射野大小对全脑调强放疗计划EPID验证结果的影响

目的:利用电子射野影像系统（EPID）对全脑调强放疗计划进行γ测试,寻找计划设计对测试结果的影响,以此分析如何优化全脑调强计划以及推测EPID在剂量验证方面的局限性。方法:选取67例全脑放疗患者,对其放疗计划用加速器自带的EPID进行计划验证,对于容积旋转调强放疗（VMAT）计划统计并分析X方向射野大小与γ（3 mm/3%）通过率的关系,对于调强放疗（IMRT）对比分析大野调强和分野调强计划γ（3 mm/3%）通过率的差异。结果:VMAT计划验证结果发现X方向小于15 cm的射野γ（3 mm/3%）通过率普遍优于大于等于15 cm的射野,利用SPSS软件进行t检验,发现结果具有统计学意义（t=-3.828, P<0.05）;IMRT验证结果发现,X方向大于等于15 cm的射野会包含两个子野,合野验证时其交叠部分γ（3 mm/3%）通过率较差,而采用分野验证时,由于无交叠则通过率普遍较好。结论:全脑放疗VMAT计划将X方向射野控制在15 cm以内可以提升多叶准直器调节能力,并提高EPID验证的γ（3 mm/3%）通过率;EPID原件对低剂量区的响应偏差会导致全脑IMRT大野调强计划两子野交叠处γ（3 mm/3%）通过率较差,改用分野验证可以显著消除这种影响。     

带金属植入物患者调强放射治疗计划中射野角度设置对剂量计算准确度的影响

目的:对金属植入物及其伪影在放疗计划中产生的剂量计算偏差进行测试,评估射野角度设置对调强放射治疗(IMRT)计划剂量计算准确度的影响。方法:模拟髋关节置换患者,在CIRS调强模体中插入两根不锈钢金属棒。将指形电离室分别置于金属棒所在平面内中间位置的3个点,采用带金属伪影消减技术的CT模拟定位机获取未经校正和校正后的模体图像。在Monaco计划系统中利用两种CT图像,在0o～360o内每隔5o设置一个射野(10 cm×10 cm, 100 MU),检测不同入射路径射野的剂量计算偏差。勾画靶区和危及器官,设计5野和7野IMRT计划,每种计划分别设有0、1或2个射野的入射路径通过金属区域,检测射野角度设置对IMRT计划剂量计算偏差的影响。结果:入射路径没有通过金属区域的单个射野在未校正及校正后图像中的剂量计算偏差分别为3.24%和1.56%,入射路径通过金属区域时剂量计算偏差分别达到5.51%～72.14%和5.32%～48.19%。在5野IMRT计划中,当有0、1和2个入射路径通过金属区域的射野时,未校正图像的计划剂量计算偏差分别为3.15%、8.75%和13.33%,校正图像的计划剂量计算偏差分别为1.54%、5.93%和9.06%;7野IMRT计划中,未校正图像的计划剂量计算偏差分别为3.03%、5.28%和10.71%,校正图像的计划剂量计算偏差分别为1.29%、4.38%和7.75%。结论:放射治疗计划中入射路径通过金属区域的射野会严重影响剂量计算的准确度,应尽量避免使用这种射野。虽然采用金属伪影消减技术校正CT图像能够改善这种影响,但在IMRT计划中存在两个及以上的这种射野可能导致临床上不可接受的剂量计算偏差。     

瓦里安IGRT治疗床对调强放疗剂量的影响

目的:射线经过加速器治疗床有一定的衰减,对靶区和危及器官的剂量有一定影响。本文探讨Varian Exact IGRT治疗床对调强放射治疗(IMRT)剂量的影响。方法:选取10例肺癌患者,使用Eclipse计划系统设计两组IMRT计划,第一组计划考虑治疗床的影响,第二组计划去掉治疗床后重新计算剂量分布,比较两组计划剂量分布。并对两组计划进行剂量验证,比较γ因子符合度指数(GAI)。结果:去掉治疗床后,计划靶区平均剂量升高3.1%,其余重要危及器官受到剂量也有增加。剂量验证的GAI结果表明,当射野先经过治疗床再到达靶区时,考虑治疗床的影响通过率更好(GAI为93.9%~100%)。结论:在设计IMRT计划时,有必要加入治疗床板,使计算结果与实际剂量分布更加吻合。     

非晶硅电子射野影像装置在宫颈癌剂量验证中的应用

目的:非晶硅电子射野影像装置(a-Si EPID)分别与Arc CHECK和二维电离室矩阵(PTW729)两种验证技术在宫颈癌剂量验证中的应用比较。方法:随机选取40例宫颈癌容积旋转调强技术(VMAT)和调强放射治疗技术(IMRT)病例。在相应的模体上分别设计出验证计划,将验证计划分为VMAT和IMRT两组,利用a-Si EPID和Arc CHECK验证VMAT计划,a-Si EPID和PTW729验证IMRT计划,在UNIQUE加速器上进行验证。采用γ分析方法(3%,3 mm,10%标准),比较两组验证计划的相对剂量与绝对剂量通过率和X、Y方向的profile。结果:VMAT组:Arc CHECK的绝对剂量通过率为(97.73±1.98)%,相对剂量通过率为(96.96±2.34)%;a-Si EPID的绝对剂量通过率为(97.58±1.88)%,相对剂量通过率为(98.13±1.47)%。IMRT组:PTW729的绝对剂量通过率为(98.48±1.89)%,相对剂量通过率为(97.32±1.56)%;a-Si EPID的绝对剂量通过率为(98.74±1.77)%,相对剂量通过率为(97.98±1.65)%。同时两组X、Y方向的profile理论与实测很相近,理论剂量分布图与实测计算剂量分布图在高低剂量点分布上重合度较高。结论:3种验证技术的结果在剂量学上没有明显差异,但a-Si EPID具有成像分辨率高、图像处理快捷、使用方便等优点。     

射野区域的剂量验证研究

目的:实现射野区域剂量分布Gamma（[γ]）通过率的计算,对治疗传输的准确性进行评估。方法:从Oncentra Masterplan治疗计划系统中随机提取6位完全匿名患者的调强放射治疗验证计划,导出DICOM格式的验证计划并利用Matlab软件重建多叶准直器区域和剂量。然后将验证计划移植到MatriXX模体并测量剂量分布。用Matlab代码对验证计划剂量分布和模体测量的绝对剂量分布进行分析。结果:传统方法[γ]通过率受计算区域选择影响较大,而以射野区域作为计算区域则避免了这个问题,两种方法计算得到的[γ]通过率有统计学差异（[P]<0.05）。结论:射野区域的剂量验证避免了[Dn]值对[γ]通过率的影响,而且对射野区域利用剂量面积直方图分析其剂量特性,有利于评估治疗计划系统临床治疗的准确性和指导临床工作。     

调强放疗计划床板修正的剂量学分析

【摘 要】 目的:分析调强放疗计划加速器床板和定位床板修正的剂量学影响。 方法:分别选取碳素板胸部定位食管癌患者14例和盆腔有机板定位宫颈癌术后患者14例,在修正床板和不修正床板条件下设计调强计划,得到4组计划数据;再将调强计划导入验证模体,做验证计划,各选取等中心层面5个剂量验证参考点,用电离室剂量仪测量参考点的吸收剂量,对测得的10组剂量数据进行统计分析。 结果:实测床板的衰减平均为3.4%～6.6%。胸部食管癌组床板修正前后剂量误差3.0%～4.5%,平均3.8%,P=0.000;盆腔宫颈癌术后组床板修正前后剂量误差3.5%～4.6%,平均4.1%,P=0.000。 结论:调强放疗计划设计过程中,有后野或后斜野射线穿过床板时,应充分考虑床板对剂量的衰减,必须对加速器床板和定位床板进行剂量修正。     

基于深度学习方法预测IMRT计划射野的γ通过率

目的:建立卷积神经网络（CNN）模型预测IMRT计划射野的γ通过率（GPR）。方法:从Eclipse治疗计划系统中选取48例脑胶质瘤患者的IMRT计划,共计260个照射野,制作每个计划基于电子射野影像系统测量的验证计划,并在Varian 23EX直线加速器上执行。利用portal dosimetry剂量测定软件包对计划剂量的计算值和电子射野影像系统实测值进行γ分析,得到射野在2%（global）/2 mm标准下的GPR。选取portal dosimetry系统计算的剂量分布图作为输入数据,并将数据集划分为训练集208个射野,验证集和测试集各26个射野。基于tensorflow框架建立CNN模型去学习射野的剂量分布图与GPR之间的相关性,并使用平均绝对误差对模型的预测效果进行评估。结果:在验证集和测试集上,96%样本的GPR预测误差都小于±3%,最大误差分别为3.09%和3.54%,平均绝对误差分别为0.99%和1.17%,模型预测和实际测量的GPR之间的皮尔逊相关性系数r分别为0.96和0.90。结论:深度学习CNN模型可以准确地预测脑胶质瘤患者IMRT计划射野的GPR,有助于物理师提前识别可能不能通过QA测量的计划,有效地促进临床放疗的QA工作。     

AAA和PDIP算法在非均整模式容积调强放射治疗剂量预测方面的差异

【摘要】目的:探究各项异性算法（AAA）和射野剂量图像预测（PDIP）算法在非均整模式（FFF）容积调强放射治疗计划治疗前验证γ分析中的差异以及计划复杂程度对这种差异的影响,为临床上基于电子射野影像系统（EPID）的剂量预测算法的选择提供依据。方法:选取能量为6 MV FFF的两种测试野和16例头颈部肿瘤治疗计划,利用PDIP和AAA两种算法分别生成预测数据并与EPID实测数据进行γ分析,统计两种算法在不同γ评判标准下的通过率并计算通过率差异（Delta γ）。计算上述病例每个射野的复杂系数,分析不同标准下两种算法的Delta γ与复杂系数的相关性;利用γmean、γsd、γ1和γ通过率共同描述γ分布,并分析其与复杂系数间的相关性。结果:当评判标准为3%/3 mm或2%/2 mm时,不同算法下测试射野的Delta γ较小。当评判标准为1%/1 mm,不同开野的Delta γ变化明显:射野较小时,PDIP算法的通过率低于AAA;当射野增大到（10×10） cm2时,通过率基本一致;当射野继续增大时,PDIP算法的通过率逐渐高于AAA。全部射野的通过率与评判标准的关系类似:在3%/3 mm标准下,两种算法的结果基本一致;随着标准的提高,两种算法的通过率逐渐下降,二者之间的差异也逐渐明显。复杂系数与Delta γ、γmean、γsd和γ1为正相关,与γ通过率为负相关。结论:PDIP算法对于有机械臂支撑的EPID的剂量预测更准确;AAA则适用于无机械臂支撑的EPID或机械臂反散射影响较小的射野。当计划复杂程度或评判标准提高时,两种算法的差异也增大。计划复杂程度对FFF计划验证结果的影响是负面的。上述结果提示临床应针对性地选择计划验证工具来确保治疗的安全有效。     

二维电离室矩阵中射野边界位置对调强验证Gamma通过率的影响

目的:针对(10×10)cm~2射野,探讨改变射野边界在矩阵中的位置对测量射野大小及Gamma(γ)通过率的影响。方法:使用MatriXX二维电离室矩阵测量(10×10)cm~2射野剂量分布,保持射野大小不变,移动X方向准直器和在Y方向移动治疗床两种方式改变射野边界在矩阵中的位置,用OmniPro I'mRT(1.7)软件分析每次移动0.1 cm时射野边长的改变量,同时用实测剂量分布和XiO(4.40)治疗计划系统相应射野剂量分布对比,记录3%/3 mm评估标准下的γ通过率和γ为100%时的评估标准。结果:在矩阵电离室腔外间隙射野边长改变量低于0.1 cm,且在每两个电离室腔外间隙正中改变量最小接近0.05 cm;在电离室腔体内改变量高于0.1 cm,且在每一个电离室腔体中心接近最大值0.2 cm。3%/3 mm下的γ结果显示射野边界不通过点数随位置变化明显不同,在射野边长改变量最大和最小附近通过率高,全部通过的评估标准范围是2%/2 mm至6%/3 mm。结论:选取射野边界在矩阵电离室腔体中心或腔外间隙正中位置时,所测射野大小偏差最小。同时上述射野边界位置γ通过率最高,因此,在调强计划剂量分布验证中要充分考虑射野剂量梯度较大处在电离室矩阵的位置对γ通过率的影响,可调整剂量分布在矩阵中位置或改变不同评估标准详细分析γ通过率差异,从而提高γ通过率的有效性,对临床工作具有一定的指导作用。     

快速容积旋转调强治疗系统的验收测试

目的:检验新型快速容积旋转调强(VMAT)治疗系统的计划剂量计算和执行精度,为系统投入临床应用提供质量保证。方法:根据美国医学物理师协会第142号工作组报告中的要求,检测加速器机械和剂量学精度,并基于仿真人体不均质模型设置8个包括标准方野、楔形野、多叶准直器菱形野、不规则野、组合野等射野,以及斜入射、切线照射、共面和非共面照射等照射条件对测试例进行多点剂量验证。针对VMAT的特点特别设置固定方野、适形射野、静态和动态调强射野的旋转照射测试计划,对系统进行端到端的验证测试,最后采用实际VMAT治疗计划检验系统的执行精度。结果:加速器的机架、准直器和床旋转等中心误差≤1.5 mm,所有旋转运动的角度刻度误差≤0.5°,多叶准直器到位误差≤1.0mm,光野射野重合度误差≤1.0 mm。8个测试例中多点剂量验证的误差为-2.57%~2.30%,各旋转照射验证测试的剂量误差为-1.83%~1.19%。实际VMAT治疗计划验证的γ(3.00%,3 mm)通过率优于95.00%。结论:所设置的验收测试能保证快速VMAT治疗系统机械和剂量学精度达到临床要求,可作为同类系统临床前质量保证的验证方法。     

综合复杂性特征和剂量学评估指标提高剂量验证结果预测模型的性能

目的:构建随机森林模型预测调强计划剂量验证结果，研究综合复杂性特征和剂量学评估指标提高模型性能的可行性。方法:选取269例IMRT计划，共2 558个射野，采用电子射野影像系统进行剂量验证，γ通过率（2%/2 mm标准）阈值为95%，将剂量验证结果分为“通过”和“不通过”。提取计划的剂量学评估指标和射野的复杂性特征，分别构建剂量模型（基于剂量学评估指标）、计划模型（基于计划复杂性特征）和混合模型（综合剂量学评估指标和计划复杂性特征）。计算AUC值、特异性和敏感性评估模型性能。结果:剂量模型、计划模型和混合模型的AUC值分别为0.68、0.80和0.82，混合模型优于其他两个模型。混合模型的特异性和敏感性为0.70和0.79，均高于其他两个模型。剂量模型、计划模型和混合模型达到最佳性能所需的样本量分别为1 200、900和700。结论:剂量学评估指标与计划复杂性特征综合，可以提高模型的预测性能，同时在一定程度上弥补样本数量的不足，为预测剂量验证结果的机器学习模型性能的改善提供参考。     

Clinical experience with EPID dosimetry for prostate IMRT pre-treatment dose verification

The aim of this study was to demonstrate how dosimetry with an amorphous silicon electronic portal imaging device (a-Si EPID) replaced film and ionization chamber measurements for routine pre-treatment dosimetry in our clinic. Furthermore, we described how EPID dosimetry was used to solve a clinical problem. IMRT prostate plans were delivered to a homogeneous slab phantom. EPID transit images were acquired for each segment. A previously developed in-house back-projection algorithm was used to reconstruct the dose distribution in the phantom mid-plane (intersecting the isocenter). Segment dose images were summed to obtain an EPID mid-plane dose image for each field. Fields were compared using profiles and in two dimensions with the y evaluation (criteria: 3%/3 mm). To quantify results, the average gamma (gamma avg), maximum gamma (gamma max), and the percentage of points with gamma < 1(P gamma < 1) were calculated within the 20% isodose line of each field. For 10 patient plans, all fields were measured with EPID and film at gantry set to 0 degrees. The film was located in the phantom coronal mid-plane (10 cm depth), and compared with the back-projected EPID mid-plane absolute dose. EPID and film measurements agreed well for all 50 fields, with (gamma avg) =0.16, (gamma max)=1.00, and (P gamma < 1)= 100%. Based on these results, film measurements were discontinued for verification of prostate IMRT plans. For 20 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original gantry angles. The planned isocenter dose (plan(iso)) was verified with the EPID (EPID(iso)) and an ionization chamber (IC(iso)). The average ratio, (EPID(iso)/IC(iso)), was 1.00 (0.01 SD). Both measurements were systematically lower than planned, with (EPID(iso)/plan(iso)) and (IC(iso)/plan(iso))=0.99 (0.01 SD). EPID mid-plane dose images for each field were also compared with the corresponding plane derived from the three dimensional (3D) dose grid calculated with the phantom CT scan. Comparisons of 100 fields yielded (gamma avg)=0.39, gamma max=2.52, and (P gamma < 1)=98.7%. Seven plans revealed under-dosage in individual fields ranging from 5% to 16%, occurring at small regions of overlapping segments or along the junction of abutting segments (tongue-and-groove side). Test fields were designed to simulate errors and gave similar results. The agreement was improved after adjusting an incorrectly set tongue-and-groove width parameter in the treatment planning system (TPS), reducing (gamma max) from 2.19 to 0.80 for the test field. Mid-plane dose distributions determined with the EPID were consistent with film measurements in a slab phantom for all IMRT fields. Isocenter doses of the total plan measured with an EPID and an ionization chamber also agreed. The EPID can therefore replace these dosimetry devices for field-by-field and isocenter IMRT pre-treatment verification. Systematic errors were detected using EPID dosimetry, resulting in the adjustment of a TPS parameter and alteration of two clinical patient plans. One set of EPID measurements (i.e., one open and transit image acquired for each segment of the plan) is sufficient to check each IMRT plan field-by-field and at the isocenter, making it a useful, efficient, and accurate dosimetric tool.     

Performance analysis of a film dosimetric quality assurance procedure for IMRT with regard to the employment of quantitative evaluation methods

A system for dosimetric verification of intensity-modulated radiotherapy (IMRT) treatment plans using absolute calibrated radiographic films is presented. At our institution this verification procedure is performed for all IMRT treatment plans prior to patient irradiation. Therefore clinical treatment plans are transferred to a phantom and recalculated. Composite treatment plans are irradiated to a single film. Film density to absolute dose conversion is performed automatically based on a single calibration film. A software application encompassing film calibration, 2D registration of measurement and calculated distributions, image fusion, and a number of visual and quantitative evaluation utilities was developed. The main topic of this paper is a performance analysis for this quality assurance procedure, with regard to the specification of tolerance levels for quantitative evaluations. Spatial and dosimetric precision and accuracy were determined for the entire procedure, comprising all possible sources of error. The overall dosimetric and spatial measurement uncertainties obtained thereby were 1.9% and 0.8 mm respectively. Based on these results, we specified 5% dose difference and 3 mm distance-to-agreement as our tolerance levels for patient-specific quality assurance for IMRT treatments.     

Implementation and validation of portal dosimetry with an amorphous silicon EPID in the energy range from 6 to 25 MV

The purpose of this study was to develop, implement and validate a method for portal dosimetry with an amorphous silicon EPID for a wide energy range. Analytic functions were applied in order to correct for nonlinearities in detector response with dose rate, irradiation time and total dose. EPID scattering processes were corrected for by means of empirically determined convolution kernels. For a variety of rectangular and irregularly shaped fields, head scatter factors determined from central axis portal dose values and those measured with an ionization chamber showed a maximum deviation of 0.5%. The accuracy of our method was further investigated for pretreatment IMRT verification (i.e. without absorbers in the beam). The agreement between EPID and film dosimetry was quantified using gamma (gamma) evaluation, with 2% dose and 2 mm distance-to-agreement criteria. All gamma-distributions showed a gamma(mean) < 0.5, a 99th percentile <1.5 and a fraction of pixels with gamma > 1 smaller than 7%. The number of monitor units delivered by single segments of the IMRT fields could be extracted from the portal images with high accuracy. Measured and delivered doses were within +/-3% for more than 98% of data points. Ghosting effects were found to have limited effects on dosimetric IMRT verification.     

Radiochromic film dosimetry with flatbed scanners: a fast and accurate method for dose calibration and uniformity correction with single film exposure

Film dosimetry is an attractive tool for dose distribution verification in intensity modulated radiotherapy (IMRT). A critical aspect of radiochromic film dosimetry is the scanner used for the readout of the film: the output needs to be calibrated in dose response and corrected for pixel value and spatial dependent nonuniformity caused by light scattering; these procedures can take a long time. A method for a fast and accurate calibration and uniformity correction for radiochromic film dosimetry is presented: a single film exposure is used to do both calibration and correction. Gafchromic EBT films were read with two flatbed charge coupled device scanners (Epson V750 and 1680Pro). The accuracy of the method is investigated with specific dose patterns and an IMRT beam. The comparisons with a two-dimensional array of ionization chambers using a 18 x 18 cm2 open field and an inverse pyramid dose pattern show an increment in the percentage of points which pass the gamma analysis (tolerance parameters of 3% and 3 mm), passing from 55% and 64% for the 1680Pro and V750 scanners, respectively, to 94% for both scanners for the 18 x 18 open field, and from 76% and 75% to 91% for the inverse pyramid pattern. Application to an IMRT beam also shows better gamma index results, passing from 88% and 86% for the two scanners, respectively, to 94% for both. The number of points and dose range considered for correction and calibration appears to be appropriate for use in IMRT verification. The method showed to be fast and to correct properly the nonuniformity and has been adopted for routine clinical IMRT dose verification.     

Monte Carlo dose calculation of segmental IMRT delivery to a moving phantom using dynamic MLC and gating log files

Respiratory gating is emerging as a tool to limit the effect of motion for liver and lung tumors. In order to study the impact of target motion and gated intensity modulated radiation therapy (IMRT) delivery, a computer program was developed to simulate segmental IMRT delivery to a moving phantom. Two distinct plans were delivered to a rigid-motion phantom with a film insert in place under four conditions: static, sinusoidal motion, gated sinusoidal motion with a duty cycle of 25% and gated sinusoidal motion with duty cycle of 50% under motion conditions of a typical patient (A = 1 cm, T = 4 s). The MLC controller log files and gating log files were retained to perform a retrospective Monte Carlo dose calculation of the plans. Comparison of the 2D planar dose distributions between simulation and measurement demonstrated that our technique had at least 94% of the points passing gamma criteria of 3% for dose difference and 3 mm as the distance to agreement. This note demonstrates that the use of dynamic multi-leaf collimator and respiratory monitoring system log files together with a fast Monte Carlo dose calculation algorithm is an accurate and efficient way to study the dosimetric effect of motion for gated or non-gated IMRT delivery on a rigidly-moving body.     

新型国产二维矩阵剂量验证系统的临床测试

目的:对新型国产二维矩阵剂量验证系统在临床条件下进行测试,检验其是否能够满足临床使用需要。方法:参照GB15213-94对用来检测国产二维矩阵剂量验证系统的医用直线加速器进行检测调整,使其达到国家标准。使用新型国产二维矩阵剂量验证系统,对标准照射野下的绝对剂量重复性,标准照射野下的剂量线性,平坦度、对称性,真实病例放疗计划验证进行测试。结果:标准照射野下的绝对剂量重复性检测,其变异系数小于0.7%,符合测试要求;标准照射野下的剂量线性检测与电离室检测结果相比,无明显差异;平坦度检测±3%以内、对称性检测±2%以内,均满足临床使用要求;真实病例计划验证γ通过率均大于98%,完全满足临床放疗计划验证要求。结论:新型国产二维矩阵剂量验证系统具备点剂量、面剂量测量功能,能够对加速器基本剂量性能进行检测,达到临床使用要求;能够实现放疗计划系统的DICOM数据导入,与实际测量结果比较分析,达到临床计划验证要求。     

Two-dimensional measurement of photon beam attenuation by the treatment couch and immobilization devices using an electronic portal imaging device

In our institution, an individualized dosimetric quality assurance protocol for intensity modulated radiotherapy (IMRT) is being implemented. This protocol includes dosimetric measurements with a fluoroscopic electronic portal imaging device (EPID) for all IMRT fields while the patient is being irradiated. For some of the first patients enrolled in this protocol, significant beam attenuation by (carbon fiber) components of the treatment couch was observed. To study this beam attenuation in two-dimensional, EPID images were also acquired in absence of the patient, both with and without treatment couch and immobilization devices, as positioned during treatment. For treatments of head and neck cancer patients with a 6 MV photon beam, attenuation of up to 15% was detected. These findings led to the development of new tools and procedures for planning and treatment delivery to avoid underdosages in the tumor.