背景:随着最近对放射性药物治疗(RPT)和选择性内部放射治疗(SIRT)的患者特异性剂量学的兴趣,正在评估越来越多的基于体素的算法。蒙特卡罗(MC)辐射传输,通常被认为是在不同的体素水平吸收剂量估计方法中最准确的,对于常规临床使用,计算效率可能很低。
目的:这项工作展示了最近实现的基于网格的线性玻尔兹曼输运方程(LBTE)求解器,用于RPT和SIRT中快速准确的基于体素的剂量测定,并将其与MC进行基准测试。
方法:在商业RPT剂量测定软件包(Velocity4.1)中实施了确定性LBTE求解器(AcurosMRT)。使用自适应网格细化网格直接离散化LBTE,然后在指定体积内迭代求解耦合的光子-电子辐射传输,以估计来自异质介质中光子和带电粒子的辐射剂量。为了评估RPT和SIRT应用的LBTE求解器的性能,177路SPECT/CT,90YPET/CT,并使用131ISPECT/CT图像的体模和患者。对于每个研究描绘多个病变(2-1052mL)和正常器官。用LBTE求解器进行体素剂量测定,剂量体素核(DVK)卷积与密度校正,以及经过验证的内部MC代码,使用相同的时间集成活动和密度图作为不同剂量引擎的输入。由此产生的剂量图,差异地图,和剂量-体积-直方图(DVH)指标进行了比较,来评估体素级别的协议。平均吸收剂量的评估包括与OLINDA的结构水平估计值的比较。
结果:在幻影插入物/隔室中,LBTE求解器与MC和DVK卷积显示出与平均吸收剂量和DVH指标一致在5%以内的良好一致性,但90Y的极低活性浓度插入的D90和D70指标的一致性在15%以内。在患者研究中(五名患者在177LuDOTATATERPT后成像,90YSIRT后五个,和两个在131I放射免疫疗法后),总的来说,对于平均吸收剂量和体素水平评估,LBTE求解器与MC之间的一致性优于LBTE求解器与DVK卷积之间的一致性.在所有患者中,所有三种放射性核素,对于软组织结构(肾脏,肝脏,病变),来自LBTE求解器的平均吸收剂量估计与来自MC的平均吸收剂量估计非常吻合(中位数差异<1%,最大9%)和来自DVK的(中位数差异<5%,最大9%)。LBTE和OLINDA估计肾脏和肝脏的平均吸收剂量在10%以内,但是病变的差异更大,177Lu的差异最大为14%,对于90年,23%,131I的比例为26%。对于骨骼区域,LBTE与MC和DVK的平均吸收剂量一致(中位数<11%,最大11%),而对于肺,LBTE和MC之间的一致性(中位数<1%,max8%)显著优于LBTE和DVK(中位数<16%,最大33%)。软组织结构的体素水平估计也显示了LBTE求解器与MC和DVK之间的良好一致性,所有三种放射性核素的DVH指标的中值差异<5%(最大<13%)。DVH指标的最大差异是肺和骨骼中的D90和D70指标,其中摄取较低。这里,LBTE和MC之间的差异具有骨的中值<14%(最大23%)和肺的中值<4%(最大37%),而LBTE和DVK之间的相应差异<23%(最大31%)和<67%(最大313%),分别。对于矩阵尺寸为166×166×129(体素尺寸为3×3×3mm3)的典型患者,使用LBTE解算器的体素剂量测定在台式计算机上的速度高达2分钟。
结论:对于RPT和SIRT应用,LBTE求解器和MC之间建立了良好的协议,LBTE求解器是体素剂量测定的可行选择,可以比MC更快。正在进行进一步的分析,以涵盖广泛的放射性核素和临床遇到的条件。
BACKGROUND: With recent interest in patient-specific dosimetry for radiopharmaceutical therapy (RPT) and selective internal radiation therapy (SIRT), an increasing number of voxel-based algorithms are being evaluated. Monte Carlo (MC) radiation transport, generally considered to be the most accurate among different methods for voxel-level absorbed dose estimation, can be computationally inefficient for routine clinical use.
OBJECTIVE: This work demonstrates a recently implemented grid-based linear Boltzmann transport equation (LBTE) solver for fast and accurate voxel-based dosimetry in RPT and SIRT and benchmarks it against MC.
METHODS: A deterministic LBTE solver (Acuros MRT) was implemented within a commercial RPT dosimetry package (Velocity 4.1). The LBTE is directly discretized using an adaptive mesh refined grid and then the coupled photon-electron radiation transport is iteratively solved inside specified volumes to estimate radiation doses from both photons and charged particles in heterogeneous media. To evaluate the performance of the LBTE solver for RPT and SIRT applications, 177Lu SPECT/CT, 90Y PET/CT, and 131I SPECT/CT images of phantoms and patients were used. Multiple lesions (2-1052 mL) and normal organs were delineated for each study. Voxel dosimetry was performed with the LBTE solver, dose voxel kernel (DVK) convolution with density correction, and a validated in-house MC code using the same time-integrated activity and density maps as input to the different dose engines. The resulting dose maps, difference maps, and dose-volume-histogram (DVH) metrics were compared, to assess the voxel-level agreement. Evaluation of mean absorbed dose included comparison with structure-level estimates from OLINDA.
RESULTS: In the phantom inserts/compartments, the LBTE solver versus MC and DVK convolution demonstrated good agreement with mean absorbed dose and DVH metrics agreeing to within 5% except for the D90 and D70 metrics of a very low activity concentration insert of 90Y where the agreement was within 15%. In the patient studies (five patients imaged after 177Lu DOTATATE RPT, five after 90Y SIRT, and two after 131I radioimmunotherapy), in general, there was better agreement between the LBTE solver and MC than between LBTE solver and DVK convolution for mean absorbed dose and voxel-level evaluations. Across all patients for all three radionuclides, for soft tissue structures (kidney, liver, lesions), the mean absorbed dose estimates from the LBTE solver were in good agreement with those from MC (median difference < 1%, maximum 9%) and those from DVK (median difference < 5%, maximum 9%). The LBTE and OLINDA estimates for mean absorbed dose in kidneys and liver agreed to within 10%, but differences for lesions were larger with a maximum 14% for 177Lu, 23% for 90Y, and 26% for 131I. For bone regions, the agreement in mean absorbed doses between LBTE and both MC and DVK were similar (median < 11%, max 11%) while for lung the agreement between LBTE and MC (median < 1%, max 8%) was substantially better than between LBTE and DVK (median < 16%, max 33%). Voxel level estimates for soft tissue structures also showed good agreement between the LBTE solver and both MC and DVK with a median difference < 5% (maximum < 13%) for the DVH metrics with all three radionuclides. The largest difference in DVH metrics was for the D90 and D70 metric in lung and bone where the uptake was low. Here, the difference between LBTE and MC had a median value < 14% (maximum 23%) for bone and < 4% (maximum 37%) for lung, while the corresponding differences between LBTE and DVK were < 23% (maximum 31%) and < 67% (maximum 313%), respectively. For a typical patient with a matrix size of 166 × 166 × 129 (voxel size 3 × 3 × 3 mm3), voxel dosimetry using the LBTE solver was as fast as ∼2 min on a desktop computer.
CONCLUSIONS: Having established good agreement between the LBTE solver and MC for RPT and SIRT applications, the LBTE solver is a viable option for voxel dosimetry that can be faster than MC. Further analysis is being performed to encompass the broad range of radionuclides and conditions encountered clinically.