For stereotactic irradiation, both, penumbra and MLC leaf width make an impact on the sparing of healthy tissue around the target. Mostly, MLC design is regarded as the one influenceable parameter. However, also penumbra can be varied by choosing different distances between the source of radiation and the patient. The authors investigate the distance- dependent penumbra effects of idealized collimators as well as for real 5 mm MLCs. Test objects are small spherical targets of varying diameters to be irradiated under differing prescription conditions. A method to calculate exact stereotactic radial dose distributions from beam profiles or 2D dose distributions of single beams is developed for circular and MLC shaped targets. Also, a planning study is performed using a Pinnacle3? planning system. Also, in a theoretical analysis perfect top hat profile beams and beams with varying penumbra are compared for better understanding of penumbra effects with respect to radial dose distributions. It is shown, that the penumbra changes for small targets are more relevant than the beam shaping by 5 mm MLCs. Quasi-isotropic irradiated MLC shaped (quadratic) beams at virtual SAD 700 mm produce steeper radial dose decrease than ideal circular beam shapes with a penumbra typical for SAD 1000 mm. A reduced source-to-patient distance allows better sparing of healthy tissue because of two reasons: The smaller effective leaf width but even more due to steeper penumbra. First, the authors suggest for future recommendations on stereotactic irradiations to specify not only MLC widths but also penumbra characteristics. Second, a so-called “virtual isocentre” could be useful to take advantage of the penumbra effect: Dependent on gantry angle and isocentric couch angle, the couch should be steered automatically in a way that the central axes of all beams always intersect in the same point at the same distance from the source.
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机译：对于立体定向照射，半影和MLC叶的宽度都会影响靶周围健康组织的保留。通常，MLC设计被视为一个可影响的参数。但是，也可以通过选择辐射源和患者之间的不同距离来改变半影。作者研究了理想准直器以及实际5 mm MLC的距离相关半影效应。测试对象是直径不同的小球形目标，需要在不同的处方条件下进行辐照。针对圆形和MLC形状的目标，开发了一种从光束轮廓或单光束的2D剂量分布计算精确的立体定向径向剂量分布的方法。另外，使用Pinnacle3？进行计划研究。规划系统。而且，在理论分析中，比较了理想的高顶轮廓光束和具有不同半影的光束，以更好地了解半影效果对径向剂量分布的影响。结果表明，小目标的半影变化比5 mm MLC的光束整形更重要。在虚拟SAD为700 mm时，准各向同性辐照的MLC形（二次）光束产生的径向剂量减少要比理想半圆形（典型值为1000 mm SAD的半圆形）光束更陡。减少源到患者的距离可以更好地保留健康的组织，这有两个原因：有效叶宽较小，但由于半影较陡，甚至更大。首先，作者提出了有关立体定向辐射的未来建议，不仅要规定MLC宽度，还要规定半影特征。其次，所谓的“虚拟等中心点”可能会利用半影效应：取决于龙门角度和等中心卧榻角度，应以所有光束的中心轴始终相交的方式自动转向卧榻。距源相同距离的相同点。
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