Abdel-Hakim Khaled, Nishimura Tetsuo, Takaih Michikatsu, Suzuki Shuji, Sakahara Harumi
Department of Radiology, Hamamatsu University School of Medicine, Handayama, Hamamatsu, Japan.
Strahlenther Onkol. 2003 May;179(5):312-9. doi: 10.1007/s00066-003-1024-1.
The use of conventional asymmetric collimators for junctioning of abutted fields can lead to significant dose inhomogeneity, due to jaw misalignment. However, recent technologic advances enable us to fabricate much finer leafpositioning accuracy. Consequently, it is anticipated that the use of multileaf collimator (MLC) will potentially improve dose homogeneity at the junction of abutted fields. In this work, we evaluated the dose inhomogeneities at the match-plane in monoisocentric three-field head and neck setups, using MLC for field abutment.
To define either the anterior or the lateral fields, the MLC was used with either the longitudinal (0 degree angle) or the transverse (90 degree angle) settings. For 0 degree setting, each leaf moves in a direction perpendicular to the gantry rotation axis, hence the "tongue and groove" (T&G) design can effect matching-area dose at the side of the leaf (Figure 1a). For 90 degree setting, the rounded shape of the leaf produces its effect at the leaf end (Figure 1b). Four combinations of abutted anterior field and abutted lateral field defined by MLC, i.e., abutted using MLC side-by-side, side-by-end, end-by-side and end-by-end, were compared (Table 1). Dose inhomogeneity was measured at the junction of the two abutted fields with films in a solid water phantom (Figure 2). The effect of jaw settings as a backup diaphragm on the dose distribution was also studied. Reproducibility of the results was confirmed by repeated measurements over a 1-year period.
Abutted fields using MLC side-by-side caused underdose of approximately 15% (Figure 3a). Abutted fields using MLC side-by-end produced > 10% overdose that could be improved to +/- 1% for 0.5 mm overlap of the leaf end from the lateral portals (Figure 4). When using end-by-side, an overdose of approximately 15% was observed. However, the dose improved to a homogeneous dose for 0.8 mm overlap of leaf end from the anterior portal (Figure 5). End-by-end showed an overdose of > 20% (Figure 3b). This overdose could be smoothed out by overlaps of both leaf ends by 0.8 mm from both lateral and anterior portals (Figure 6). The ideal jaw position was found to be at 1 mm away from the beam central axis in any combination (Table 2).
The use of MLCs for photon field junction matching is appropriate and represents an alternative approach to the problem of field matching using the asymmetric jaws in head and neck treatments.
由于准直器错位，使用传统非对称准直器对接相邻射野会导致显著的剂量不均匀性。然而，最近的技术进步使我们能够制造出叶片定位精度更高的准直器。因此，预计使用多叶准直器（MLC）可能会改善相邻射野交界处的剂量均匀性。在本研究中，我们评估了在单等中心头颈部三野照射设置中，使用MLC进行射野对接时，匹配平面处的剂量不均匀性。
为了定义前野或侧野，MLC分别采用纵向（0度角）或横向（90度角）设置。对于0度设置，每个叶片沿垂直于机架旋转轴的方向移动，因此“舌槽”（T&G）设计会影响叶片侧面的匹配区域剂量（图1a）。对于90度设置，叶片的圆形形状在叶片末端产生影响（图1b）。比较了由MLC定义的相邻前野和相邻侧野的四种组合，即MLC并排对接、端对侧对接、侧对端对接和端对端对接（表1）。在固体水模体中，用胶片在两个相邻射野的交界处测量剂量不均匀性（图2）。还研究了作为备用光阑的准直器设置对剂量分布的影响。通过1年期间的重复测量证实了结果的可重复性。
MLC并排对接的相邻射野导致约15%的剂量不足（图3a）。MLC端对侧对接的相邻射野产生超过10%的剂量过量，当叶片末端从侧野入口处重叠0.5 mm时，可改善至±1%（图4）。使用侧对端对接时，观察到约15%的剂量过量。然而，当叶片末端从前野入口处重叠0.8 mm时，剂量改善为均匀剂量（图5）。端对端对接显示超过20%的剂量过量（图3b）。通过叶片末端从侧野和前野入口处均重叠0.8 mm，可使这种剂量过量变得平滑（图6）。发现在任何组合中，理想的准直器位置是距离射束中心轴1 mm处（表2）。
在头颈部治疗中，使用MLC进行光子射野对接匹配是合适的，并且是使用非对称准直器解决射野匹配问题的一种替代方法。
