Gete Ermias, Teke Tony, Kwa William
British Columbia Cancer Agency-Vancouver Center, Vancouver, BC, Canada.
Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada.
J Med Imaging Radiat Sci. 2012 Mar;43(1):26-33. doi: 10.1016/j.jmir.2011.09.002. Epub 2011 Nov 30.
To evaluate the dose calculation accuracy of the Varian Eclipse anisotropic analytical algorithm (AAA) for stereotactic body radiation therapy (SBRT), and to investigate the dosimetric consequences of not applying tissue heterogeneity correction on complex SBRT lung plans.
Nine cases of non-small-cell lung cancer (NSCLC) that were previously treated with SBRT at our center were selected for this study. Following Radiation Therapy Oncology Group 0236, the original plans were calculated using pencil beam without heterogeneity correction (PBNC). For this study, these plans were recalculated by applying tissue heterogeneity correction with the AAA algorithm and with the Monte Carlo (MC) method, keeping the number of monitor units the same as the original plans. Two kinds of plan comparison were made. First, the AAA calculations were compared with MC. Second, the treatment plans that were calculated with AAA were compared with the original PBNC calculations. The following dose-volume parameters were used for the comparison: V; V; the maximum, the minimum, and the mean planning target volume (PTV) doses (D, D, and D, respectively); V, V, V, V; D for the lung; and D for the critical organs.
Comparable results were obtained for AAA and MC calculations: except for Dmax, Dmin, and Dmean, the differences in the patient-average values of all of the PTV dose parameters were less than 2%. The largest average difference was observed for Dmin (3.8 ± 5.4%). Average differences in all the lung dose parameters were under 0.2%, and average differences in normal tissue Dmax were under 0.3 Gy, except for the skin dose. There were appreciable differences in the PTV and normal tissue dose-volume parameters when comparing AAA and PBNC calculations. Except for V and V, PBNC calculations on average underestimated the dose to the PTV. The largest discrepancy was in the PTV maximum dose, with a patient-averaged difference of 11.1 ± 4.6%.
Based on our MC investigation, we conclude that the Eclipse AAA algorithm is sufficiently accurate for dose calculations of lung SBRT plans involving small 6-MV photon fields. Our results also demonstrate that, although dose calculations at the periphery of the PTV showed good agreement when comparing PBNC with both AAA and MC calculations, there is a potential to significantly underestimate the dose inside the PTV and doses to critical structures if tissue heterogeneity correction is not applied to lung SBRT plans.
评估瓦里安医科达公司的Eclipse各向异性分析算法（AAA）在立体定向体部放射治疗（SBRT）中的剂量计算准确性，并研究在复杂的SBRT肺部计划中不应用组织不均匀性校正的剂量学后果。
本研究选取了9例此前在我们中心接受过SBRT治疗的非小细胞肺癌（NSCLC）病例。按照放射肿瘤学组0236号方案，原始计划采用未进行不均匀性校正的笔形束算法（PBNC）进行计算。在本研究中，这些计划通过应用AAA算法和蒙特卡罗（MC）方法进行组织不均匀性校正后重新计算，保持监测单位数量与原始计划相同。进行了两种计划比较。首先，将AAA计算结果与MC结果进行比较。其次，将用AAA计算的治疗计划与原始PBNC计算结果进行比较。以下剂量体积参数用于比较：V；V；计划靶体积（PTV）的最大、最小和平均剂量（分别为D、D和D）；V、V、V、V；肺的D；以及关键器官的D。
AAA和MC计算得到了可比的结果：除Dmax、Dmin和Dmean外，所有PTV剂量参数的患者平均值差异均小于2%。观察到Dmin的平均差异最大（3.8±5.4%）。所有肺剂量参数的平均差异均低于0.2%，正常组织Dmax的平均差异除皮肤剂量外均低于0.3 Gy。比较AAA和PBNC计算结果时，PTV和正常组织剂量体积参数存在明显差异。除V和V外，PBNC计算平均低估了PTV的剂量。最大差异在于PTV最大剂量，患者平均差异为11.1±4.6%。
基于我们的MC研究，我们得出结论，Eclipse AAA算法对于涉及小6-MV光子射野的肺部SBRT计划的剂量计算足够准确。我们的结果还表明，尽管在比较PBNC与AAA和MC计算结果时，PTV周边的剂量计算显示出良好的一致性，但如果肺部SBRT计划不应用组织不均匀性校正，则有可能显著低估PTV内部的剂量以及关键结构的剂量。
