Rosenthal D I, McDonough J, Kassaee A
Department of Radiation Oncology, The University of Pennsylvania Medical Center, Philadelphia 19104-4283, USA.
Radiother Oncol. 1998 Dec;49(3):273-8. doi: 10.1016/s0167-8140(98)00128-5.
Independent collimation conveniently allows for the junctioning of abutting fields with non-diverging beam edges. When this technique is used at the junction of multiple fields, e.g. lateral and low anterior fields in three-field head and neck set-ups, there should be a dosimetric match with no overdose or underdose at the matchline. We set out to evaluate the actual dosimetry at the central match plane.
Independent jaws were used to mimic two half-beam blocked fields abutting at the central axis. X-Ray verification film was exposed in a water-equivalent phantom and the dose at the matchline was evaluated with laser densitometry. Collimators were then programmed to force a gap or overlap of the radiation fields to evaluate the effect of jaw misalignment within the tolerance of the manufacturer's specification. Diode measurements of the field edges were also performed. Four beam energies from four different linear accelerators were evaluated.
Small systematic inhomogeneities were found along the matchline in all linear accelerators tested. The maximum dose on the central axis varied linearly with small programmed jaw misalignments. For a gap or overlap of 2 mm between the jaws, the matchline dose increased or decreased by 30-40%. The region of overdose or underdose around the matchline is 3-4 mm wide. The discrepancy between the width of jaw separation and the width of the region of altered dose is explained by a penumbra effect.
We recommend that independent jaw alignment be evaluated routinely and provide a simple method to estimate dose inhomogeneity at the match plane. If there is a field gap or overlap resulting in a clinically significant change in dosimetry, jaw misalignment should be corrected. If it cannot be corrected, part of the benefit of asymmetric collimation is lost and other methods of field junctioning may have to be considered. We routinely use a small block over the spinal cord at the mono-isocenter set-up plane for three-field head and neck treatments to prevent an overdose.
独立准直能够方便地实现相邻射野的拼接，且射野边缘无发散。当该技术用于多个射野的拼接时，如在头颈三野照射设置中的侧野和下前野，在拼接线处应实现剂量匹配，避免出现剂量过高或过低的情况。我们旨在评估中央匹配平面处的实际剂量学情况。
使用独立准直器模拟在中心轴处相邻的两个半束遮挡射野。在水等效模体中曝光X射线验证片，并通过激光密度计评估拼接线处的剂量。然后对准直器进行编程，使辐射野产生间隙或重叠，以评估在制造商规定公差范围内准直器错位的影响。还进行了射野边缘的二极管测量。评估了来自四台不同直线加速器的四种射束能量。
在所有测试的直线加速器中，沿拼接线均发现了小的系统性不均匀性。中心轴上的最大剂量随准直器的小错位呈线性变化。当准直器之间的间隙或重叠为2毫米时，拼接线剂量增加或减少30%至40%。拼接线周围剂量过高或过低的区域宽度为3至4毫米。准直器间距宽度与剂量改变区域宽度之间的差异可通过半影效应来解释。
我们建议定期评估独立准直器的对准情况，并提供一种简单方法来估计匹配平面处的剂量不均匀性。如果存在射野间隙或重叠导致剂量学出现临床显著变化，则应校正准直器错位。如果无法校正，则会失去不对称准直的部分优势，可能不得不考虑其他射野拼接方法。在头颈三野治疗的单等中心设置平面，我们常规在脊髓上方使用一个小铅块，以防止剂量过高。
