Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona 6500, Switzerland.
Med Phys. 2013 Oct;40(10):101706. doi: 10.1118/1.4820443.
The accuracy of photon dose calculation algorithms in out-of-field regions is often neglected, despite its importance for organs at risk and peripheral dose evaluation. The present work has assessed this for the anisotropic analytical algorithm (AAA) and the Acuros-XB algorithms implemented in the Eclipse treatment planning system. Specifically, the regions shielded by the jaw, or the MLC, or both MLC and jaw for flattened and unflattened beams have been studied.
The accuracy in out-of-field dose under different conditions was studied for two different algorithms. Measured depth doses out of the field, for different field sizes and various distances from the beam edge were compared with the corresponding AAA and Acuros-XB calculations in water. Four volumetric modulated arc therapy plans (in the RapidArc form) were optimized in a water equivalent phantom, PTW Octavius, to obtain a region always shielded by the MLC (or MLC and jaw) during the delivery. Doses to different points located in the shielded region and in a target-like structure were measured with an ion chamber, and results were compared with the AAA and Acuros-XB calculations. Photon beams of 6 and 10 MV, flattened and unflattened were used for the tests.
Good agreement between calculated and measured depth doses was found using both algorithms for all points measured at depth greater than 3 cm. The mean dose differences (± 1SD) were -8% ± 16%, -3% ± 15%, -16% ± 18%, and -9% ± 16% for measurements vs AAA calculations and -10% ± 14%, -5% ± 12%, -19% ± 17%, and -13% ± 14% for Acuros-XB, for 6X, 6 flattening-filter free (FFF), 10X, and 10FFF beams, respectively. The same figures for dose differences relative to the open beam central axis dose were: -0.1% ± 0.3%, 0.0% ± 0.4%, -0.3% ± 0.3%, and -0.1% ± 0.3% for AAA and -0.2% ± 0.4%, -0.1% ± 0.4%, -0.5% ± 0.5%, and -0.3% ± 0.4% for Acuros-XB. Buildup dose was overestimated with AAA, while Acuros-XB gave results more consistent with measurements. From RapidArc plan analysis the average difference between calculation and measurement in the shielded region was -0.3% ± 0.4% and -2.5% ± 1.2% for AAA and Acuros-XB, respectively, relative to the mean target dose value (1.6% ± 2.3%, -12.7% ± 4.0% if relative to each local value). These values were compared with the corresponding differences in the target structure: -0.7% ± 2.3% for AAA, and -0.5% ± 2.3% for Acuros-XB.
The two algorithms analyzed showed encouraging results in predicting out-of-field region dose for clinical use.
尽管在评估危及器官和周围剂量方面，场外区域光子剂量计算算法的准确性很重要，但通常会忽略其准确性。本研究评估了 Eclipse 治疗计划系统中使用的各向异性解析算法（AAA）和 Acuros-XB 算法的准确性。具体来说，研究了被屏蔽的区域，包括颌骨、MLC 或 MLC 和颌骨，以及平坦和非平坦射束的屏蔽区域。
研究了两种不同算法在不同条件下的场外剂量的准确性。在水中比较了不同射野大小和离射束边缘不同距离的场外深度剂量的测量值与相应的 AAA 和 Acuros-XB 计算值。在水等效体模（PTW Octavius）中，优化了四个容积调强弧形治疗计划（以 RapidArc 形式），以获得在输送过程中始终被 MLC（或 MLC 和颌骨）屏蔽的区域。使用电离室测量位于屏蔽区域和靶样结构中的不同点的剂量，并将结果与 AAA 和 Acuros-XB 计算值进行比较。使用 6 和 10 MV 的平坦和非平坦光子束进行了测试。
对于所有在深度大于 3 cm 的测量点，两种算法的计算深度剂量与测量深度剂量都吻合得很好。对于 6X、6 无均整滤过器自由射束（FFF）、10X 和 10FFF 射束，测量值与 AAA 计算值的平均剂量差异（±1SD）分别为-8%±16%、-3%±15%、-16%±18%和-9%±16%，与 Acuros-XB 的分别为-10%±14%、-5%±12%、-19%±17%和-13%±14%。对于与开放束中心轴剂量的剂量差异，相应的数字分别为-0.1%±0.3%、0.0%±0.4%、-0.3%±0.3%和-0.1%±0.3%（AAA）和-0.2%±0.4%、-0.1%±0.4%、-0.5%±0.5%和-0.3%±0.4%（Acuros-XB）。AAA 高估了建成剂量，而 Acuros-XB 给出的结果与测量值更为一致。从 RapidArc 计划分析来看，在屏蔽区域中，计算值与测量值的平均差异分别为 AAA 为-0.3%±0.4%和 Acuros-XB 为-2.5%±1.2%，相对于平均靶剂量值（如果相对于每个局部值，分别为 1.6%±2.3%和-12.7%±4.0%）。将这些值与靶结构中的相应差异进行比较：AAA 为-0.7%±2.3%，Acuros-XB 为-0.5%±2.3%。
两种分析的算法在预测临床应用中的场外区域剂量方面显示出令人鼓舞的结果。
