锥形束CT图像引导放疗系统的质量保证

全文综述了锥形束CT图像引导放疗系统用于校正器官移动和摆位带来的误差时所需要的质量保证(QA)内容,包括安全性QA、图像质量QA、硬件参数QA、几何特性QA.     

加速器附加锥形束CT图像质量评价

目的评价瓦里安23EX加速器附加kV X线锥形束CT的图像质量。方法应用安装在直线加速器上的锥形束CT系统,重复扫描Catphan 504体模,用图像分析软件评价获取图像的质量;并用电离室测量扫描体模时等中心点的辐射剂量;并扫描5例头颈部肿瘤患者,进行临床评价。结果图像均一性较好,大部分测量点的CT值偏差在±3 HU以内,图像边缘均一性略差,最大偏差值16 HU;空间分辨率达0．63 mm;体模锥形束CT图像的CT值具有较好的准确性和稳定性,实测值与标称值的最大差值为22 HU;空间线性距离的标称值和实测值的最大差别为0．4%,几何精度较高;低对比度分辨率为1．0%的对比度为4 mm,0．5%的对比度为5 mm;扫描体模时等中心焦点的辐射剂量为（2．97±0．19）cGy。5例头颈部肿瘤患者的锥形束CT图像结果显示,鼻咽部肿瘤和脑膜瘤显示清晰,颅内高分化胶质瘤显示清晰度欠佳。结论瓦里安23EX加速器附加kV X线锥形束CT图像均匀性好,有较准确而稳定的CT值,几何精度高,辐射剂量低,但低对比度分辨率欠佳。锥形束CT可以实现肿瘤和软组织成像,是图像引导放疗的有效工具。     

机载锥形束CT的质量控制

机载锥形束CT主要通过放疗在线获得图象,并根据在线与计划参考图象对比,校正位置误差和离线分析肿瘤及器官运动,以辅助提高和确保精度放疗的效果。因此同样需要规范的质量控制,并定期执行。文章简要介绍目前形成的较为合理规范的机载锥形束CT质量控制内容、基准和执行频度。     

千伏级锥形束CT影像进行放疗剂量计算的可行性研究

目的:研究千伏级锥形束CT（kilovoltage cone-beam CT,KVCBCT）影像进行放疗剂量计算的可行性及精确性。方法:用Elekta Synergy医用直线加速器及多层螺旋CT（德国Siomonos AG,SOMATOM Definition AS 40层）分别扫描CIRS-062电子密度模体,获取KVCBCT及扇形束CT（fan beam CT,FBCT）特定区域亨氏单位值（hounsfield unit,HU）,重新刻度亨氏单位值-相对电子密（HU-RED）表。选取我院行调强放疗的肿瘤患者80例（鼻咽癌、肺癌、胃癌及宫颈癌各20例）,将在FBCT影像上进行的三维适形调强放疗（intensity modulated radiation therapy,IMRT）计划在相对应的CBCT影像上以相同的条件再次进行剂量的计算,并将两种影像条件下的计算结果行配对t检验,比较其剂量分布有无明显差异。结果:在KVCBCT及FBCT两种影像条件下的放疗计划的比较中,鼻咽癌、胃癌、宫颈癌的95%PTV无明显差异,而在肺癌的计划中有着明显差异,在脊髓最大剂量(Dmax)、脑干Dmax、腮腺 V30、眼球Dmax、肺 V20、肺 V5、心脏 V30、肝脏平均剂量（Dmean）、直肠 V40、膀胱 V50、小肠Dmax的比较中无明显差异。结论:经过修订HU-RED表后,CBCT影像用于放疗计划的计算是可行的,但在胸部肿瘤即肺癌患者的放疗中还需要进一步研究找到更合适的方法去减少伪影的干扰。CBCT影像能较准确的反应出患者在治疗中的组织结构变化,并能根据变化实时的制定放疗计划,最终为实现自适应放疗（ART）提供准确的影像及剂量保证。     

兆伏级锥形束CT成像剂量及其致继发肿瘤终生归因风险研究

目的 评估胸部影像引导放射治疗中,兆伏级锥形束CT(MV-CBCT)成像在不同年龄患者各重要器官沉积的剂量,并预测成像剂量致继发肿瘤的终生归因风险(LAR)。方法 回顾性评估2018-02-01-2021-10-31山东省肿瘤防治研究院收治的206例年龄为3～70岁患者胸部MV-CBCT计划系统模拟的心脏、双侧肺、双侧乳腺等器官平均剂量和脊髓最大点剂量。建立成像野内器官剂量与胸围尺寸的线性关系。使用美国电离辐射生物效应委员会(BEIR)Ⅶ模型,预测MV-CBCT致不同年龄患者继发肿瘤的风险值。结果 206例患者以10 cm(头脚方向)×28 cm(左右方向)的开野范围模拟MV-CBCT扫描,其心脏、双侧肺、双侧乳腺和脊髓单次扫描的中位剂量分别为4.3(3.6～5.3)、2.7(1.9～5.0)、4.5(3.6～5.8)和3.7(2.8～4.9) cGy。射野内各器官平均剂量随着胸围的增大而单调减小。不同肿瘤LAR均随暴露年龄增大而降低。暴露年龄段为3～<10、10～<20、30～<40、50～<60和60～<70岁的男性肺癌LAR分别为2 539/10万人...     

锥形束CT在肿瘤放射治疗中的作用

目的探讨锥形束CT(cone beam,CT)在肿瘤放射治疗中的作用。方法对236例放疗患者,应用CBCT对摆位误差进行测量。对未在允许误差范围内的患者进行重新摆位,使其误差在允许范围内。将患者首次摆位误差记为Ⅰ组,重新摆位后的误差记为Ⅱ组,并对两组数据进行统计学分析。结果对数据比较分析发现,首次测量中有180例(76.3%)在允许误差范围内;48例(20.3%)未在允许误差范围内,需要重新摆位;8例(3.4%)出现不同原因的摆位错误。通过对未在允许误差范围内的48例患者进行重新摆位后,均使其误差控制在允许范围内,且重新摆位后各部位的摆位误差Ⅱ组均小于Ⅰ组。结论在肿瘤放疗中,应用CBCT对摆位误差进行测量,不仅能发现每位患者体位是否准确,且能及时发现治疗操作中的错误,而且为临床对PTV外放提供参考数据。     

兆伏级锥形束CT在食管癌放射治疗中摆位误差的分析

目的:评估兆伏级锥形束CT(CBCT)图像引导食管癌三维适形放射治疗(3DRT)的摆位误差,计算临床靶体积(CTV)到计划靶体积(PTV)的外放边界.方法:用西门子配备有MVision兆伏级CBCT的直线加速器,对32例三维适形放疗(3DRT)的食管癌患者,在治疗的5周内每周1次,分别对治疗前、摆位误差调整后行CBCT扫描.通过计划CT图像与治疗图像进行匹配,获取左右(X)、头脚(Y)、前后(Z)的摆位误差,计算CTV到PTV的外放边界.结果:32例患者共获取320幅CBCT图像.在校正前,患者的摆位误差分别为左右(-1.25±3.28)mm、头脚(-0.63±5.00)mm、前后(0.84±3.26) mm;根据Van等提供的公式,CTV至PTV的外放边界为左右9.38mm,头脚12.28mm,前后7.70mm.摆位误差调整后:误差分别为左右(0.19±1.89) mm、头脚(-0.56±3.71)mm和前后(0.53±1.54)mm,与调整前相比在三维方向均有降低,且有统计学差异(P＜0.05).摆位误差调整后PTV外扩边界,左右3.68mm,头脚4.83mm,前后4.24mm.结论:通过CBCT获取食管癌患者的摆位误差并对其进行纠正,能显著降低分次间的摆位误差,提高放疗精确度,减小PTV外放边界.     

兆伏级锥形束CT:系统说明及IGRT临床应用介绍

阐述临床用兆伏级锥形束CT（MVCBCT）系统，介绍图像采集和患者摆位的过程，并就摆位精度和图像质量进行讨论，并通过所选病例介绍该系统在影像引导下的放射治疗（IGRT）中的潜在应用价值。兆伏级锥形束CT系统由一台标准直线加速器外加配备以非晶硅平板型EPID，能够适合兆伏级光子射线。一体化的计算机工作平台能够支持投照图像的自动采集，图像重建，CT到锥形束CT图像的注册，以及治疗床的移动计算。系统具有亚毫米级的定位精度和足够的软组织分辨率，能够显示前列腺等软组织结构。在我院，已经使用兆伏级锥形束CT系统检测脊髓的非刚性形变，监控肿瘤的生长和消退，以及肺内固定肿瘤的定位等。对于受到金属伪影干扰的结构，兆伏级锥形束CT还提高了其勾画的准确性。在过去的几年中，兆伏级锥形束CT有了较快的发展。现在的图像质量完全可以满足IGRT应用的需要。此外，我们希望兆伏级锥形束CT在临床上的应用将随着影像技术的发展而不断扩大。     

应用锥形束CT研究摆位误差对头颈部调强放疗剂量分布的影响

[目的]应用IGRT机载千伏级锥形束CT(CBCT)研究调强放疗时摆位的线性误差和旋转误差对靶区和危及器官剂量分布的影响。[方法]应用CBCT扫描MED-TEC头体模获得X线容积影像,再利用计算机模拟,研究单纯线性误差、单纯旋转误差以及两者同时存在时对靶区和危及器官剂量分布的影响。[结果]计算机模拟结果表明线性误差和旋转误差对靶区及危及器官的剂量分布均有影响,两者同时存在时影响更大,即使较小的线性误差与旋转误差相叠加也能对剂量分布产生影响。[结论]旋转误差会增加线性误差对靶区及危及器官受照剂量的影响,只纠正线性误差不能很好地纠正剂量分布的改变,因此旋转误差的纠正具有重要的临床意义。     

使用兆伏级锥形束CT进行头颈部剂量计算的研究

目的 考察使用兆伏级锥形束CT(MVCBCT)进行头颈部剂量计算的可行性和准确性.方法 采用可进行MVCBCT扫描的西门子ONCOR直线加速器,扫描MiniCTQC模体,建立MVCBCT值密度曲线;分别采集模体和鼻咽癌病例的常规CT和MVCBCT图像.在模体的常规CT上设计两种单野计划,在鼻咽癌的常规CT上没计IMRT计划,将完全相同的计划分别应用于模体和鼻咽癌的MVCBCT图像.应用MVCBCT值密度校正曲线计算靶区及正常组织的受量,并与使用常规CT计算所得的剂量分布做比较.结果 绝对剂量和剂量点距离比较结果显示,在单野方案中所有MVCBCT与常规CT剂量偏差<3%和剂量点距离<3mm.在IMRT计划中DVH显示常规CT和MVCBCT计划曲线有很好的一致性,常规CT与MVCBCT计算所得最大差值为95 cGy,误差为1.4%.绝对剂量和剂量点距离比较结果显示在等中心平面机架角度0°、45°、90°、120°、160°、200°、240°、280°、320°的通过率为95.5%、99.4%、93.8%、98.7%、100%、94.5%、97.3%、95.6%、99.3%、99.4%.结论 使用兆伏级锥形束CT进行头颈部剂量计算可行且可得到与常规CT基本一致的剂量分布.     

Dose calculation using megavoltage cone-beam CT

PURPOSE: To demonstrate the feasibility of performing dose calculation on megavoltage cone-beam CT (MVCBCT) of head-and-neck patients in order to track the dosimetric errors produced by anatomic changes. METHODS AND MATERIALS: A simple geometric model was developed using a head-size water cylinder to correct an observed cupping artifact occurring with MVCBCT. The uniformity-corrected MVCBCT was calibrated for physical density. Beam arrangements and weights from the initial treatment plans defined using the conventional CT were applied to the MVCBCT image, and the dose distribution was recalculated. The dosimetric inaccuracies caused by the cupping artifact were evaluated on the water phantom images. An ideal test patient with no observable anatomic changes and a patient imaged with both CT and MVCBCT before and after considerable weight loss were used to clinically validate MVCBCT for dose calculation and to determine the dosimetric impact of large anatomic changes. RESULTS: The nonuniformity of a head-size water phantom ( approximately 30%) causes a dosimetric error of less than 5%. The uniformity correction method developed greatly reduces the cupping artifact, resulting in dosimetric inaccuracies of less than 1%. For the clinical cases, the agreement between the dose distributions calculated using MVCBCT and CT was better than 3% and 3 mm where all tissue was encompassed within the MVCBCT. Dose-volume histograms from the dose calculations on CT and MVCBCT were in excellent agreement. CONCLUSION: MVCBCT can be used to estimate the dosimetric impact of changing anatomy on several structures in the head-and-neck region.     

Feasibility of contrast-enhanced cone-beam CT for target localization and treatment monitoring

A dog with a spontaneous maxillary tumour was given 40 Gy of fractionated radiotherapy. At five out of 10 fractions cone-beam CT (CBCT) imaging before and after administration of an iodinated contrast agent were performed. Contrast enhancement maps were overlaid on the pre-contrast CBCT images. The tumour was clearly visualized in the images thus produced.     

肺癌锥形束CT图像引导放疗最优图像配准方法的筛选与评价

目的 筛选、评价适合应用于千伏特锥形束CT(KVCBCT)在线引导肺癌放疗的图像配准方法.方法 选择16例行根治性放疗的非小细胞肺癌患者进入研究,每例患者每周行KVCBCT在线引导体位校正1次,共96幅KVCBCT图像用于研究.分别采用基于灰度的自动配准法、骨性结构的自动配准法、人工图像配准法和自动加手动图像配准法配准患者KVCBCT图像与计划没计CT图像.配准由1名医生完成,配准结果由另外1名医牛在对所用配准方法设盲的情况下进行评价与筛选,并对筛选出的最优图像配准方法的可重复性进行评价.结果 基于灰度的自动配准法、骨性结构的自动配准法、人工图像配准法和自动加手动图像配准法的平均得分分别为2.7、2.4、3.0和3.7分,4个组评分差异有统计学意义(F=42.20,P<0.001).采用自动加手动图像配准法左右(LR)、头脚(SI)和前后(AP)方向上,同一医牛2次配准结果差值>3 mm所占比例分别为0、3%和6%,不同医生的分别为0、14%和0,医生与技术员的分别为8%、14%和8%.结论 自动加手动图像配准法以其配准结果好、所需时间较短、临床应用可重复性强等特点适合应用于肺癌KVCBCT在线引导放疗.     

Reducing metal artifacts in cone-beam CT images by preprocessing projection data

PURPOSE: Computed tomography (CT) streak artifacts caused by metallic implants remain a challenge for the automatic processing of image data. The impact of metal artifacts in the soft-tissue region is magnified in cone-beam CT (CBCT), because the soft-tissue contrast is usually lower in CBCT images. The goal of this study was to develop an effective offline processing technique to minimize the effect. METHODS AND MATERIALS: The geometry calibration cue of the CBCT system was used to track the position of the metal object in projection views. The three-dimensional (3D) representation of the object can be established from only two user-selected viewing angles. The position of the shadowed region in other views can be tracked by projecting the 3D coordinates of the object. Automatic image segmentation was used followed by a Laplacian diffusion method to replace the pixels inside the metal object with the boundary pixels. The modified projection data were then used to reconstruct a new CBCT image. The procedure was tested in phantoms, prostate cancer patients with implanted gold markers and metal prosthesis, and a head-and-neck patient with dental amalgam in the teeth. RESULTS: Both phantom and patient studies demonstrated that the procedure was able to minimize the metal artifacts. Soft-tissue visibility was improved near or away from the metal object. The processing time was 1-2 s per projection. CONCLUSION: We have implemented an effective metal artifact-suppressing algorithm to improve the quality of CBCT images.     

ExacTrac X-ray 6 degree-of-freedom image-guidance for intracranial non-invasive stereotactic radiotherapy: Comparison with kilo-voltage cone-beam CT

Background and purpose

To compare the residual setup errors measured with ExacTrac X-ray 6 degree-of-freedom (6D) and cone-beam computed tomography (CBCT) for a head phantom and patients receiving intracranial non-invasive fractionated stereotactic radiotherapy (SRT).

Materials and methods

Setup data were collected on a Novalis Tx treatment unit for an anthropomorphic head phantom and 18 patients with intracranial tumors. Initial corrections were determined and corrected with the ExacTrac system only, and then the residual setup error was determined by means of three different procedures. These procedures included registrations of ExacTrac X-ray images with the corresponding digitally reconstructed radiographs (DRRs) using the ExacTrac 6D fusion, and registrations of CBCT images with the planning CT using both online 3D fusion and offline 6D fusion. The difference in residual setup errors between ExacTrac system and CBCT was computed. The impact of rotations on the difference was evaluated.

Results

A modest difference in residual setup errors was found between ExacTrac system and CBCT. The root-mean-square (RMS) of the differences observed for translations was typically <0.5 mm for phantom, and <1.5 mm for patients, respectively. The RMS of the differences for rotation(s) was however <0.2 degree for phantom, and <1.0 degree for patients, respectively. The impact of rotation on the setup difference was minor but not negligible.

Conclusions

This study indicates that there is a general agreement between ExacTrac system and CBCT.     

肺肿瘤在线与离线结合锥形束CT图像引导放疗的可行性研究

目的 探讨肺肿瘤在、离线结合锥形束CT(CBCT)图像引导放疗的可行性.方法 14例行三维适形放疗的肺肿瘤患者入组.放疗前后分别行在线CBCT扫描1次,并与计划CT图像配准,记录各个方向的配准差值.放疗前后配准获得的平移矢量分别作为分次间误差和分次内误差,利用CTV外放公式分别计算未行在线校正以及在线校正后的cTV外放.分别以0.5、1.5 mm为允许的最大残余系统摆位误差,计算预测总系统摆位误差所需的最少CBCT图像数以及离线校正系统摆位误差后的CTV外放.结果 未行在线校正时,左右、头脚、前后方向上群体化CTV外放分别为5.7、8.0、7.8 mm;每分次放疗均行在线校正时,3个方向上群体化CTV外放分别为2.4、2.4、2.3 mm.分别以0.5 mm或1.5 mm为允许的最大残余系统误差,计算预测系统摆位误差所需的最少CBCT图像数为9套或7套,对系统摆位误差进行离线校正后,左右、头脚和前后方向上群体化CTV外放分别为3.3 mm或3.9 mm、3.7 mm或4.3 mm和3.6 mm或4.3 mm.结论 基于CBCT图像分析的在线校正和离线校正均能明显减小摆位误差,并有助于缩小CTV外放.肺肿瘤患者进行在线、离线相结合的图像引导放疗是可行的.     

锥形束CT图像拼接技术的实现及其临床应用研究

目的 自主编写图像拼接软件"CBCT Pasting"实现kV锥形束CT(kV-CBCT)图像的无缝拼接,达到CBCT扫描宽度的拓展,实现CBCT评价肿瘤靶区及危及器官的完整性,为在CBCT图像上设计和评价放疗计划提供条件.方法 在计划CT图像空间分别制定两段CBCT扫描计划;在获得1套CBCT图像后手动平移治疗床,再次行CBCT拍摄以获取相邻分段的CBCT图像.将2段图像分别导入治疗计划系统,三维重建后分别与计划CT进行图像融合,通过对融合后骨性标志的分析来选取拼接层面.将2套CBCT图像依次导入"CBCT Pasting"进行拼接,通过体模验证拼接图像的几何学完整性.用同样方法实现10例胸部肿瘤患者自由呼吸状态下CBCT图像的拼接,经三维重建后评估患者肺体积并将其与计划CT中的相同参数进行比较.结果 无论体模还足人体,利用"CBCT Pasring"软件很容易实现分段CBCT的拼接.经拼接CBCT图像测定,体模外轮廓体积与计划CT测量结果相差0.26%(28.34cm3),体模两侧肺体积与计划CT测得结果分别相差1.87%(12.82cm3)和1.47%(10.07cm3).10例患者拼接后CBCT图像与计划CT图像全肺体积平均相差1.97%±0.42%[(64.53±26.07)cm3],左肺2.30%±0.78%[(33.32±17.03)cm3],右肺1.75%±0.2%[(31.21±12.51)cm3].结论 利用笔者编写的"CBCT Pasting"软件可实现相邻部位分段扫描所得CBCT图像的拼接,扩大了靶区及危及器官的观察范围,保证了以CBCT制定治疗计划剂量体积参数的完整性,有广泛的临床应用前景.     

Comparison of kilovoltage cone-beam computed tomography with megavoltage projection pairs for paraspinal radiosurgery patient alignment and position verification

PURPOSE: Implanted gold markers and megavoltage (MV) portal imaging are commonly used for setup verification of paraspinal tumors treated with high-dose, single-fraction radiotherapy. We investigated whether the use of kilovoltage cone-beam computed tomography (CBCT) imaging eliminates the need for marker implantation. METHODS AND MATERIALS: Patients with paraspinal disease who were eligible for single-fraction stereotactic body radiotherapy were accrued to an institutional review board-approved protocol. Each of 16 patients underwent implantation of fiducial markers near the target. The markers were visible on the MV images. Three MV image pairs were acquired for each patient (initial, verification, and final) and were registered to the reference images. Every MV pair was complemented by a CBCT scan. CBCT image registration was performed automatically by maximizing the mutual information using a region of interest that excluded the markers. The corrections, as determined from the MV images, were compared with these from CBCT and were used for actual patient setup. RESULTS: The mean and standard deviation of the absolute values of the differences between the CBCT and MV corrections were 1.0 +/- 0.7, 1.0 +/- 0.6, and 1.0 +/- 0.8 mm for the left-right, anteroposterior, and superoinferior directions, respectively. The absolute differences between the corresponding pre- and post-treatment kilovoltage CBCT image registration were 0.6 +/- 0.5, 0.6 +/- 0.5, and 1.0 +/- 0.8 mm. CONCLUSION: The setup corrections found using CBCT without the use of implanted markers were consistent with the marker registration on MV projections. CBCT has additional advantages, including better positioning precision and robust automatic three-dimensional registration, as well as eliminating the need for invasive marker implantation. We have adopted CBCT for the setup of all single-fraction paraspinal patients. Our data have also demonstrated that target displacements during treatment are insignificant.     

Dose recalculation in megavoltage cone-beam CT for treatment evaluation: Removal of cupping and truncation artefacts in scans of the thorax and abdomen

Purpose

To correct megavoltage cone-beam CT (MVCBCT) images of the thorax and abdomen for cupping and truncation artefacts to reconstruct the 3D-delivered dose distribution for treatment evaluation.

Materials and methods

MVCBCT scans of three phantoms, three lung and two rectal cancer patients were acquired. The cone-beam projection images were iteratively corrected for cupping and truncation artefacts and the resulting primary transmission was used for cone-beam reconstruction. The reconstructed scans were merged into the planning CT scan (MVCBCT+). Dose distributions of clinical IMRT, stereotactic and conformal treatment plans were recalculated on the uncorrected and corrected MVCBCT+ scans using the treatment planning system and compared to the planned dose distribution.

Results

The dose distributions on the corrected MVCBCT+ of the phantoms were accurate for 99% of the voxels within 2% or 2 mm. Using this method the errors in mean GTV dose reduced from about 10% to 1% for the patients.

Conclusions

The method corrects cupping and truncation artefacts in cone-beam scans of the thorax and abdomen in addition to head-and-neck (demonstrated previously). The corrected scans can be used to calculate the influence of anatomical changes on the 3D-delivered dose distribution.