Muralidhar K R, Murthy P Narayana, Sresty N V N M, Dixit Pramod Kumar, Kumar Rajneesh, Raju A K
Department of Physics, Indo-American Cancer Institute and Research Center, Banjara Hills, Hyderabad, Andhra Pradesh, India.
J Med Phys. 2007 Apr;32(2):65-7. doi: 10.4103/0971-6203.33243.
Measurements designed to find the collimator backscatter into the beam monitor chamber from Micro Multileaf collimator of 6 MV photon beams of the Siemens Primus linear accelerator were made with the help of dose rate feedback control. The photons and electrons backscattered from the upper and lower secondary collimator jaws give rise to a significant increase in the ion charge measured by monitor chamber. This increase varies between the different accelerators. The output measurements were carried out in air at the isocenter. The effect of collimator backscatter was investigated by measuring the pulse width, number of beam pulses per monitor unit, monitor unit rate and dose for different mMLC openings. These measurements were made with and without dose rate feedback control, i.e., with constant electron beam current in the accelerator. Monitor unit rate (MU/min) was almost constant for all field sizes. The maximum variation between the open and the closed feedback control circuits was 2.5%. There was no difference in pulse width and negligible difference in pulse frequency. Maximum value of backscattered radiation from the micro Multileaf collimator into the beam monitor chamber was found to be 0.5%.
借助剂量率反馈控制，对西门子Primus直线加速器6 MV光子束的微型多叶准直器向束流监测电离室的准直器反向散射进行了测量。从上下二级准直器钳口反向散射的光子和电子，会导致监测电离室测量的离子电荷显著增加。这种增加在不同加速器之间有所不同。输出测量在等中心的空气中进行。通过测量不同微型多叶准直器开口情况下的脉冲宽度、每个监测单位的束流脉冲数、监测单位率和剂量，研究了准直器反向散射的影响。这些测量在有和没有剂量率反馈控制的情况下进行，即在加速器中电子束流恒定的情况下进行。对于所有射野大小，监测单位率（MU/分钟）几乎恒定。开环和闭环反馈控制电路之间的最大变化为2.5%。脉冲宽度没有差异，脉冲频率差异可忽略不计。发现微型多叶准直器向束流监测电离室的反向散射辐射最大值为0.5%。
