Department of Radiation Oncology, Montefiore Medical Center, Bronx, NY, United States of America. Albert Einstein College of Medicine, Bronx, NY, United States of America.
Phys Med Biol. 2018 Jul 24;63(15):155003. doi: 10.1088/1361-6560/aacfb1.
This study investigates the dosimetric impact of using population-based Hounsfield units (HUs) and ICRU-based HUs as a function of the number of tissue segments for bulk density assignment toward MRI-based treatment planning in the abdomen. To avoid potential geometric differences between CT and MR images, CT images rather than MR images were chosen to simulate an MRI-only planning scenario. A retrospective study was performed utilizing 18 patients that had previously undergone stereotactic body radiation therapy for liver or pancreas cancer. HU values in the CT datasets were collected for various tissue types, and compared with the HUs derived from ICRU report 46. Doses were recalculated using the fluence obtained from clinical plans and with (1) homogeneous assignment, (2) ICRU-based HU assignment and (3) population-based HU assignment using three, four, five, nine or ten tissue segments. Dose-volume metrics for targets and organs-at-risk for all scenarios were compared with those obtained using the clinical CT. For the planning target volume (PTV) D99.9%, the mean differences from clinical CT plans were -2.1% ± 3.9%, -0.6% ± 0.3% and -0.1% ± 0.3% for homogeneity, ICRU-HUs and population-HUs using ten tissue segments, respectively. The population-HU method resulted in better dosimetric accuracy compared to the ICRU-HU method (p-value < 0.05). The dosimetric accuracy of homogeneity plans was comparable to that of both ICRU-HU and population-HU plans when targets were far from the lungs but deteriorated when targets were close to the lungs. As the number of tissue segments decreased, the dosimetric accuracy for PTV D99.9% reduced for the population-HU method, from -0.1% for ten tissue segments to -0.4% for three tissue segments, while no such dependence was observed for the ICRU-HU method. Hence, to generate a clinically acceptable plan when using MRI to synthesize CT in the abdomen for treatment planning, it might be sufficient for electron density assignment with either the population-HU or ICRU-HU method to only use three tissue segments.
本研究调查了在腹部基于 MRI 的治疗计划中，使用基于人群的 Hounsfield 单位（HU）和 ICRU 基于 HU 作为体素密度分配的函数时的剂量学影响。为了避免 CT 和 MR 图像之间的潜在几何差异，选择 CT 图像而不是 MR 图像来模拟仅 MRI 计划场景。利用先前接受肝或胰腺癌症立体定向体部放射治疗的 18 名患者进行了回顾性研究。为各种组织类型收集了 CT 数据集的 HU 值，并与 ICRU 报告 46 中得出的 HU 值进行了比较。使用从临床计划中获得的通量重新计算剂量，并使用（1）均匀分配，（2）ICRU 基于 HU 分配和（3）基于人群的 HU 分配，使用三个、四个、五个、九个或十个组织段。所有情况下的靶区和危及器官的剂量-体积指标均与使用临床 CT 获得的指标进行了比较。对于计划靶区（PTV）D99.9%，与临床 CT 计划相比，均匀分配、ICRU-HU 和人群-HU 分别使用十个组织段时的平均差异分别为-2.1%±3.9%、-0.6%±0.3%和-0.1%±0.3%。与 ICRU-HU 方法相比，人群-HU 方法的剂量学准确性更好（p 值<0.05）。当靶区远离肺部时，均匀分配计划的剂量学准确性与 ICRU-HU 和人群-HU 计划相当，但当靶区靠近肺部时，情况则恶化。随着组织段数的减少，人群-HU 方法的 PTV D99.9%的剂量学准确性降低，从十个组织段的-0.1%降低到三个组织段的-0.4%，而 ICRU-HU 方法则没有观察到这种依赖性。因此，在使用 MRI 合成腹部 CT 进行治疗计划时，为了生成临床可接受的计划，对于电子密度分配，使用人群-HU 或 ICRU-HU 方法仅使用三个组织段可能就足够了。
