Svensson R, Asell M, Näfstadius P, Brahme A
Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, Sweden.
Phys Med Biol. 1998 May;43(5):1091-112. doi: 10.1088/0031-9155/43/5/004.
A new method for producing very narrow and intense 50 MV bremsstrahlung beams with a half-width as low as 35 mm at a distance of 1 m from the target is presented. Such a beam is well suited for intensity modulation using scanned photon beams. An algorithm has been developed to minimize the width of the bremsstrahlung beam generated in a multilayer target by varying the individual layer thicknesses and atomic numbers under given constraints on the total target thickness and the mean energy of the transmitted electrons. Under such constraints the narrowest possible bremsstrahlung beam is obtained with a target composed of layers of monotonically increasing atomic number starting with the lowest possible value at the entrance side where the electrons impinge. It is also shown that the narrowest photon beam profile is associated with the highest possible forward photon yield. To be able to use the optimized target clinically it is desirable to be able to collect and stop all the electrons that are transmitted through the target. The electrons are most efficiently collected if they are kept close together, i.e. by minimizing the multiple scatter of the electrons and consequently the half-width of the generated bremsstrahlung beam. This is achieved by a thin low-atomic-number target. A dedicated electron stopper has been developed and integrated with the purging magnet. When the electron stopper is combined with a purging magnet, a primary photon collimator and a multileaf collimator, almost all of the transmitted electrons and their associated bremsstrahlung contamination can effectively be collected. The narrow photon beams from thin low-atomic-number targets have the additional advantage of producing the hardest and most penetrative photon spectrum possible, which is ideal for treating large deep-seated tumours.
本文提出了一种产生极窄且高强度50兆伏轫致辐射束的新方法，在距靶1米处半高宽低至35毫米。这样的束非常适合用于扫描光子束的强度调制。已开发出一种算法，通过在给定的总靶厚度和透射电子平均能量的约束下改变各层厚度和原子序数，使多层靶中产生的轫致辐射束宽度最小化。在这种约束下，由原子序数从电子入射的入口侧最低可能值开始单调增加的层组成的靶可获得尽可能窄的轫致辐射束。还表明，最窄的光子束轮廓与最高的向前光子产额相关。为了能够在临床上使用优化后的靶，希望能够收集并阻止所有透射过靶的电子。如果电子保持靠近，即通过最小化电子的多次散射以及由此产生的轫致辐射束的半高宽，就能最有效地收集电子。这可通过薄的低原子序数靶来实现。已开发出一种专用电子阻挡器并将其与清除磁铁集成在一起。当电子阻挡器与清除磁铁、初级光子准直器和多叶准直器结合使用时，几乎所有透射电子及其相关的轫致辐射污染都能被有效收集。来自薄的低原子序数靶的窄光子束还有一个额外优势，即能产生尽可能硬且穿透性最强的光子能谱，这对于治疗大型深部肿瘤非常理想。
