Fontenot Jonas D, Newhauser Wayne D, Bloch Charles, White R Allen, Titt Uwe, Starkschall George
Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030, USA.
Med Phys. 2007 Feb;34(2):489-98. doi: 10.1118/1.2428406.
Current protocols for the measurement of proton dose focus on measurements under reference conditions; methods for measuring dose under patient-specific conditions have not been standardized. In particular, it is unclear whether dose in patient-specific fields can be determined more reliably with or without the presence of the patient-specific range compensator. The aim of this study was to quantitatively assess the reliability of two methods for measuring dose per monitor unit (DIMU) values for small-field treatment portals: one with the range compensator and one without the range compensator. A Monte Carlo model of the Proton Therapy Center-Houston double-scattering nozzle was created, and estimates of D/MU values were obtained from 14 simulated treatments of a simple geometric patient model. Field-specific D/MU calibration measurements were simulated with a dosimeter in a water phantom with and without the range compensator. D/MU values from the simulated calibration measurements were compared with D/MU values from the corresponding treatment simulation in the patient model. To evaluate the reliability of the calibration measurements, six metrics and four figures of merit were defined to characterize accuracy, uncertainty, the standard deviations of accuracy and uncertainty, worst agreement, and maximum uncertainty. Measuring D/MU without the range compensator provided superior results for five of the six metrics and for all four figures of merit. The two techniques yielded different results primarily because of high-dose gradient regions introduced into the water phantom when the range compensator was present. Estimated uncertainties (approximately 1 mm) in the position of the dosimeter in these regions resulted in large uncertainties and high variability in D/MU values. When the range compensator was absent, these gradients were minimized and D/MU values were less sensitive to dosimeter positioning errors. We conclude that measuring D/MU without the range compensator present provides more reliable results than measuring it with the range compensator in place.
当前质子剂量测量方案主要关注参考条件下的测量；针对患者特定条件下剂量测量的方法尚未标准化。具体而言，尚不清楚在有或没有患者特定范围补偿器的情况下，患者特定射野中的剂量是否能更可靠地确定。本研究的目的是定量评估两种测量小射野治疗射束每监测单位剂量（DIMU）值方法的可靠性：一种使用范围补偿器，另一种不使用范围补偿器。创建了休斯顿质子治疗中心双散射喷嘴的蒙特卡罗模型，并从一个简单几何形状患者模型的14次模拟治疗中获得了DIMU值的估计。使用剂量仪在有水模体且有和没有范围补偿器的情况下模拟特定射野的DIMU校准测量。将模拟校准测量得到的DIMU值与患者模型中相应治疗模拟得到的DIMU值进行比较。为评估校准测量的可靠性，定义了六个指标和四个品质因数来表征准确性、不确定性、准确性和不确定性的标准偏差、最差一致性以及最大不确定性。不使用范围补偿器测量DIMU在六个指标中的五个以及所有四个品质因数方面都提供了更好的结果。这两种技术产生不同结果主要是因为存在范围补偿器时引入水模体的高剂量梯度区域。剂量仪在这些区域位置的估计不确定性（约1毫米）导致DIMU值存在较大不确定性和高变异性。当没有范围补偿器时，这些梯度最小化，并且DIMU值对剂量仪定位误差不太敏感。我们得出结论，不使用范围补偿器测量DIMU比使用范围补偿器时能提供更可靠的结果。
