基于盆腔迭代锥形束CT图像的剂量学可行性分析

目的 研究利用盆腔迭代锥形束CT（CBCT）图像用于治疗计划剂量计算的可行性分析,为自适应放疗提供图像保障。方法 使用Varian Halcyon 2.0环形加速器对CIRS 062 M模体（CIRS,Norfolk,VA,USA）进行扫描,测量其不同散射条件下的CT值并计算其平均值,建立锥形束CT-电子密度转换曲线（iterative Cone-beam CT to electron density,ICBCT-ED）。采集CIRS 002PRA盆腔调强专用模体的CT和不同位置的ICBCT图像,设计基于CT图像的VMAT计划,移植至ICBCT图像上,重新进行剂量计算,比较靶区、危及器官及三维体积剂量γ通过率的差异。以患者实际治疗计划为基准,回顾性分析10例盆腔患者全流程三维剂量γ通过率的差异。结果 无散射体的孤立模式与全散射中心位置的CT值偏差较大,最大偏差144 HU。其他全散射位置与中心位置CT值相近,最大偏差<50 HU。基于盆腔模体不同位置处的ICBCT图像的计算结果,无论靶区还是危及器官的剂量偏差均<1 Gy。与基于CT图像的计划相比,基于ICBCT图像的三维剂量γ通过率1%/1 mm和2%/2 mm的平均值分别为（88.86±1.18）%和（98.38±0.89）%。10例盆腔肿瘤患者2%/2 mm和3%/3 mm的平均值范围分别为90.03%~95.43%和93.58%~97.78%。最差结果为膀胱过充盈引起的外轮廓变化造成的剂量差异,2%/2 mm和3%/3 mm的三维剂量通过率仅为85.90%和92.90%。结论 在足够的散射条件下,重建ICBCT-ED转换曲线,利用Halcyon直线加速器的ICBCT图像进行治疗计划设计,其精度是可以满足临床应用的标准的,为将来的自适应放疗提供了保障。

Dosimetric feasibility of iterative kV CBCT for radiation therapy planning for pelvis

Objective To study the feasibility of using pelvic iterative cone beam CT images for dose calculation of radiation therapy planning, so as to provide support for adaptive radiotherapy. Methods The CIRS 062 M phantom was scanned by Varian Halcyon v2.0 o-ring accelerator, and the average CT number under different scattering conditions was calculated, and then the ICBCT-ED conversion curve was established. CT images of CIRS 002PRA pelvic IMRT phantom and ICBCT images at different positions were collected. Treatment plan using VMAT technique based on CT image was designed and transplanted into ICBCT image with dose recalculated. The differences of gamma passing rate among target volume, organs at risk and 3-dimensional dose were compared. Based on the actual treatment plan of patients, the differences of 3-dimensional dose gamma passing rate in 10 pelvic patients were analyzed retrospectively. Results There was a large CT value deviation at central position between the isolated no-scattering condition and the full-scattering condition, and the maximum deviation was 144 HU. The CT values of other positions in full-scattering condition were similar to those of the central position, and the maximum deviation was less than 50 HU. Based on the calculated result of ICBCT images at different positions of the pelvic phantom, the dose deviation of the target volume or organs at risk was less than 1 Gy. Compared with the plan based on CT images, the average 3-dimensional dose gamma passing rate under the criteria of 1% dose difference (DD)/1 mm distance-to-agreement (DTA) and 2% DD/2 mm DTA in plan based on ICBCT images were (88.86 ±1.18)% and (98.38±0.89)%, respectively. The ranges of average 3-dimensional dose gamma passing rate under the criteria of 2% DD/2 mm DTA and 3% DD/3 mm DTA in 10 patients with pelvic tumors were 90.03%-95.43% and 93.58%-97.78%, respectively, and the worst result was only 85.90% and 92.90%, respectively. The main reason of the worst result was the dose difference caused by large variation of bladder contour due to over-filling. Conclusions Under comprehensive scattering conditions, the ICBCT-ED conversion curve is reconstructed and the treatment plan can be designed by using the ICBCT image of Halcyon v2.0 linear accelerator. The accuracy meets the standards of clinical application, which provides assurance for adaptive radiotherapy in the future.