Density calibrated cone beam CT as a tool for adaptive radiotherapy

Visual inspections of anatomical changes observed on daily cone-beam CT (CBCT) images are often used as triggers for radiotherapy plan adaptation to avoid unacceptable dose levels to the target or OARs. Direct CBCT dose calculations would improve the ability to adapt only those plans where dosimetric changes are observed. This study investigates the accuracy of dose calculations on CBCTs. Calibration curves were obtained for CBCT imagers at nine identical accelerators. CBCT scans of a phantom with different density inserts were recorded for two scan modes (Head-Neck and Pelvis) and mean calibration curves were calculated. Subsequently, CBCT scans of the phantom with six different density inserts were recorded, the dose distributions on the CBCTs were calculated and compared to dose on the planning CT (pCT). The uncertainty was quantified by the dosimetric difference between the pCT and the CBCT. The two mean calibration curves were used to calculate the daily delivered CBCT dose for ten Head-Neck-, eleven Lung-, and ten pelvic patients. Additional patient calculations were performed using low-HU empirically corrected calibration curves. Patient doses were compared on target coverage and mean dose, and D1cc for OARs. The dose differences between pCT and CBCT for phantom data were small for all DVH parameters, with mean deviations below ±0.6% for both CBCT modes. For patient data, it was found that low-HU corrected calibration curves performed the best. The mean deviations for the mean dose and coverage of the target were 0.2%±0.7% and 0.1%±0.6%, across all patient groups. Dose calculation on CBCT images results in target coverage and mean dose with an accuracy of the order of 1%, which makes this acceptable for clinical use. The CBCT mode specific calibration curves can be used at all identical imaging devices and for all patient groups.

临床中常通过对每日锥形束计算机断层扫描（cone-beam CT, CBCT）图像所观测到的解剖结构变化开展视觉评估，以此作为放射治疗计划适配的触发依据，从而避免靶区或危及器官（organs at risk, OARs）受到不可接受的剂量照射。直接基于CBCT开展剂量计算，仅可在观测到剂量学变化时优化适配计划的能力。本研究针对CBCT图像上的剂量计算精度展开了探究。研究首先为9台相同的医用直线加速器的CBCT成像系统构建校准曲线：针对两种扫描模式（头颈部与盆腔模式），采集带有不同密度等效插入体的体模CBCT扫描数据，并据此计算得到平均校准曲线。随后，采集带有6种不同密度等效插入体的体模CBCT扫描数据，对CBCT图像上的剂量分布进行计算，并与计划CT（planning CT, pCT）上的剂量分布进行对比。通过pCT与CBCT之间的剂量学差异量化计算不确定性。利用上述两条平均校准曲线，分别为10例头颈部患者、11例肺部患者以及10例盆腔患者计算其每日实际递送的CBCT剂量。此外，还采用低HU值经验校正校准曲线开展了额外的患者剂量计算。随后从靶区覆盖度、平均剂量以及危及器官的D1cc三个维度对患者剂量计算结果进行对比分析。对于体模数据，所有剂量体积直方图（dose-volume histogram, DVH）参数对应的CBCT与pCT间剂量差异均较小，两种CBCT扫描模式下的平均偏差均处于±0.6%以内。对于患者数据，结果显示低HU值经验校正校准曲线的表现最优：在所有患者组中，靶区平均剂量与覆盖度的平均偏差分别为0.2%±0.7%与0.1%±0.6%。基于CBCT图像的剂量计算可实现约1%量级的靶区覆盖度与平均剂量计算精度，足以满足临床应用需求。此外，CBCT扫描模式专属校准曲线可在所有相同的成像设备以及所有患者组中推广使用。