Perez-Andujar A, Morin O, Chuang C, Pouliot J
University of California San Francisco, San Francisco, CA.
Med Phys. 2012 Jun;39(6Part11):3731. doi: 10.1118/1.4735181.
A lower than ideal tolerance limit is used in intensity-modulated radiation therapy quality assurance (IMRT QA) with a 2D diode array due to passing rate fluctuations. The objective is to identify patterns in the passing rates to predict sources of uncertainty that can affect treatment delivery, for example, the need to re-calibrate the multileaf collimator when the passing rates start to decrease.
Five complex clinical prostate IMRT plans were evaluated with a 2D diode array. The QA for each plan was repeated five times during one and a half month period. One of the plans was randomly selected and repeated the same day five consecutive times. The planar doses calculated by the treatment planning system were compared to the measurements of the 2D diode array. The individual passing rates per beam per plan were compared.
The average passing rate for each plan ranged from 94% to 97%. While the average percent difference of this ranged between -7.67% to 17.61%. Additionally, the minimum and maximum standard deviation among all beams was 0.13% and 9.63% respectively. We also compared the standard deviation of a plan QA repeated during different days versus a plan QA repeated during the same day. For the former the highest standard deviation was 6.05 % while for the later 0.21%. We noticed that the largest discrepancy between the passing rates was for angles at around 155° and 205°.
These results show some inconsistency in the IMRT QA passing rates from one day to the next. Moreover, lower passing rates for a specific angle like the ones shown here can represent possible mechanical or tuning problems with the linear accelerator at these specific locations. Early identification of these sources of uncertainty can greatly improve the precision of the treatment delivery.
由于通过率波动，在使用二维二极管阵列进行调强放射治疗质量保证（IMRT QA）时采用了低于理想的公差极限。目的是识别通过率中的模式，以预测可能影响治疗实施的不确定性来源，例如，当通过率开始下降时重新校准多叶准直器的必要性。
使用二维二极管阵列对五个复杂的临床前列腺IMRT计划进行评估。在一个半月的时间内，对每个计划的QA重复进行五次。随机选择其中一个计划并在同一天连续重复五次。将治疗计划系统计算的平面剂量与二维二极管阵列的测量值进行比较。比较每个计划中每束光的个体通过率。
每个计划的平均通过率在94%至97%之间。而其平均百分比差异在-7.67%至17.61%之间。此外，所有光束中的最小和最大标准差分别为0.13%和9.63%。我们还比较了在不同日期重复的计划QA与在同一天重复的计划QA的标准差。前者的最高标准差为6.05%，而后者为0.21%。我们注意到，通过率之间的最大差异出现在大约155°和205°的角度处。
这些结果表明IMRT QA通过率在一天到下一天之间存在一些不一致性。此外，如此处所示特定角度的较低通过率可能表示直线加速器在这些特定位置存在可能的机械或调谐问题。尽早识别这些不确定性来源可以大大提高治疗实施的精度。
