Dose evaluation of MRI-based synthetic CT generated using a machine learning method for prostate cancer radiotherapy

Magnetic resonance imaging (MRI)-only radiotherapy treatment planning is attractive since MRI provides superior soft tissue contrast over computed tomographies (CTs), without the ionizing radiation exposure. However, it requires the generation of a synthetic CT (SCT) from MRIs for patient setup and dose calculation. In this study, we aim to investigate the accuracy of dose calculation in prostate cancer radiotherapy using SCTs generated from MRIs using our learning-based method. We retrospectively investigated a total of 17 treatment plans from 10 patients, each having both planning CTs (pCT) and MRIs acquired before treatment. The SCT was registered to the pCT for generating SCT-based treatment plans. The original pCT-based plans served as ground truth. Clinically-relevant dose volume histogram (DVH) metrics were extracted from both ground truth and SCT-based plans for comparison and evaluation. Gamma analysis was performed for the comparison of absorbed dose distributions between SCT- and pCT-based plans of each patient. Gamma analysis of dose distribution on pCT and SCT within 1%/1 mm at 10% dose threshold showed greater than 99% pass rate. The average differences in DVH metrics for planning target volumes (PTVs) were less than 1%, and similar metrics for organs at risk (OAR) were not statistically different. The SCT images created from MR images using our proposed machine learning method are accurate for dose calculation in prostate cancer radiation treatment planning. This study also demonstrates the great potential for MRI to completely replace CT scans in the process of simulation and treatment planning. However, MR images are needed to further analyze geometric distortion effects. Digitally reconstructed radiograph (DRR) can be generated within our method, and their accuracy in patient setup needs further analysis.     

Dosimetric study on learning-based cone-beam CT correction in adaptive radiation therapy

IntroductionCone-beam CT (CBCT) image quality is important for its quantitative analysis in adaptive radiation therapy. However, due to severe artifacts, the CBCTs are primarily used for verifying patient setup only so far. We have developed a learning-based image quality improvement method which could provide CBCTs with image quality comparable to planning CTs (pCTs). The accuracy of dose calculations based on these CBCTs is unknown. In this study, we aim to investigate the dosimetric accuracy of our corrected CBCT (CCBCT) in brain stereotactic radiosurgery (SRS) and pelvic radiotherapy.Materials and MethodsWe retrospectively investigated a total of 32 treatment plans from 22 patients, each of whom with both original treatment pCTs and CBCTs acquired during treatment setup. The CCBCT and original CBCT (OCBCT) were registered to the pCT for generating CCBCT-based and OCBCT-based treatment plans. The original pCT-based plans served as ground truth. Clinically-relevant dose volume histogram (DVH) metrics were extracted from the ground truth, OCBCT-based and CCBCT-based plans for comparison. Gamma analysis was also performed to compare the absorbed dose distributions between the pCT-based and OCBCT/CCBCT-based plans of each patient.ResultsCCBCTs demonstrated better image contrast and more accurate HU ranges when compared side-by-side with OCBCTs. For pelvic radiotherapy plans, the mean dose error in DVH metrics for planning target volume (PTV), bladder and rectum was significantly reduced, from 1% to 0.3%, after CBCT correction. The gamma analysis showed the average pass rate increased from 94.5% before correction to 99.0% after correction. For brain SRS treatment plans, both original and corrected CBCT images were accurate enough for dose calculation, though CCBCT featured higher image quality.ConclusionCCBCTs can provide a level of dose accuracy comparable to traditional pCTs for brain and prostate radiotherapy planning and the correction method proposed here can be useful in CBCT-guided adaptive radiotherapy.     

Initial experience of ArcCHECK and 3DVH software for RapidArc treatment plan verification

The purpose of this study was to perform delivery quality assurance with ArcCHECK and 3DVH system (Sun Nuclear, FL) and to evaluate the suitability of this system for volumetric-modulated arc therapy (VMAT) (RapidArc [RA]) verification. This software calculates the delivered dose distributions in patients by perturbing the calculated dose using errors detected in fluence or planar dose measurements. The device is tested to correlate the gamma passing rate (%GP) and the composite dose predicted by 3DVH software. A total of 28 patients with prostate cancer who were treated with RA were analyzed. RA treatments were delivered to a diode array phantom (ArcCHECK), which was used to create a planned dose perturbation (PDP) file. The 3DVH analysis used the dose differences derived from comparing the measured dose with the treatment planning system (TPS)-calculated doses to perturb the initial TPS-calculated dose. The 3DVH then overlays the resultant dose on the patient׳s structures using the resultant “PDP” beams. Measured dose distributions were compared with the calculated ones using the gamma index (GI) method by applying the global (Van Dyk) normalization and acceptance criteria, i.e., 3%/3 mm. Paired differences tests were used to estimate statistical significance of the differences between the composite dose calculated using 3DVH and %GP. Also, statistical correlation by means of logistic regression analysis has been analyzed. Dose-volume histogram (DVH) analysis for patient plans revealed small differences between treatment plan calculations and 3DVH results for organ at risk (OAR), whereas planning target volume (PTV) of the measured plan was systematically higher than that predicted by the TPS. The t-test results between the planned and the estimated DVH values showed that mean values were incomparable (p < 0.05). The quality assurance (QA) gamma analysis 3%/3 mm showed that in all cases there were only weak-to-moderate correlations (Pearson r: 0.12 to 0.74). Moreover, clinically relevant differences increased with increasing QA passing rate, indicating that some of the largest dose differences occurred in the cases of high QA passing rates, which may be called “false negatives.” The clinical importance of any disagreement between the measured and the calculated dose is often difficult to interpret; however, beam errors (either in delivery or in TPS calculation) can affect the effectiveness of the patient dose. Further research is needed to determinate the role of a PDP-type algorithm to accurately estimate patient dose effect.     

加速器MVCBCT扫描图像在自适应放疗中应用的可行性研究

目的 探讨加速器成像射束影像系统（IBL）的全扇形束和大射野（EFOV）两种模式扫描得到的兆伏级锥形束断层（MV CBCT）图像可否用于剂量计算。方法 利用大孔径CT和在IBL的全扇形束和EFOV模式下对CIRS 062M型电子密度模体进行扫描,在Pinnacle计划系统中分别建立电子密度曲线。用CT和加速器MV级CBCT模式扫描头颈、胸、腹盆腔部仿真模体,利用CT图像制作调强计划,并将计划移植于MV CBCT的图像中,利用相应的电子密度曲线计算剂量,比较靶区及危及器官剂量分布。结果 MV CBCT图像中剂量分布比参考计划剂量偏低,并且在头颈、胸、腹盆腔模体中偏差依次增大。与参考计划相比,头颈部靶区剂量和危及器官剂量分布一致,偏差均在3%以内。胸部和腹盆腔靶区和危及器官的剂量分布均有大幅度的降低,偏差分别达到5%和10%,超出了临床接受范围。结论 在加速器IBL中全扇形束模式条件下,头颈部患者扫描得到的MV CBCT图像可在自适应放疗中用于剂量计算,胸、腹盆腔部位在EFOV模式下仅可用于图像引导,不能用于剂量计算。     

Validation of a commercial software dose calculation for Y-90 microspheres

PURPOSESeveral new commercial software packages have become available that can calculate the tumor and normal tissue dose distributions from post-treatment PET-CT scans for Y-90 microsphere treatments of liver lesions. This work seeks to validate the MIM SurePlan Liver Y90 software by comparing its results to a previously developed Monte Carlo derived voxel dose kernel calculation method.METHODSWe analyzed 10 patients who had treatments for metastatic liver cancer and created contours on post Y-90 treatment PET-CT images. We then performed dose calculations using three methods and compared the results. The first two methods calculated the dose using MIM SurePlan Liver Y90’s LDM (Local Deposition Method) and the VSV (Voxel S Value) algorithms. The third method calculated the dose using a publicly available Fluka Monte Carlo-derived dose kernel (MCK) calculation (used as ground truth). We investigated 3D Gamma passing rates and several dosimetric parameters.RESULTSA total of 3%/3 mm 3D gamma passing rates averaged 99.3% for the VSV and 78.9% for LDM. Compared to the MCK distribution, the differences for combined target GTV V_70Gy and normal liver and/or lobe mean doses were small. Larger differences were seen in GTV mean doses and D₉₅, likely due to large dose gradients in the treated regions combined with differences in dose kernel, dose grid and finite volume effects.CONCLUSIONSThe MIM SurePlan Liver Y90 VSV algorithm agreed well with the MCK calculation for patients treated with Y-90 microspheres based on the gamma analysis and several dosimetric parameters. Larger dosimetric differences in lesion mean doses and D₉₅ suggests that these metrics are less robust to changes in calculation grid location and finite volume effects for small lesions.     

磁共振弥散加权成像在食管癌放疗中的应用价值

 目的 探讨CT图像和CT/MR弥散加权成像(diffusion weighted imaging,DWI)图像融合在食管癌靶区勾画中的应用价值。方法 收集2017-08至2018-12医院收治的26例食管鳞癌患者,进行常规CT扫描及 MR DWI,扩散敏感梯度b值取为600 s/mm²,分别在CT图像和CT/MR DWI融合图像上勾画食管原发肿瘤大体体积(gross tumor volume,GTV)和PTV,计算GTV和PTV的长度,并利用TiGRT治疗计划系统获得GTV与PTV体积,以同样的处方剂量及危及器官限制剂量分别在两种图像上制定治疗计划,比较不同计划的GTV和PTV长度、体积、处方剂量和危及器官的剂量差异。结果 两种条件下计划的剂量分布和各项参数均达到了临床处方剂量要求。而在CT/MR DWI计划下的PTV、GTV的长度和体积均小于CT计划(P=0.00、0.00、0.01、0.03)。基于DWI计划的危及器官肺部平均剂量明显低于CT计划(P=0.00)。结论 DWI在食管癌放疗中的应用更精确,能够保护正常组织。     

食管癌调强计划三维剂量验证中γ通过率最佳阈值的研究

目的 基于三维剂量验证系统EDose^TM,寻找食管癌调强放疗计划验证中利用γ分析方法判别计划是否通过的最佳阈值。方法 回顾性地选取25例经临床批准的食管癌7野调强计划,使用Edose^TM在计划CT中重建三维剂量,并进行γ分析和剂量体积直方图（DVH）评价。然后,按照以下两个准则对计划分类：DVH特定剂量学指标绝对百分剂量差异<5%视为临床可接受计划。3%/3 mm基准下,γ通过率>90%,认为计划通过。根据分类结果绘制出受试者工作特征（receiver operating characteristic,ROC）曲线,通过计算约登系数的方式寻找全局γ在5%/3 mm、3%/3 mm和2%/2 mm基准下的最佳通过率阈值,并对最佳阈值在计划判别中的敏感性和特异性进行分析。结果 全局5%/3 mm、3%/3 mm和2%/2 mm基准下的最佳阈值分别为98.66%、94.84%和78.56%。在常规阈值90%处,全局3%/3 mm基准下的敏感性和特异性分别为0.17和0.84,而最佳阈值同样条件下的敏感性和特异性分别为0.85和0.27;全局5%/3 mm和2%/2 mm基准下最佳阈值的敏感性分别为0.89和0.65,特异性分别为0.23和0.47。结论 在3%/3 mm基准下最佳阈值较常规阈值（90%）敏感性得到显著提升,提高了阻止不可接受计划通过的能力;在2%/2 mm基准下,最佳阈值的敏感性和特异性更为平衡（相对3%/3 mm基准）,一定程度上减少了不可接受计划通过的概率,同时也降低了可接受计划被误判的机会。     

Dosimetric comparison and validation of Eclipse Anisotropic Analytical Algorithm (AAA) and AcurosXB (AXB) algorithms in RapidArc-based radiosurgery plans of patients with solitary brain metastasis

Stereotactic radiosurgery (SRS) is increasingly being used to manage solitary or multiple brain metastasis. This study aims to compare and validate Anisotropic Analytical Algorithm (AAA) and AcurosXB (AXB) algorithms of Eclipse Treatment Planning System (TPS) in RapidArc-based SRS plans of patients with solitary brain metastasis. Twenty patients with solitary brain metastasis who have been already treated with RapidArc SRS plans calculated using AAA plans were selected for this study. These plans were recalculated using AXB algorithm keeping the same arc orientations, multi-leaf collimator apertures, and monitor units. The two algorithms were compared for target coverage parameters, isodose volumes, plan quality metrics, dose to organs at risk and integral dose. The dose calculated by the TPS using AAA and AXB algorithms was validated against measured dose for all patient plans using an in-house developed cylindrical phantom. An Exradin A14SL ionization chamber was positioned at the center of this phantom to measure the in-field dose. NanoDot Optically Stimulated Luminescent Dosimeters (OSLDs) (Landauer Inc.) were placed at distances 3.0 cm, 4.0 cm, 5.0 cm, and 6.0 cm respectively from the center of the phantom to measure the non-target dose. In addition, the planar dose distribution was measured using amorphous silicon aS1000 Electronic Portal Imaging Device. The measured 2D dose distribution was compared against AAA and AXB estimated 2D distribution using gamma analysis. All results were tested for significance using the paired t-test at 5% level of significance. Significant differences between the AAA and AXB plans were found only for a few parameters analyzed in this study. In the experimental verification using cylindrical phantom, the difference between the AAA calculated dose and the measured dose was found to be highly significant (p < 0.001). However, the difference between the AXB calculated dose and the measured dose was not significant (p = 0.197). The difference between AAA/AXB calculated and measured at non-target locations was statistically insignificant at all four non-target locations and the dose calculated by both AAA and AXB algorithms shows a strong positive correlation with the measured dose. The results of the gamma analysis show that the AXB calculated planar dose is in better agreement with measurements compared to the AAA. Even though the results of the dosimetric comparison show that the differences are mostly not significant, the measurements show that there are differences between the two algorithms within the target volume. The AXB algorithm may be therefore more accurate in the dose calculation of VMAT plans for the treatment of small intracranial targets. For non-target locations either algorithm can be used for the estimation of dose accounting for their limitations in non-target dose estimations.     

Advances in the implementation of helical tomotherapy–based total marrow irradiation with a novel field junction technique

Given the limitations in the travel ability of the helical tomotherapy (HT) couch, total marrow irradiation (TMI) has to be split in 2 segments, with the lower limbs treated with feet first orientation. The aim of this work is to present a planning technique useful to reduce the dose inhomogeneity resulting from the matching of the 2 helical dose distributions. Three HT plans were generated for each of the 18 patients enrolled. Upper TMI (UTMI) and lower TMI (LTMI) were planned onto the whole-body computed tomography (CT) and on the lower-limb CT, respectively. A twin lower TMI plan (tLTMI) was designed on the whole-body CT. Agreement between LTMI and tLTMI plans was assessed by computing for each dose-volume histogram (DVH) structure the γ index scored with 1% of dose and volume difference thresholds. UTMI and tLTMI plans were summed together on the whole-body CT, enabling the evaluation of dose inhomogeneity. Moreover, a couple of transition volumes were used to improve the dose uniformity in the abutment region. For every DVH, a number of points >99% passed the γ analysis, validating the method used to generate the twin plan. The planned dose inhomogeneity at the junction level resulted within ±10% of the prescribed dose. Median dose reduction to organs at risk ranged from 30–80% of the prescribed dose. Mean conformity index was 1.41 (range 1.36–1.44) for the whole-body target. The technique provided a “full helical” dose distribution for TMI treatments, which can be considered effective only if the dose agreement between LTMI and tLTMI plans is met. The planning of TMI with HT for the whole body with adequate dose homogeneity and conformity was shown to be feasible.     

CT图像重建视野大小对放射治疗计划剂量计算及体积评估的影响

目的 探讨CT图像重建视野(FOV)大小对放射治疗计划剂量计算及体积评估可能存在的影响.方法 对16例鼻咽癌患者的CT原始扫描数据分别行45 cm常规FOV和65 cm扩展视野(EFOV)重建并传输至放射治疗计划系统,所有病例均在常规FOV重建的CT图像上勾画肿瘤体积(GTV)、临床靶区(CTV)及脑干、晶体、腮腺、脊髓等危及器官,并制定7野等角动态调强放射治疗计划(GTV处方剂量70 Gy).两种重建方法图像按照医学数字影像通信3.0标准(DICOM 3.0)坐标方式融合后,拷贝常规FOV图像上的靶区及危及器官至EFOV图像,并将治疗计划移植至EFOV图像,治疗计划中心为两种重建方法图像的同一DICOM坐标,利用剂量体积直方图(DVH)工具计算两种重建方法图像上GTV、CTV和脑干、晶体、腮腺、脊髓的体积、最大剂量(Dmax)、平均剂量(Dmean)及最小剂量(Dmin).将入组病例的每个治疗计划7野分别导入常规45 cm FOV和65 cmEFOV重建的二维通量图验证设备Mapchek 1175的模体,距离通过协议(DTA)分析5 cm深度平面绝对剂量的计算和实测结果通过率.结果 两种重建方法图像上的靶区和危及器官的体积差异具有统计学意义,所有入组病例靶区和危及器官在常规FOV图像上的体积均大于EFOV图像上的体积.较小体积的晶体最大剂量Dmax常规FOV与EFOV图像之间差异有统计学意义(t=-3.14,P<0.007),其余靶区及危及器官的最大剂量Dmax差异无统计学意义.CTV和GTV平均剂量Dmean在EFOV图像上大于FOV图像,差异有统计学意义(t=-6.45、-5.65,P<0.001),危及器官的平均剂量Dmean和靶区及危及器官最小剂量Dmin差异均无统计学意义.两种重建方法图像上治疗计划的7野通过率之间差异无统计学意义.结论 在放射治疗CT模拟定位过程中图像重建FOV的大小对于靶区及部分危及器官的体积及剂量计算结果和治疗计划的评价存在影响;观察和验证二维通 量图通过率,两者之间的差异并不显著.     

CT图像重建视野大小对放射治疗计划剂量计算及体积评估的影响

目的 探讨CT图像重建视野(FOV)大小对放射治疗计划剂量计算及体积评估可能存在的影响.方法 对16例鼻咽癌患者的CT原始扫描数据分别行45 cm常规FOV和65 cm扩展视野(EFOV)重建并传输至放射治疗计划系统,所有病例均在常规FOV重建的CT图像上勾画肿瘤体积(GTV)、临床靶区(CTV)及脑干、晶体、腮腺、脊髓等危及器官,并制定7野等角动态调强放射治疗计划(GTV处方剂量70 Gy).两种重建方法图像按照医学数字影像通信3.0标准(DICOM 3.0)坐标方式融合后,拷贝常规FOV图像上的靶区及危及器官至EFOV图像,并将治疗计划移植至EFOV图像,治疗计划中心为两种重建方法图像的同一DICOM坐标,利用剂量体积直方图(DVH)工具计算两种重建方法图像上GTV、CTV和脑干、晶体、腮腺、脊髓的体积、最大剂量(Dmax)、平均剂量(Dmean)及最小剂量(Dmin).将入组病例的每个治疗计划7野分别导入常规45 cm FOV和65 cmEFOV重建的二维通量图验证设备Mapchek 1175的模体,距离通过协议(DTA)分析5 cm深度平面绝对剂量的计算和实测结果通过率.结果 两种重建方法图像上的靶区和危及器官的体积差异具有统计学意义,所有入组病例靶区和危及器官在常规FOV图像上的体积均大于EFOV图像上的体积.较小体积的晶体最大剂量Dmax常规FOV与EFOV图像之间差异有统计学意义(t=-3.14,P<0.007),其余靶区及危及器官的最大剂量Dmax差异无统计学意义.CTV和GTV平均剂量Dmean在EFOV图像上大于FOV图像,差异有统计学意义(t=-6.45、-5.65,P<0.001),危及器官的平均剂量Dmean和靶区及危及器官最小剂量Dmin差异均无统计学意义.两种重建方法图像上治疗计划的7野通过率之间差异无统计学意义.结论 在放射治疗CT模拟定位过程中图像重建FOV的大小对于靶区及部分危及器官的体积及剂量计算结果和治疗计划的评价存在影响;观察和验证二维通 量图通过率,两者之间的差异并不显著.     

Derivation of in vivo source tracking error thresholds for TRUS-based HDR prostate brachytherapy through simulation of source positioning errors

PurposeThe purpose of this study was to simulate treatment planning source positioning errors in transrectal ultrasound–based real-time high-dose-rate prostate brachytherapy treatments and determine appropriate in vivo source tracking error thresholds.Methods and MaterialsTreatment planning source positioning errors were simulated for 20 patient plans in the brachytherapy treatment planning system by manually adjusting the dwell position coordinates within selected catheters without plan reoptimization. The change in dose-volume histogram (DVH) indices was calculated as a function of the source positioning error. The magnitude of the change in the DVH indices was then used to derive appropriate in vivo source tracking error thresholds.ResultsSource positioning error thresholds to prevent potentially significant changes in prostate (target) DVH metrics ranged from 2 to 5 mm, dependent on the direction of the source positioning error, as well as the relative weight of the dwell position within the plan, and its position relative to the patient anatomy. Source positioning error thresholds to prevent potentially clinically significant changes in organ at risk DVH metrics were found to be complex and patient-dependent.ConclusionsIn vivo source tracking error thresholds for transrectal ultrasound–based real-time high-dose-rate prostate brachytherapy were investigated via the simulation of treatment planning source positioning errors. These error thresholds were found to be dependent not only on the direction of the error, but also on the endpoint. There is still the potential for larger changes in DVH indices to occur for catheter shifts smaller than the proposed threshold levels in this study.     

The Dosimetric Effect of Zipper Artifacts on TomoTherapy Adaptive Dose Calculation—A Phantom Study

Tomotherapy adaptive dose calculation offers the ability to verify and adjust the therapeutic plan during the treatment. Using tomotherapy adaptive dose calculation, the planned fluence pattern can be used to recalculate the dose distribution on pretreatment megavoltage computed tomography (MVCT) images. Zipper artifacts, which appear as increased density in the central region of MVCT images, may affect the accuracy of adaptive dose recalculation. The purpose of this study was to evaluate the dosimetric effects of zipper artifacts on tomotherapy adaptive dose calculation. MVCT images of a cylindrical water phantom of 22-cm diameter were acquired on a tomotherapy system. The zipper artifacts were enclosed by a cylindrical planning target volume (PTV) contoured on these images. For comparison, artifact-free images were created by replacing the computed tomography (CT) numbers of zipper artifacts with the mean CT number of water. Treatment plans were generated by giving a uniform dose of 2 Gy to the PTV based on these modified images; it was then applied to the images that have the zipper artifacts. The impacts of different pitch ratios on the artifacts were assessed. The dose distribution differences between the 2 sets of images were compared. The absorbed dose that covered 95% volume of PTV and maximum dose, minimum dose, and mean dose of the PTV were also calculated and compared. The water phantom was scanned on the tomotherapy system twice per week for 12 consecutive weeks. The mean CT number of zipper artifacts (101 HU) was three times higher than that of water (34 HU). The CT number value and location of zipper artifacts were not affected by the pitch ratio. Gamma analysis was performed between the original and recalculated dose distributions. The discrepancies between the isodose distributions calculated by two sets of images were within 1%/1-mm tolerance. The dosimetric impact from zipper artifacts was found insignificant such that the recalculated dose was underestimated by less than 0.5%.     

不同CT值赋值法对脑转移瘤放疗剂量计算影响的研究

目的研究不同CT值赋值法对脑转移瘤放疗计划剂量计算的影响,为基于磁共振(MR)图像进行放疗计划设计提供基础。方法选取35例接受放疗的脑转移瘤患者,每位患者在放疗前同一天分别进行CT和MR模拟定位,基于CT图像制定三维适形放射治疗(3D-CRT)或调强放射治疗(IMRT)计划为原计划Plan1。将CT图像和MR图像刚性配准,在CT和MR图像上勾画主要的组织和器官,计算各组织器官的群体化CT值。基于CT图像,采用3种CT值赋值法生成3组伪CT,分别为:全组织赋予140 HU;空腔、骨骼和软组织分别赋予-700、700和20 HU;不同组织器官分别赋予群体化的CT值。Plan1在3组伪CT上重新计算剂量分布,分别获得Plan2、Plan3、Plan4,然后比较这3组计划和Plan1的剂量学差异。结果骨骼、空腔平均CT值分别为(735.3±68.0)、(-723.9±27.0)HU,软组织的平均CT值基本分布在-70~70 HU。Plan2、Plan3、Plan4相比Plan1的剂量差异依次减小,在剂量指标比较中,眼晶状体最大剂量差异最大,分别可达5.0%以上、1.5%~2.0%、1.0%~1.5%,其余剂量指标差异的95%置信区间上限基本不超过2.0%、1.2%、0.8%。在像素点剂量比较中,局部靶区病例中差异>1%的区域主要分布在靠近射野的皮肤处,而全脑靶区病例中主要分布在骨骼与空腔、软组织交界处,以及靠近射野的皮肤处。此外,CT值赋值法在3D-CRT的剂量学差异大于IMRT,在全脑靶区病例大于局部靶区病例。结论不同CT值赋值法对脑转移瘤放疗计划剂量计算的影响显著,对骨骼、空腔和软组织赋予合适CT值,剂量计算偏差可基本控制于1.2%以内,而对各组织器官赋予群体化的CT值,可进一步将偏差控制于0.8%以内,满足临床要求。     

Feasibility of using post-contrast dual-energy CT for pediatric radiation treatment planning and dose calculation

Objectives:When iodinated contrast is administered during CT simulation, standard practice requires a separate non-contrast CT for dose calculation. The objective of this study is to validate our hypothesis that since iodine affects Hounsfield units (HUs) more than electron density (ED), the information from post-contrast dual-layer CT (DLCT) would be sufficient for accurate dose calculation for both photon and proton therapy.Methods and materials:10 pediatric patients with abdominal tumors underwent DLCT scans before and after iodinated contrast administration for radiotherapy planning. Dose distributions with these DLCT-based methods were compared to those with conventional calibration-curve methods that map HU images to ED and stopping-power ratio (SPR) images.Results:For photon plans, conventional and DLCT approaches based on post-contrast scans underestimated the PTV D99 by 0.87 ± 0.70% (p = 0.18) and 0.36 ± 0.31% (p = 0.34), respectively, comparing to their non-contrast optimization plans. Renal iodine concentration was weakly associated with D99 deviation for both conventional (R² = 0.10) and DLCT (R² = 0.02) approaches. For proton plans, the clinical target volume D99 errors were 3.67 ± 2.43% (p = 0.0001) and 0.30 ± 0.25% (p = 0.40) for conventional and DLCT approaches, respectively. The proton beam range changed noticeably with the conventional approach. Renal iodine concentration was highly associated with D99 deviation for the conventional approach (R² = 0.83) but not for DLCT (R² = 0.007).Conclusion:Conventional CT with iodine contrast resulted in a large dosimetric error for proton therapy, compared to true non-contrast plans, but the error was less for photon therapy. These errors can be greatly reduced in the case of the proton plans if DLCT is used, raising the possibility of using only a single post-contrast CT for radiotherapy dose calculation, thus reducing the time and imaging dose required.Advances in knowledge:This study is the first to compare directly the differences in the calculated dose distributions between pre- and post-contrast CT images generated by single-energy CT and dual-energy CT methods for photon and proton therapy.     

Image reconstruction and the effect on dose calculation for hip prostheses.

High atomic number inserts, such as hip prostheses and dental fillings, cause streak artifacts on computed tomography (CT) images when filtered back-projection (FBP) methods are used. These streak artifacts severely degrade our ability to differentiate the tumor volume. Also, incorrect Hounsfield numbers yield incorrect electron density information that may lead to erroneous dose calculations, and, as a result, compromise clinical outcomes. The aim of this research was to evaluate the dosimetric consequences of artifacts during radiotherapy planning of a prostate patient containing a hip prosthesis. The CT numbers corresponding to an iron prosthesis were inserted into the right femoral head of an existing CT image set. This artifact-free image was used as the standard image set. CT projections through the image set formed the sinogram, from which filtered back projection and iterative deblurring methods were used to create reconstructed image sets. These reconstructed image sets contained artifacts. Prostate treatment plans were then calculated using a Monte Carlo system for the standard and reconstructed CT image sets. Close to the prosthesis, the CT numbers between the reconstructed and standard image sets differed substantially. However, because the CT number differences covered only a small area, the dose distributions on the reconstructed and standard image sets were not significantly different. The dose-volume histograms for the prostate, rectum, and bladder were virtually identical. Our results indicate that even though CT image artifacts restrict our ability to differentiate tumors and critical structures, the dose distributions for a prostate plan containing a hip prosthesis, calculated on both artifact-free image sets and image sets containing artifacts, are not significantly different.     

CT图像重建视野大小对放射治疗计划剂量计算及体积评估的影响

目的 探讨CT图像重建视野(FOV)大小对放射治疗计划剂量计算及体积评估可能存在的影响。方法 对16例鼻咽癌患者的CT原始扫描数据分别行45 cm常规FOV和65 cm扩展视野(EFOV)重建并传输至放射治疗计划系统,所有病例均在常规FOV重建的CT图像上勾画肿瘤体积(GTV)、临床靶区(CTV)及脑干、晶体、腮腺、脊髓等危及器官,并制定7野等角动态调强放射治疗计划(GTV处方剂量70 Gy)。两种重建方法图像按照医学数字影像通信3.0标准(DICOM 3.0)坐标方式融合后,拷贝常规FOV图像上的靶区及危及器官至EFOV图像,并将治疗计划移植至EFOV图像,治疗计划中心为两种重建方法图像的同一DICOM坐标,利用剂量体积直方图(DVH)工具计算两种重建方法图像上GTV、CTV和脑干、晶体、腮腺、脊髓的体积、最大剂量(D_max)、平均剂量(D_mean)及最小剂量(D_min)。将入组病例的每个治疗计划7野分别导入常规45 cm FOV和65 cm EFOV重建的二维通量图验证设备Mapchek 1175的模体,距离通过协议(DTA)分析5 cm深度平面绝对剂量的计算和实测结果通过率。结果 两种重建方法图像上的靶区和危及器官的体积差异具有统计学意义,所有入组病例靶区和危及器官在常规FOV图像上的体积均大于EFOV图像上的体积。较小体积的晶体最大剂量D_max常规FOV与EFOV图像之间差异有统计学意义(t =-3.14, P<0.007),其余靶区及危及器官的最大剂量D_max差异无统计学意义。CTV和GTV平均剂量D_mean在EFOV图像上大于FOV图像,差异有统计学意义(t=-6.45、-5.65, P< 0.001),危及器官的平均剂量D_mean和靶区及危及器官最小剂量D_min差异均无统计学意义。两种重建方法图像上治疗计划的7野通过率之间差异无统计学意义。结论 在放射治疗CT模拟定位过程中图像重建FOV的大小对于靶区及部分危及器官的体积及剂量计算结果和治疗计划的评价存在影响;观察和验证二维通量图通过率,两者之间的差异并不显著。     

Integrating external beam and prostate seed implant dosimetry for intermediate and high-risk prostate cancer using biologically effective dose: Impact of image registration technique

PURPOSECombining external beam radiation therapy (EBRT) and prostate seed implant (PSI) is efficacious in treating intermediate- and high-risk prostate cancer at the cost of increased genitourinary toxicity. Accurate combined dosimetry remains elusive due to lack of registration between treatment plans and different biological effect. The current work proposes a method to convert physical dose to biological effective dose (BED) and spatially register the dose distributions for more accurate combined dosimetry.METHODS AND MATERIALSA PSI phantom was CT scanned with and without seeds under rigid and deformed transformations. The resulting CTs were registered using image-based rigid registration (RI), fiducial-based rigid registration (RF), or b-spline deformable image registration (DIR) to determine which was most accurate. Physical EBRT and PSI dose distributions from a sample of 91 previously-treated combined-modality prostate cancer patients were converted to BED and registered using RI, RF, and DIR. Forty-eight (48) previously-treated patients whose PSI occurred before EBRT were included as a “control” group due to inherent registration. Dose-volume histogram (DVH) parameters were compared for RI, RF, DIR, DICOM, and scalar addition of DVH parameters using ANOVA or independent Student's t tests (α = 0.05).RESULTSIn the phantom study, DIR was the most accurate registration algorithm, especially in the case of deformation. In the patient study, dosimetry from RI was significantly different than the other registration algorithms, including the control group. Dosimetry from RF and DIR were not significantly different from the control group or each other.CONCLUSIONSCombined dosimetry with BED and image registration is feasible. Future work will utilize this method to correlate dosimetry with clinical outcomes.