Prior Phil, Chen Xinfeng, Botros Maikel, Paulson Eric S, Lawton Colleen, Erickson Beth, Li X Allen
Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, WI, USA.
Phys Med Biol. 2016 May 21;61(10):3819-42. doi: 10.1088/0031-9155/61/10/3819. Epub 2016 Apr 18.
The treatment planning in radiation therapy (RT) can be arranged to combine benefits of computed tomography (CT) and magnetic resonance imaging (MRI) together to maintain dose calculation accuracy and improved target delineation. Our aim is study the dosimetric impact of uniform relative electron density assignment on IMRT treatment planning with additional consideration given to the effect of a 1.5 T transverse magnetic field (TMF) in MR-Linac. A series of intensity modulated RT (IMRT) plans were generated for two representative tumor sites, pancreas and prostate, using CT and MRI datasets. Representative CT-based IMRT plans were generated to assess the impact of different electron density (ED) assignment on plan quality using CT without the presence of a 1.5 T TMF. The relative ED (rED) values used were taken from the ICRU report 46. Four types of rED assignment in the organs at risk (OARs), the planning target volumes (PTV) and in the non-specified tissue (NST) were considered. Dose was recalculated (no optimization) using a Monaco 5.09.07a research planning system employing Monte Carlo calculations with an option to include TMF. To investigate the dosimetric effect of different rED assignment, the dose-volume parameters (DVPs) obtained from these specific rED plans were compared to those obtained from the original plans based on CT. Overall, we found that uniform rED assignment results in differences in DVPs within 3% for the PTV and 5% for OAR. The presence of 1.5 T TMF on IMRT DVPs resulted in differences that were generally within 3% of the Gold St for both the pancreas and prostate. The combination of uniform rED assignment and TMF produced differences in DVPs that were within 4-5% of the Gold St. Larger differences in DVPs were observed for OARs on T2-based plans. The effects of using different rED assignments and the presence of 1.5 T TMF for pancreas and prostate IMRT plans are generally within 3% and 5% of PTV and OAR Gold St values. There are noticeable dosimetric differences between the CT- and MRI-based IMRT plans caused by a combination of anatomical changes between the two image acquisition times, uniform rED assignment and 1.5 T TMF.
放射治疗（RT）中的治疗计划可安排为将计算机断层扫描（CT）和磁共振成像（MRI）的优势结合起来，以保持剂量计算的准确性并改善靶区勾画。我们的目的是研究均匀相对电子密度赋值对调强放射治疗（IMRT）治疗计划的剂量学影响，并额外考虑1.5 T横向磁场（TMF）在磁共振直线加速器（MR-Linac）中的作用。使用CT和MRI数据集，为两个具有代表性的肿瘤部位（胰腺和前列腺）生成了一系列调强放射治疗（IMRT）计划。生成基于CT的代表性IMRT计划，以评估在不存在1.5 T TMF的情况下，不同电子密度（ED）赋值对计划质量的影响。所使用的相对电子密度（rED）值取自ICRU报告46。考虑了在危及器官（OARs）、计划靶区（PTV）和非特定组织（NST）中的四种类型的rED赋值。使用Monaco 5.09.07a研究计划系统，采用蒙特卡罗计算并选择包含TMF的选项，重新计算剂量（不进行优化）。为了研究不同rED赋值的剂量学效应，将从这些特定rED计划中获得的剂量体积参数（DVPs）与基于CT的原始计划中获得的参数进行比较。总体而言，我们发现均匀的rED赋值导致PTV的DVPs差异在3%以内，OAR的差异在5%以内。1.5 T TMF对IMRT DVPs的影响导致的差异，对于胰腺和前列腺来说，通常在金标准（Gold St）的3%以内。均匀rED赋值和TMF的组合在DVPs上产生的差异在金标准的4 - 5%以内。在基于T2的计划中，观察到OARs的DVPs差异更大。对于胰腺和前列腺IMRT计划，使用不同rED赋值和存在1.5 T TMF的影响通常在PTV和OAR金标准值的3%和5%以内。由于两次图像采集时间之间的解剖结构变化、均匀rED赋值和1.5 T TMF的综合作用，基于CT和MRI的IMRT计划之间存在明显的剂量学差异。
