Hossain Murshed
Fox Chase Cancer Center.
J Appl Clin Med Phys. 2014 Jul 8;15(4):4783. doi: 10.1120/jacmp.v15i4.4783.
The purpose of this study is to characterize and understand the long-term behavior of the output from megavoltage radiotherapy linear accelerators. Output trends of nine beams from three linear accelerators over a period of more than three years are reported and analyzed. Output, taken during daily warm-up, forms the basis of this study. The output is measured using devices having ion chambers. These are not calibrated by accredited dosimetry laboratory, but are baseline-compared against monthly output which is measured using calibrated ion chambers. We consider the output from the daily check devices as it is, and sometimes normalized it by the actual output measured during the monthly calibration of the linacs. The data show noisy quasi-periodic behavior. The output variation, if normalized by monthly measured "real' output, is bounded between ± 3%. Beams of different energies from the same linac are correlated with a correlation coefficient as high as 0.97, for one particular linac, and as low as 0.44 for another. These maximum and minimum correlations drop to 0.78 and 0.25 when daily output is normalized by the monthly measurements. These results suggest that the origin of these correlations is both the linacs and the daily output check devices. Beams from different linacs, independent of their energies, have lower correlation coefficient, with a maximum of about 0.50 and a minimum of almost zero. The maximum correlation drops to almost zero if the output is normalized by the monthly measured output. Some scatter plots of pairs of beam output from the same linac show band-like structures. These structures are blurred when the output is normalized by the monthly calibrated output. Fourier decomposition of the quasi-periodic output is consistent with a 1/f power law. The output variation appears to come from a distorted normal distribution with a mean of slightly greater than unity. The quasi-periodic behavior is manifested in the seasonally averaged output, showing annual variability with negative variations in the winter and positive in the summer. This trend is weakened when the daily output is normalized by the monthly calibrated output, indicating that the variation of the periodic component may be intrinsic to both the linacs and the daily measurement devices. Actual linac output was measured monthly. It needs to be adjusted once every three to six months for our tolerance and action levels. If these adjustments are artificially removed, then there is an increase in output of about 2%-4% per year.
本研究的目的是描述和理解兆伏级放射治疗直线加速器输出的长期行为。报告并分析了三台直线加速器的九条射束在三年多时间内的输出趋势。日常预热期间获取的输出构成了本研究的基础。输出是使用带有电离室的设备测量的。这些设备未由认可的剂量测定实验室校准，而是与每月使用校准电离室测量的输出进行基线比较。我们直接考虑日常检查设备的输出，有时会根据直线加速器每月校准期间测量的实际输出对其进行归一化处理。数据显示出有噪声的准周期行为。如果按每月测量的“实际”输出进行归一化，输出变化范围在±3%之间。对于一台特定的直线加速器，同一台直线加速器不同能量的射束之间的相关系数高达0.97，而对于另一台直线加速器则低至0.44。当按每月测量值对每日输出进行归一化时，这些最大和最小相关系数分别降至0.78和0.25。这些结果表明，这些相关性的来源既在于直线加速器，也在于每日输出检查设备。来自不同直线加速器的射束，无论其能量如何，相关系数都较低，最大值约为0.50，最小值几乎为零。如果按每月测量的输出对输出进行归一化，最大相关性几乎降至零。同一台直线加速器的射束输出对的一些散点图显示出带状结构。当按每月校准输出对输出进行归一化时，这些结构会变得模糊。准周期输出的傅里叶分解与1/f幂律一致。输出变化似乎来自均值略大于1的扭曲正态分布。准周期行为体现在季节性平均输出中，呈现出年度变化，冬季为负变化，夏季为正变化。当按每月校准输出对每日输出进行归一化时，这种趋势会减弱，这表明周期分量的变化可能对于直线加速器和日常测量设备来说都是固有的。直线加速器的实际输出每月测量一次。根据我们的公差和行动水平，每三到六个月需要进行一次调整。如果人为去除这些调整，那么每年输出会增加约2% - 4%。
