Svensson R, Lind B, Brahme A
Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, Sweden.
Med Phys. 1998 Dec;25(12):2358-69. doi: 10.1118/1.598446.
Not until the last decade has flexible intensity modulated three-dimensional dose delivery techniques with photon beams become a clinical reality, first in the form of heavy metal transmission blocks and other beam compensators, then in dynamic and segmented multileaf collimation, and most recently by scanning high-energy narrow electron and photon beams. The merits of various treatment unit and bremsstrahlung target designs for high-energy photon therapy are investigated theoretically for two clinically relevant target sites, a cervix and a larynx cancer both in late stages. With an optimized bremsstrahlung target it is possible to generate photon beams with a half-width of about 3 cm at a source to axis distance (SAD) of 100 cm and an initial electron energy of 50 MeV. By making a more compact treatment head and shortening the SAD, it is possible to reduce the half-width even further to about 2 cm at a SAD of 70 cm and still have sufficient clearance between the collimator head and the patient. One advantage of a reduced SAD is that the divergence of the beam for a given field size on the patient is increased, and thus the exit dose is lowered by as much as 1%/cm of the patient cross section. A second advantage of a reduced SAD is that the electron beam on the patient surface will be only about 8 mm wide and very suitable for precision spot beam scanning. It may also be possible to reduce the beamwidth further by increasing the electron energy up to about 60 MeV to get a photon beam of around 15 mm half-width and an electron beam as narrow as 5 mm. The compact machine will be more efficient and easy to work with, due to the small gantry and the reduced isocentric height. For a given target volume and optimally selected static multileaf collimator, it is no surprise that the narrowest possible scanned elementary bremsstrahlung beam generates the best possible treatment outcome. In fact, by delivering a few static field segments with individually optimized scan patterns, it is possible to combine the advantage of being able to fine tune the fluence distribution by the scanning system with the steeper dose gradients that can be delivered by a few static multileaf collimator segments. It is demonstrated that in most cases a few collimator segments are sufficient and often a single segment per beam portal may suffice when narrow scanned photon beams are employed, and they can be delivered sequentially with a negligible time delay. A further advantage is the increase of therapeutically useful photons and improved patient protection, since the pencil beam is only scanned where the leaf collimator is open. Consequently, some of the problems associated with dynamic multileaf collimation such as the tongue and groove and edge leakage effects are significantly reduced. Fast scanning beam techniques combined with good treatment verification systems allow interesting future possibilities to counteract patient and internal organ motions in real time.
直到最近十年，基于光子束的灵活强度调制三维剂量输送技术才成为临床现实，最初是通过重金属透射挡块和其他射束补偿器的形式，然后是动态和分段多叶准直，最近则是通过扫描高能窄电子束和光子束。针对两个临床相关的靶区，即晚期宫颈癌和喉癌，从理论上研究了用于高能光子治疗的各种治疗单元和轫致辐射靶设计的优点。使用优化的轫致辐射靶，在源轴距（SAD）为100 cm且初始电子能量为50 MeV时，可以产生半高宽约为3 cm的光子束。通过制造更紧凑的治疗头并缩短源轴距，可以在源轴距为70 cm时将半高宽进一步减小至约2 cm，并且在准直器头与患者之间仍有足够的间隙。减小源轴距的一个优点是，对于患者身上给定的射野尺寸，射束的发散度增加，因此出射剂量可降低多达1%/cm的患者横截面。减小源轴距的第二个优点是，患者表面的电子束宽度仅约为8 mm，非常适合精确的点束扫描。通过将电子能量提高到约60 MeV，还可能进一步减小束宽，以获得半高宽约为15 mm的光子束和窄至5 mm的电子束。由于机架较小且等中心高度降低，紧凑型机器将更高效且易于操作。对于给定的靶体积和最佳选择的静态多叶准直器，毫不奇怪，尽可能窄的扫描基本轫致辐射束能产生最佳的治疗效果。实际上，通过以单独优化的扫描模式输送几个静态野段，可以将扫描系统能够微调注量分布的优点与几个静态多叶准直器段能够提供的更陡剂量梯度相结合。结果表明，在大多数情况下，当使用窄扫描光子束时，几个准直器段就足够了，而且每个射野入口通常一个段就足够了，并且它们可以以可忽略的时间延迟顺序输送。另一个优点是治疗有用光子的增加和患者防护的改善，因为笔形束仅在叶准直器打开的地方进行扫描。因此，与动态多叶准直相关的一些问题，如舌槽效应和边缘泄漏效应，会显著减少。快速扫描束技术与良好的治疗验证系统相结合，为未来实时抵消患者和内部器官运动带来了有趣的可能性。
