan Sathiyan, Chandraraj Varatharaj, Kumar A Sathish, George Seby, Raman Arun, Singh Rabi Raja
Department of Radiation Oncology, Christian Medical College, Vellore, Tamil Nadu, India.
Department of Radiotherapy, Sri Shankara Cancer Hospital and Research Centre, Bengaluru, Karnataka, India.
J Med Phys. 2024 Jan-Mar;49(1):73-83. doi: 10.4103/jmp.jmp_148_23. Epub 2024 Mar 30.
Accurate dose measurements are difficult in small fields due to charge particle disequilibrium, partial source occlusion, steep dose gradient, and the finite size of the detector.
The study aims to determine the output factor using various detectors oriented in parallel and perpendicular orientations for three different tertiary collimating systems using 15 MV photon beams. In addition, this study analyzes how the output factor could be affected by different configurations of X and Y jaws above the tertiary collimators.
Small field output factor measurements were carried out with three detectors for different tertiary collimating systems such as BrainLab stereotactic cones, BrainLab mMLC and Millennium MLC namely. To analyze the effect of jaw position on output factor, measurements have been carried out by positioning the jaws at the edge, 0.25, 0.5, and 1.0 cm away from the tertiary collimated field.
The data acquired with 15 MV photon beams show significant differences in output factor obtained with different detectors for all collimating systems. For smaller fields when compared to microDiamond, the SRS diode underestimates the output by up to -1.7% ± 0.8% and -2.1% ± 0.3%, and the pinpoint ion chamber underestimates the output by up to -8.1% ± 1.4% and -11.9% ± 1.9% in their parallel and perpendicular orientation respectively. A large increase in output factor was observed in the small field when the jaw was moved 0.25 cm symmetrically away from the tertiary collimated field.
The investigated data on the effect of jaw position inferred that the position of the X and Y jaw highly influences the output factors of the small field. It also confirms that the output factor highly depends on the configuration of X and Y jaw settings, the tertiary collimating system as well as the orientation of the detectors in small fields.
由于带电粒子不平衡、部分源遮挡、陡峭的剂量梯度以及探测器的有限尺寸，小射野中的准确剂量测量较为困难。
本研究旨在使用15 MV光子束，针对三种不同的三级准直系统，确定采用平行和垂直方向放置的各种探测器时的输出因子。此外，本研究分析了三级准直器上方X和Y光阑的不同配置如何影响输出因子。
使用三种探测器对不同的三级准直系统（即BrainLab立体定向锥、BrainLab mMLC和Millennium MLC）进行小射野输出因子测量。为分析光阑位置对输出因子的影响，通过将光阑定位在距三级准直射野边缘、0.25、0.5和1.0 cm处进行测量。
用15 MV光子束获得的数据表明，对于所有准直系统，不同探测器获得的输出因子存在显著差异。与微钻石探测器相比，在较小射野中，SRS二极管在平行和垂直方向上分别低估输出达-1.7%±0.8%和-2.1%±0.3%，针点电离室分别低估输出达-8.1%±1.4%和-11.9%±1.9%。当光阑对称地从三级准直射野移开0.25 cm时，小射野中的输出因子大幅增加。
关于光阑位置影响的研究数据推断，X和Y光阑的位置对小射野的输出因子有很大影响。这也证实了输出因子高度依赖于X和Y光阑设置的配置、三级准直系统以及小射野中探测器的方向。
