Siman W, Mawlawi O R, Mourtada F, Kappadath S C
Department of Radiology, The University of Colorado School of Medicine, Denver, CO, USA.
Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, USA.
Med Phys. 2020 Jun;47(6):2441-2449. doi: 10.1002/mp.14117. Epub 2020 Mar 28.
The objective was to characterize both systematic and random errors in Positron Emission Tomography (PET)-based Y three-dimensional (3D) dose quantification.
A modified NEMA-IEC phantom was used to emulate Y-microsphere PET imaging conditions: sphere activity concentrations of 1.6 and 4.8 MBq/cc, sphere-to-background ratios of 4 and 13, and sphere diameters of 13, 17, and 37 mm. PET data were acquired using a GE D690 PET/CT scanner for 300 min on days 0-11. The data were downsampled to 60-5 min for multiple realizations to evaluate the count starvation effect. The image reconstruction algorithm was 3D-OSEM with PSF + TOF modeling; the parameters were optimized for dose-volume histogram (DVH), as a Y 3D dose quantification. Y-PET images were converted to dose maps using the local deposition method, then the sphere DVHs were calculated. The ground truth for the DVH was calculated using convolution method. Dose linearity was evaluated in decaying Y activity (reduced count rate and total count) and decreasing acquisition durations (reduced total count only). Finally, the impacts of the low 32-ppm positron yield on PET-based 3D Y-dose quantification were evaluated; the bias and variability of resulting DVHs were characterized.
We observed nonlinear errors that depended on the Y activity (count rate) and not on the total true prompt counts. These nonlinear errors in mean dose underestimated the measured mean dose by> 20% for a measured dose range of 40-230 Gy; although the shapes of the DVH were not altered. Compensation based on empirical models reduced the nonlinearity errors to be within 5% for measured dose range of 40-230 Gy. In contrast, the errors due to nonuniformity introduced by image noise dominated the systematic errors in the DVH and stretched the DVH on both tails. For the 37-mm sphere, the magnitude of errors in D increased from -25% to -36% when acquisition duration was decreased from 300 to 10 min. The effect of image noise on DVH was more extensive in smaller spheres; for the 17-mm sphere, the magnitude of errors in D increased from -29% to -45% acquisition duration was decreased from 300 to 10 min. For the 37-mm sphere, the errors in D increased from +3.5% to only +10.5% when the acquisition duration was decreased from 300 to 10 min; in the 17-mm sphere, the errors in D were 6.5% for both 300- and 10-min sphere images.
Count-starved Y-PET data introduce both systematic and random errors. The systematic error increases the apparent nonuniformity of the DVH, while the random error increases the uncertainty in the DVH. The systematic errors were larger than the random errors. Lower count rate of Y-PET also introduces systematic bias, which is scanner specific. The errors of bias-compensated mean tumor dose were <10% when Y-PET scan time was >15 min/bed for tumors >37 mm. D and D were the most stable dose metrics. An acquisition duration of 30 min is recommended to keep the random errors < 10% for a typical tumor with sphere equivalent diameter >17 mm and average tumor dose >40 Gy.
本研究旨在描述基于正电子发射断层扫描（PET）的钇三维（3D）剂量定量中的系统误差和随机误差。
使用改良的NEMA-IEC体模模拟钇微球PET成像条件：球体活度浓度为1.6和4.8MBq/cc，球体与背景比为4和13，球体直径为13、17和37mm。在第0至11天使用GE D690 PET/CT扫描仪采集PET数据300分钟。为评估计数饥饿效应，对数据进行多次下采样至60 - 5分钟。图像重建算法为带点扩散函数（PSF）+飞行时间（TOF）建模的3D-有序子集期望最大化（OSEM）算法；参数针对剂量体积直方图（DVH）进行优化，作为钇3D剂量定量。使用局部沉积法将钇PET图像转换为剂量图，然后计算球体DVH。使用卷积法计算DVH的真实值。在钇活度衰减（计数率和总计数降低）以及采集时长缩短（仅总计数降低）的情况下评估剂量线性。最后，评估低32ppm正电子产额对基于PET的3D钇剂量定量的影响；对所得DVH的偏差和变异性进行特征描述。
我们观察到非线性误差，其取决于钇活度（计数率）而非总真实符合计数。对于40 - 230Gy的测量剂量范围，这些平均剂量中的非线性误差使测量平均剂量低估超过20%；尽管DVH的形状未改变。基于经验模型的补偿将40 - 230Gy测量剂量范围内的非线性误差降低至5%以内。相比之下，由图像噪声引入的不均匀性导致的误差在DVH的系统误差中占主导，并使DVH在两端拉伸。对于37mm球体，当采集时长从300分钟减少到10分钟时，D的误差幅度从 - 25%增加到 - 36%。图像噪声对DVH的影响在较小球体中更广泛；对于17mm球体，当采集时长从300分钟减少到10分钟时，D的误差幅度从 - 29%增加到 - 45%。对于37mm球体，当采集时长从300分钟减少到10分钟时，D的误差从 + 3.5%增加到仅 + 10.5%；在17mm球体中，300分钟和10分钟球体图像的D误差均为6.5%。
计数饥饿的钇PET数据会引入系统误差和随机误差。系统误差增加了DVH的表观不均匀性，而随机误差增加了DVH的不确定性。系统误差大于随机误差。钇PET的较低计数率也会引入系统偏差，这是扫描仪特定的。当钇PET扫描时间>15分钟/床位且肿瘤>37mm时，偏差补偿后的平均肿瘤剂量误差<10%。D和D是最稳定的剂量指标。对于等效球体直径>17mm且平均肿瘤剂量>40Gy的典型肿瘤，建议采集时长为30分钟以保持随机误差<10%。
