Validation of a dose warping algorithm using clinically realistic scenarios

Objective:

Dose warping following deformable image registration (DIR) has been proposed for interfractional dose accumulation. Robust evaluation workflows are vital to clinically implement such procedures. This study demonstrates such a workflow and quantifies the accuracy of a commercial DIR algorithm for this purpose under clinically realistic scenarios.

Methods:

12 head and neck (H&N) patient data sets were used for this retrospective study. For each case, four clinically relevant anatomical changes have been manually generated. Dose distributions were then calculated on each artificially deformed image and warped back to the original anatomy following DIR by a commercial algorithm. Spatial registration was evaluated by quantitative comparison of the original and warped structure sets, using conformity index and mean distance to conformity (MDC) metrics. Dosimetric evaluation was performed by quantitative comparison of the dose–volume histograms generated for the calculated and warped dose distributions, which should be identical for the ideal “perfect” registration of mass-conserving deformations.

Results:

Spatial registration of the artificially deformed image back to the planning CT was accurate (MDC range of 1–2 voxels or 1.2–2.4 mm). Dosimetric discrepancies introduced by the DIR were low (0.02 ± 0.03 Gy per fraction in clinically relevant dose metrics) with no statistically significant difference found (Wilcoxon test, 0.6 ≥ p ≥ 0.2).

Conclusion:

The reliability of CT-to-CT DIR-based dose warping and image registration was demonstrated for a commercial algorithm with H&N patient data.

Advances in knowledge:

This study demonstrates a workflow for validation of dose warping following DIR that could assist physicists and physicians in quantifying the uncertainties associated with dose accumulation in clinical scenarios.Modern radiotherapy aims to move towards a personalized treatment for each patient with cancer, requiring reliable predictions of an individual''s response to a particular therapy and accurate monitoring of treatment delivery, enabling adaptations to the treatment plan as required. To date, typical radiotherapy practice involves the preparation of a treatment plan based on an initial high resolution CT scan of the anatomy to be treated. However, since the treatment is optimized for the anatomy on planning CT (pCT), any changes in a patient''s anatomy during the treatment course itself (which may last for up to 8 weeks) could result in a suboptimal treatment. Currently, to account for interfraction movements, a low-resolution, low-dose CT image [typically cone beam CT (CBCT) or mega-voltage CT (MVCT), although other options exist] of the patient is often acquired prior to each treatment (daily images). This is termed image-guided radiotherapy (IGRT).¹In 1997, Yan et al² proposed the concept of adaptive radiotherapy (ART), suggesting the adaptation of the treatment plan to account for interfraction anatomical variations, based on these daily images. Such treatment adaptations are sometimes currently employed in routine clinical practice when significant anatomical changes are observed, such as substantial weight loss.³ State-of-the-art ART, on the other hand, aims to regularly monitor the treatment delivery and adapt when necessary (offline ART)² or even predict the result and alter it before the treatment of that day (online ART).⁴ The ability to determine the accumulated delivered dose to deforming anatomy is of vital importance not only for ART but also for the assessment and optimization of radiobiological models,⁵ since without it, these models are informed by less accurate estimates of delivered dose to each tissue or partial tissue volume. However, certain limitations such as inaccuracies in contour propagation and in reliable dose accumulation currently prevent efficient routine monitoring of delivered dose throughout the treatment.Deformable image registration (DIR) algorithms have been proposed as a method for facilitating these processes. The accuracy of DIR algorithms is therefore of critical importance and has been the subject of investigation by several researchers, with mechanical phantoms,^6–13 patient images^14–22 and digital phantoms (i.e. patient images artificially deformed with known deformations)^10,11,23 being extensively used for DIR assessment.An extension to these issues is the application of the underlying anatomical deformations to a calculated dose distribution, which is a necessary step in interfractional dose accumulation. Such dose warping process involves the direct deformation of a calculated dose distribution by applying the deformation matrix estimated during DIR between two anatomical scans, essentially warping the dose according to the reference anatomy. Dose warping and deformable dose accumulation have been employed in a number of clinical investigations, including a dose feedback technique in ART frameworks,²⁴ the assessment of planning target volume (PTV) margins²⁵ and the examination of parotid gland dose–effect relationships,²⁶ based on dose distributions recalculated on daily or weekly scans and the accumulation on a single frame of reference. Consequently, quality assurance and evaluation techniques have been investigated in order to validate the applicability of this dose warping concept. Previous work has investigated mathematical models to directly convert DIR errors into dose-warping uncertainties, through the use of patient images and mechanical or digital phantoms,^15,27–30 while a number of deformable dosimetric and non-dosimetric gel phantoms have been produced enabling the experimental evaluation of both DIR and dose warping.^31–35 Even though some of these studies revealed promising results, they have not convinced the radiotherapy community that these uncertainties are adequately understood.³⁶In one such study, Yeo et al³⁴ used a cylindrical deformable dosimetric gel phantom for the experimental validation of dose warping against actual three-dimensional (3D) measurements. The warped and measured dose distributions revealed an agreement of 3D γ_3%/3mm = 99.9%, after small deformations (approximately 9 mm), and γ_3%/3 mm = 96.7% after larger deformations (approximately 20 mm). The authors therefore concluded that “dose-warping may be justified for small deformations in particular and those that do not involve significant density changes”. On the other hand, Juang et al³⁵ exposed “substantial errors in a commercial DIR” used for dose-warping evaluation, utilizing another 3D deformable dosimetric gel, revealing a 3D γ_3%/3mm passing rate of 60%.Such studies, and especially the use of deformable dosemeters for the evaluation of dose warping, are very important as they can reveal the 3D dosimetric impact owing to the uncertainties of a given DIR algorithm. However, they possess three important limitations: first, typical physical dosimetric phantoms present limited image complexity and would not assess the performance limits of the DIR algorithm under evaluation in clinical scenarios. Second, plan delivery, intrinsic dosimetric and dose reading uncertainties are present when using any type of dosemeter in physical phantom measurements. The third limitation is the fact that even where such approaches can offer high precision dosimetric uncertainty evaluation, they cannot directly inform users about the potential extent of those uncertainties in practical clinical cases. All these issues will be addressed in this work.In the present study, a workflow for the robust validation of DIR and dose warping is presented, using patient images artificially deformed with clinically realistic deformations and clinically optimized Monte Carlo dose calculations of intensity-modulated radiotherapy (IMRT) plans, quantifying both the spatial errors in the deformable registration and their dosimetric impact when applied to dose accumulation. In contrast to previously proposed evaluation procedures, this method examines and reports dose-warping uncertainties under clinically relevant scenarios. Although the validation workflow is applicable for different DIRs and clinical indications, it is here employed specifically for the evaluation of a commercial software (OnQ rts^®; Oncology Systems Limited, Shropshire, UK) in head and neck (H&N) cancer patient cases.     

Evaluating the accumulated dose distribution of organs at risk in combined radiotherapy for cervical carcinoma based on deformable image registration

ObjectiveTo evaluate the feasibility and value of deformable image registration (DIR) in calculating the cumulative doses of organs at risk (OARs) in the combined radiotherapy of cervical cancer.Patients and methodsThirty cervical cancer patients treated with external beam radiotherapy (EBRT) combined with intracavitary brachytherapy (ICBT) were reviewed. The simulation CT images of EBRT and ICBT were imported into Varian Velocity 4.1 for the DIR-based dose accumulation. Cumulative dose-volume parameters of D2cc for rectum and bladder were compared between the direct addition (DA) and DIR methods. The quantitative parameters were measured to evaluate the accuracy of DIR.ResultsThe three-dimensional cumulative dose distribution of the tumor and OARs were graphically well illustrated by composite isodose lines. In combined EBRT and ICBT, the mean cumulative bladder D2cc calculated by DIR and DA was 86.13 Gy and 86.27 Gy, respectively. The mean cumulative rectal D2cc calculated by DIR and DA was 72.97 Gy and 73.90 Gy, respectively. No significant differences were noted between these two methods (p > 0.05). As to the parameters used to evaluate the DIR accuracy, the mean DSC, Jacobian, MDA (mm) and Hausdorff distance (mm) were 0.79, 1.0, 3.84, and 22.01 respectively for the bladder and 0.53, 1.2, 7.31, and 29.58 respectively for the rectum. In this study, the DSC seemed to be slightly lower compared with previous studies.ConclusionDose accumulation based on DIR might be an alternative method to illustrate and evaluate the cumulative doses of the OARs in combined radiotherapy for cervical cancer. However, DIR should be used with caution before overcoming the relevant limitations.     

Validation of a Monte Carlo tool for patient-specific dose simulations in multi-slice computed tomography

Estimating the dose delivered to the patient in X-ray computed tomography (CT) examinations is not a trivial task. Monte Carlo (MC) methods appear to be the method of choice to assess the 3D dose distribution. The purpose of this work was to extend an existing MC-based tool to account for arbitrary scanners and scan protocols such as multi-slice CT (MSCT) scanners and to validate the tool in homogeneous and heterogeneous phantoms. The tool was validated by measurements on MSCT scanners for different scan protocols under known conditions. Quantitative CT Dose Index (CTDI) measurements were performed in cylindrical CTDI phantoms and in anthropomorphic thorax phantoms of various sizes; dose profiles were measured with thermoluminescent dosimeters (TLD) in the CTDI phantoms and compared with the computed dose profiles. The in-plane dose distributions were simulated and compared with TLD measurements in an Alderson-Rando phantom. The calculated dose values were generally within 10% of measurements for all phantoms and all investigated conditions. Three-dimensional dose distributions can be accurately calculated with the MC tool for arbitrary scanners and protocols including tube current modulation schemes. The use of the tool has meanwhile also been extended to further scanners and to flat-detector CT.     

Effects of heterogeneities in dose distributions under nonreference conditions: Monte Carlo simulation vs dose calculation algorithms

The purpose of this study is to evaluate the performance of dose calculation algorithms used in radiotherapy treatment planning systems (TPSs) in comparison with Monte Carlo (MC) simulations in nonelectronic equilibrium conditions. MC simulations with PENELOPE package were performed for comparison of doses calculated by pencil beam convolution (PBC), analytical anisotropy algorithm (AAA), and Acuros XB TPS algorithms. Relative depth dose curves were calculated in heterogeneous water phantoms with layers of bone (1.8?g/cm³) and lung (0.3?g/cm³) equivalent materials for radiation fields between 1?×?1?cm² and 10?×?10?cm². Analysis of relative depth dose curves at the water-bone interface shows that PBC and AAA algorithms present the largest differences to MC calculations (u_MC?=?0.5%), with maximum differences of up to 4.3% of maximum dose. For the lung-equivalent material and 1?×?1?cm² field, differences can be up to 24.3% for PBC, 11.5% for AAA, and 7.5% for Acuros. Results show that Acurus presents the best agreement with MC simulation data with equivalent accuracy for modeling radiotherapy dose deposition especially in regions where electronic equilibrium does not hold. For typical (nonsmall) fields used in radiotherapy, AAA and PBC can exhibit reasonable agreement with MC results even in regions of heterogeneities.     

Evaluation of the effectiveness of novel single-intervention adaptive radiotherapy strategies based on daily dose accumulation

Parotid gland (PG) shrinkage and neck volume reduction during radiotherapy of head and neck (H&N) cancer patients is a clinical issue that has prompted interest in adaptive radiotherapy (ART). This study focuses on the difference between planned dose and delivered dose and the possible effects of an efficient replanning strategy during the course of treatment. Six patients with H&N cancer treated by tomotherapy were retrospectively enrolled. Thirty daily dose distributions (D_MVCT) were calculated on pretreatment megavoltage computed tomography (MVCT) scans. Deformable Image Registration which matched daily MVCT with treatment planning kilovoltage computed tomography was performed. Using the resulting deformation vector field, all daily D_MVCT were deformed to the planning kilovoltage computed tomography and resulting doses were accumulated voxel per voxel. Cumulative D_MVCT was compared to planned dose distribution performing γ-analysis (2 mm, 2% of 2.2 Gy). Two single-intervention ART strategies were executed on the 18th fraction whose previous data had suggested to be a suitable timepoint for a single replanning intervention: (1) replanning on the original target and deformed organ at risks (OARs) (a “safer” approach regarding tumor coverage) and (2) replanning on both deformed target and deformed OARs. D_MVCT showed differences between planned and delivered doses (3D-γ 2mm/2%-passing rate = 85 ± 1%, p < 0.001). Voxel by voxel dose accumulation showed an increase in average dose of warped PG of 3.0 Gy ± 3.3 Gy. With ART the average dose of warped PG decreased by 3.2 Gy ± 1.7 Gy in comparison to delivered dose without replanning when both target and OARs were deformed. Average dose of warped PG decreased by 2.0 Gy ± 1.4 Gy when only OARs were deformed. Anatomical variations lead to increased doses to PGs. Efficient single-intervention ART-strategies with replanning on the 18th MVCT result a reduced PG dose. A strategy with deformation of both target and OAR resulted in the lowest PG dose, while formally maintaining PTV coverage. Deformation of only OAR nevertheless reduces PG dose and has less uncertainties regarding PTV coverage.     

Practically acquired and modified cone-beam computed tomography images for accurate dose calculation in head and neck cancer

Background

On-line cone-beam computed tomography (CBCT) may be used to reconstruct the dose for geometric changes of patients and tumors during radiotherapy course. This study is to establish a practical method to modify the CBCT for accurate dose calculation in head and neck cancer.

Patients and Methods

Fan-beam CT (FBCT) and Elekta??s CBCT were used to acquire images. The CT numbers for different materials on CBCT were mathematically modified to match them with FBCT. Three phantoms were scanned by FBCT and CBCT for image uniformity, spatial resolution, and CT numbers, and to compare the dose distribution from orthogonal beams. A Rando phantom was scanned and planned with intensity-modulated radiation therapy (IMRT). Finally, two nasopharyngeal cancer patients treated with IMRT had their CBCT image sets calculated for dose comparison.

Results

With 360° acquisition of CBCT and high-resolution reconstruction, the uniformity of CT number distribution was improved and the otherwise large variations for background and high-density materials were reduced significantly. The dose difference between FBCT and CBCT was < 2% in phantoms. In the Rando phantom and the patients, the dose?Cvolume histograms were similar. The corresponding isodose curves covering ?? 90% of prescribed dose on FBCT and CBCT were close to each other (within 2 mm). Most dosimetric differences were from the setup errors related to the interval changes in body shape and tumor response.

Conclusion

The specific CBCT acquisition, reconstruction, and CT number modification can generate accurate dose calculation for the potential use in adaptive radiotherapy.     

PET成像应用于高能光子剂量验证的定量研究

目的 通过PET扫描对高能光子照射不同模体产生的正电子进行定量研究,探讨PET成像在高能光子放疗中生物剂量验证的可行性.方法 利用25和50 MV高能光子照射不同模体(水凝胶、聚乙烯和不均匀模体),照射剂量为2、4、6和8 Gy,照后1 min内立即对模体进行PET扫描,记录正电子计数率随时间的变化,扫描完成后对其进行数据拟合,推算产生核素的半衰期.将模体横截面和矢状面的活度分布图与TPS计算的剂量分布图进行对比,观察正电子活度与剂量分布的相关性.结果 根据各时刻正电子计数率进行拟合,得到水凝胶和聚乙烯中产生的正电子半衰期分别为2.23和19.47 min,与11C和15O的半衰期2.08和20.2 min相差不大.50 MV光子在水凝胶和聚乙烯中产生的正电子数量分别是25 MV光子产生的3.88和3.86倍,正电子产额随模体吸收剂量增加而成比例增加.除了在剂量建成区和模体空腔外,模体深度-活度和离轴-活度分布均与剂量分布相似.结论 高能光子与模体反应生成的正电子数量和分布与剂量关系密切,利用PET成像进行高能光子剂量验证理论上是可行的.     

利用3D打印颅脑辐射等效体模进行个性化适形放疗的剂量验证

目的 建立利用3D打印颅脑辐射等效体模对患者进行个性化放疗剂量验证的方法,为三维适形放射治疗安全提供一种可靠的剂量保证手段。方法 采集两例患者（患者1和患者2）的CT图像数据,基于患者1的图像数据,重建其颅骨与脑组织,制作颅脑体模,验证颅骨与脑组织的等效材料。基于患者2的图像数据,根据3D图像重建并选用组织等效材料重建完全的头颅结构,采用3D打印技术制作全头颅体模。通过对目标区域插入电离室剂量仪并行放射治疗方案,获得头颅体模病灶部的剂量,验证和校准实际放疗计划的安全性。结果 对所获两个体模分别进行DR、CT成像,颅脑体模的等效骨骼与患者1骨骼的X射线灰度值差异为13 721,颅脑体模的等效脑组织与患者1的脑组织的CT值差异为35~40 HU,全头颅体模等效颞肌与患者2的颞肌组织的CT值差异为18~28 HU,影像数据表明体模材质的辐射等效性与人体组织近似,并且等效剂量分布符合常规治疗范围,体模的剂量验证可以有效验证放疗计划系统的准确性。结论 基于3D打印和组织等效技术所设计的个性化放疗体模,可应用于个性化放射治疗验证。体模制作方法简单快速,个性化程度高,为三维适形放射治疗安全提供一种可靠的剂量保证手段。     

基于磁共振加速器系统头颈部肿瘤自适应放射治疗的剂量学评估

目的 探讨头颈部肿瘤患者基于磁共振加速器系统开展自适应放射治疗的可行性。方法 回顾性分析2019年在中山大学肿瘤防治中心采用磁共振加速器上开展自适应放射治疗的6例头颈部肿瘤患者,共计128个治疗分次的在线自适应治疗计划。评估分次间靶区处方剂量覆盖和危及器官最大剂量或平均剂量的变化情况。然后将每个治疗分次计划剂量叠加后,比较靶区处方剂量覆盖和各危及器官剂量与参考计划的差异。结果 分次间靶区和危及器官剂量评估结果显示,靶区处方剂量覆盖变化<1%,均满足临床要求。脑干、视交叉、视神经、眼球分次间最大剂量和平均剂量变化较小,但眼晶状体剂量变化最大可达98%。累积剂量评估结果显示,靶区处方剂量覆盖和参考计划无明显差别（<1%）,脑干、视交叉、视神经、眼球的剂量低于参考计划。眼晶状体剂量变化明显,其剂量高于参考计划最大为31.7%。结论 靶区与危及器官的累积受照剂量和分次间剂量均满足临床要求,磁共振加速器系统开展头颈部肿瘤自适应放射治疗方案是可行的。眼晶状体实际受照剂量与参考计划差异较大,应在临床中予以考虑。     

Conceptus radiation dose and risk from chest screen-film radiography

The objectives of the present study were to (a) estimate the conceptus radiation dose and risks for pregnant women undergoing posteroanterior and anteroposterior (AP) chest radiographs, (b) study the conceptus dose as a function of chest thickness of the patient undergoing chest radiograph, and (c) investigate the possibility of a conceptus to receive a dose of more than 10 mGy, the level above which specific measurements of conceptus doses may be necessary. Thermoluminescent dosimeters were used for dose measurements in anthropomorphic phantoms simulating pregnancy at the three trimesters of gestation. The effect of chest thickness on conceptus dose and risk was studied by adding slabs of lucite on the anterior and posterior surface of the phantom chest. The conceptus risk for radiation-induced childhood fatal cancer and hereditary effects was calculated based on appropriate risk factors. The average AP chest dimension (d_a) was estimated for 51 women of childbearing age from chest CT examinations. The value of d_a was estimated to be 22.3 cm (17.4–27.2 cm). The calculated maximum conceptus dose was 107×10^–3 mGy for AP chest radiographs performed during the third trimester of pregnancy with maternal chest thickness of 27.2 cm. This calculation was based on dose data obtained from measurements in the phantoms and d_a estimated from the patient group. The corresponding average excess of childhood cancer was 10.7 per million patients. The risk for hereditary effects was 1.1 per million births. Radiation dose for a conceptus increases exponentially as chest thickness increases. The conceptus dose at the third trimester is higher than that of the second and first trimesters. The results of the current study suggest that chest radiographs carried out in women at any time during gestation will result in a negligible increase in risk of radiation-induced harmful effects to the unborn child. After a properly performed maternal chest X-ray, there is no need for individual conceptus dose estimations. Electronic Publication     

Hi-ART螺旋断层放疗机MV螺旋CT剂量指数的测量

目的 探讨Hi-ART螺旋断层放疗机MV扇形束CT图像获取过程中患者接受的剂量。方法 用PTWTM30009CT电离室分别在T40017头部和T40016躯干模体中,选择扫描层厚2、4及6mm和改变扫描范围等参数,分别测量加权CT剂量指数,计算相应的剂量长度乘积,并与XVIkV锥形束CT、ACQSim模拟定位CT的结果进行比较。结果 Hi-ART螺旋断层治疗机的CT剂量指数与层厚成反比,剂量长度乘积与扫描范围成正比。临床应用条件下Hi-ART的CT剂量指数在头颈部比XVIkV锥形束CT大,但躯干较小。结论 CT剂量指数能反映患者成像过程中接受的剂量,可以作为治疗保证与控制的指标。图像引导过程中应该合理选择层厚,减少扫描范围,最大限度减少患者接受剂量。     

A PC program for estimating organ dose and effective dose values in computed tomography

Dose values in CT are specified by the manufacturers for all CT systems and operating conditions in phantoms. It is not trivial, however, to derive dose values in patients from this information. Therefore, we have developed a PC-based program which calculates organ dose and effective dose values for arbitrary scan parameters and anatomical ranges. Values for primary radiation are derived from measurements or manufacturer specifications; values for scattered radiation are derived from Monte Carlo calculations tabulated for standard anthropomorphic phantoms. Based on these values, organ doses can be computed by the program for arbitrary scan protocols in conventional and in spiral CT. Effective dose values are also provided, both with ICRP 26 and ICRP 60 tissue-weighting coefficients. Results for several standard CT protocols are presented in tabular form in this paper. In addition, potential for dose reduction is demonstrated, for example, in spiral CT and in quantitative CT. Providing realistic patient dose estimates for arbitrary CT protocols is relevant both for the physician and the patient, and it is particularly useful for educational and training purposes. The program, called WinDose, is now in use at the Erlangen University hospitals (Germany) as an information tool for radiologists and patients. Further extensions are planned. Received: 9 March 1998; Revision received: 4 June 1998; Accepted: 4 November 1998     

基于离线自适应放疗的宫颈癌病例靶区外扩边界及其剂量评估

目的 分析行容积旋转调强放射治疗宫颈癌病例在离线自适应放疗（off-line ART）中靶区的外扩边界及其剂量学参数。方法 选取50例宫颈癌病例,采用随机配对法均分成试验组与对照组,每例患者每周行2次锥形束CT （CBCT）扫描,记录整个治疗过程中,患者在左右（LR）、前后（AP）与头脚（CC）方向上的摆位误差值,利用靶区外放边界公式计算新的临床靶区体积（CTV）-计划靶区体积（PTV）的外扩边界。同时,将摆位误差值归一至等中心点（ISO）,回归治疗计划系统,对照组在原有PTV的基础上移动ISO后重新计算剂量,试验组在得到新的外扩边界后移动ISO重新计算剂量,比较评估两组计划在重新计算剂量后的CTV与危及器官剂量学参数。结果 根据靶区外放边界公式,CTV外扩边界在LR、AP和CC方向上分别为0.45、0.46和0.82 cm。治疗计划系统（TPS）剂量再计算结果显示,试验组在CTV的D_100%与D_95%上优于对照组（t=-8.16、-6.73,P<0.05）,在股骨头的V₄₀、V₃₀以及D_mean上均优于对照组（t=3.14、-9.52、-7.48,P<0.05）,在骨盆的V₃₄与D_mean上均优于对照组（t=10.14、-9.38,P<0.05）。结论 在宫颈癌的容积旋转调强放射治疗中,off-line ART技术可以有效地减少CTV-PTV的外扩边界,并且可以提高靶区的照射覆盖范围,减少相应危及器官的照射剂量。     

基于剂量预测的个性化放射治疗计划质量的定量评价方法

目的:提出一种基于剂量预测的放射治疗计划质量定量评价方法,并验证该方法的临床可行性和临床价值。方法:基于45例5年以上从业经验的物理师制定的直肠癌病例,训练3D U-Net网络。利用3D U-Net网络预测得到三维剂量分布后,基于剂量预测的剂量-体积直方图(DVH)指标,建立调强放射治疗(IMRT)直肠癌计划质量评估标...     

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.     

3D dose distribution calculation in a voxelized human phantom by means of Monte Carlo method

The aim of this work is to provide the reconstruction of a real human voxelized phantom by means of a MatLab^® program and the simulation of the irradiation of such phantom with the photon beam generated in a Theratron 780^® (MDS Nordion) ⁶⁰Co radiotherapy unit, by using the Monte Carlo transport code MCNP (Monte Carlo N-Particle), version 5. The project results in 3D dose mapping calculations inside the voxelized antropomorphic head phantom.The program provides the voxelization by first processing the CT slices; the process follows a two-dimensional pixel and material identification algorithm on each slice and three-dimensional interpolation in order to describe the phantom geometry via small cubic cells, resulting in an MCNP input deck format output. Dose rates are calculated by using the MCNP5 tool FMESH, superimposed mesh tally, which gives the track length estimation of the particle flux in units of particles/cm². Furthermore, the particle flux is converted into dose by using the conversion coefficients extracted from the NIST Physical Reference Data.The voxelization using a three-dimensional interpolation technique in combination with the use of the FMESH tool of the MCNP Monte Carlo code offers an optimal simulation which results in 3D dose mapping calculations inside anthropomorphic phantoms. This tool is very useful in radiation treatment assessments, in which voxelized phantoms are widely utilized.     

Dose Length Products for the 10 Most Commonly Ordered CT Examinations in Adults: Analysis of Three Years of the ACR Dose Index Registry

As CT use steadily rises, concern over potential risks of radiation exposure from medical imaging has received increasing attention. Since May 2011, the ACR Dose Index Registry (DIR) has been open for general participation and has been collecting CT radiation dose data from an increasing number of facilities of various types. In this introductory review, we analyze the first three years of ACR DIR data, categorize the 10 most commonly performed CT examinations nationwide, review the variability of the recorded radiation dose indices for each, and take preliminary steps toward identifying possible factors associated with variability in dose indices. We believe that disseminating such information will help prompt informed improvements in standardization of CT protocols with respect to radiation dose.     

Critical dose and toxicity index of organs at risk in radiotherapy: Analyzing the calculated effects of modified dose fractionation in non–small cell lung cancer

To increase the efficacy of radiotherapy for non–small cell lung cancer (NSCLC), many schemes of dose fractionation were assessed by a new “toxicity index” (I), which allows one to choose the fractionation schedules that produce less toxic treatments. Thirty-two patients affected by non resectable NSCLC were treated by standard 3-dimensional conformal radiotherapy (3DCRT) with a strategy of limited treated volume. Computed tomography datasets were employed to re plan by simultaneous integrated boost intensity-modulated radiotherapy (IMRT). The dose distributions from plans were used to test various schemes of dose fractionation, in 3DCRT as well as in IMRT, by transforming the dose-volume histogram (DVH) into a biological equivalent DVH (BDVH) and by varying the overall treatment time. The BDVHs were obtained through the toxicity index, which was defined for each of the organs at risk (OAR) by a linear quadratic model keeping an equivalent radiobiological effect on the target volume. The less toxic fractionation consisted in a severe/moderate hyper fractionation for the volume including the primary tumor and lymph nodes, followed by a hypofractionation for the reduced volume of the primary tumor. The 3DCRT and IMRT resulted, respectively, in 4.7% and 4.3% of dose sparing for the spinal cord, without significant changes for the combined-lungs toxicity (p < 0.001). Schedules with reduced overall treatment time (accelerated fractionations) led to a 12.5% dose sparing for the spinal cord (7.5% in IMRT), 8.3% dose sparing for V₂₀ in the combined lungs (5.5% in IMRT), and also significant dose sparing for all the other OARs (p < 0.001). The toxicity index allows to choose fractionation schedules with reduced toxicity for all the OARs and equivalent radiobiological effect for the tumor in 3DCRT, as well as in IMRT, treatments of NSCLC.     

A dose distribution overlay technique for image guidance during prostate radiotherapy

Adaptive radiotherapy involves altering the treatment plan according to variations in patient anatomy and set-up. This relies upon an accurate representation of the changing dose distribution within the patient, requiring a full dose recalculation. This work proposes a novel workflow using the planned dose distribution to assess dose coverage in three-dimensional verification CT studies acquired at the time of treatment delivery, using an overlay technique, in lieu of a recalculated dose distribution. The concept has been validated in a pilot study of 10 patients, each with 7-10 on-treatment CT studies. Differences between the geometric shape of the treatment plans for the 95% isodose and the 95% isodose obtained when the planned geometry was recalculated from the verification CT dataset were quantified. Dosimetric coverage of the verification clinical target volume (vCTV) was assessed for both the proposed overlay technique and the recalculated "delivered" dose distribution, and the conclusions on adequacy were compared. Results were consistent with geometric uncertainties of the dose calculation matrix (5 x 5 x 5 mm), suggesting that differences in the geometric shape of the 95% isodose are not significant for normal variations in patients' anatomy. Decisions on adequacy of vCTV coverage were consistent in 80 out of 87 cases, with discrepancies limited to a maximum of three axial slices per study within the range 0.5-4.5 mm (mean, 1.6 mm). The proposed dosimetric overlay technique has been validated and found to be an acceptable method of image-guided radiotherapy of the prostate suitable for effective implementation in the treatment clinic.     

Evaluation of RayStation's delivered dose and accumulated dose features for spine stereotactic radiotherapy

We investigated delivered dose and dose accumulation features in the dose tracking module of RayStation version 11B before potential integration into the spine stereotactic radiosurgery (SSRS) program at our institution. End-to-end testing with a rigid Rando phantom was performed, and 10 retrospective clinical cases were selected for evaluation. Pre-treatment cone beam CTs (pCBCT) were corrected for Hounsfield unit (HU) integrity and contours were rigidly copied from the reference plan. We then calculated the delivered dose to the corrected cone beam CT (cCBCT). A deformable image registration (DIR) was generated between cCBCT and reference planning CT (rCT) using controlling region of interests. Deformed delivered dose to the rCT was summed to generate the accumulated dose for multiple fractions. The end-to-end tests of the phantom study revealed an improvement of cCBCT HU information by > 100 HU compared to the pCBCT. When compared to the reference plan, the delivered dose and deformed delivered dose were within 1.0% for the GTV and CTV and 3.0% for the spinal cord, respectively. Nearly all 10 clinical cases demonstrated delivered dose and accumulated dose deviations < 3.0% from the reference plan. However, 2 patients rendered delivered dose deviations between 3.0% and 4.0%, showing the effectiveness of the module. The dose tracking module in RayStation version 11B could potentially be utilized to aid clinical decision-making for external body shape change or positional deviation in SSRS for rigid target and critical structures. Evaluation before clinical application under one's specific practice is required, and results must be carefully analyzed specially near the high dose gradient area.