Hernandez Victor, Vera-Sánchez Juan Antonio, Vieillevigne Laure, Saez Jordi
Department of Medical Physics, Hospital Universitari Sant Joan de Reus, IISPV, 43204 Tarragona, Spain.
Phys Med Biol. 2017 Aug 1;62(16):6688-6707. doi: 10.1088/1361-6560/aa7b1a.
Adequate modelling of the multi-leaf collimator (MLC) by treatment planning systems (TPS) is essential for accurate dose calculations in intensity-modulated radiation-therapy. For this reason modern TPSs incorporate MLC characteristics such as the leaf end curvature, MLC transmission and the tongue-and-groove. However, the modelling of the tongue-and-groove is often neglected during TPS commissioning and it is not known how accurate it is. This study evaluates the dosimetric consequences of the tongue-and-groove effect for two different MLC models using both film dosimetry and ionisation chambers. A set of comprehensive tests are presented that evaluate the ability of TPSs to accurately model this effect in (a) static fields, (b) sliding window beams and (c) VMAT arcs. The tests proposed are useful for the commissioning of TPSs and for the validation of major upgrades. With the ECLIPSE TPS, relevant differences were found between calculations and measurements for beams with dynamic MLCs in the presence of the TG effect, especially for the High Definition MLC, small gap sizes and the 1 mm calculation grid. For this combination, dose differences as high as 10% and 7% were obtained for dynamic MLC gaps of 5 mm and 10 mm, respectively. These differences indicate inadequate modelling of the tongue-and-groove effect, which might not be identified without the proposed tests. In particular, the TPS tended to underestimate the calculated dose, which may require tuning of other configuration parameters in the TPS (such as the dosimetric leaf gap) in order to maximise the agreement between calculations and measurements in clinical plans. In conclusion, a need for better modelling of the MLC by TPSs is demonstrated, one of the relevant aspects being the tongue-and-groove effect. This would improve the accuracy of TPS calculations, especially for plans using small MLC gaps, such as plans with small target volumes or high complexities. Improved modelling of the MLC would also reduce the need for tuning parameters in the TPS, facilitating a more comprehensive configuration and commissioning of TPSs.
治疗计划系统（TPS）对多叶准直器（MLC）进行充分建模，对于调强放射治疗中准确的剂量计算至关重要。因此，现代TPS纳入了MLC的特性，如叶片末端曲率、MLC透射率和舌槽效应。然而，在TPS调试过程中，舌槽效应的建模常常被忽视，其准确性也未知。本研究使用薄膜剂量测定法和电离室，评估了两种不同MLC模型的舌槽效应的剂量学后果。提出了一组综合测试，以评估TPS在（a）静态射野、（b）滑动窗口射束和（c）容积调强弧形治疗（VMAT）弧形射束中准确模拟这种效应的能力。所提出的测试对于TPS的调试和主要升级的验证很有用。使用ECLIPSE TPS时，发现在存在舌槽效应的情况下，对于具有动态MLC的射束，计算值与测量值之间存在相关差异，特别是对于高清MLC、小间隙尺寸和1毫米计算网格。对于这种组合，动态MLC间隙为5毫米和10毫米时，剂量差异分别高达10%和7%。这些差异表明舌槽效应建模不足，如果没有所提出的测试可能无法识别。特别是，TPS往往会低估计算剂量，这可能需要调整TPS中的其他配置参数（如剂量学叶片间隙），以便在临床计划中使计算值与测量值之间的一致性最大化。总之，证明了TPS需要对MLC进行更好的建模，其中一个相关方面是舌槽效应。这将提高TPS计算的准确性，特别是对于使用小MLC间隙的计划，如具有小靶体积或高复杂性的计划。改进MLC的建模也将减少TPS中调整参数的需要，便于对TPS进行更全面的配置和调试。
