Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, P.O. Box 260, SE-171 76 Stockholm, Sweden.
Med Phys. 2011 Aug;38(8):4774-84. doi: 10.1118/1.3615059.
Since the first publications on intensity modulated radiation therapy (IMRT) in the early 1980s almost all efforts have been focused on fairly time consuming dynamic or segmental multileaf collimation. With narrow fast scanned photon beams, the flexibility and accuracy in beam shaping increases, not least in combination with fast penumbra trimming multileaf collimators. Previously, experiments have been performed with full range targets, generating a broad bremsstrahlung beam, in combination with multileaf collimators or material compensators. In the present publication, the first measurements with fast narrow high energy (50 MV) scanned photon beams are presented indicating an interesting performance increase even though some of the hardware used were suboptimal.
Inverse therapy planning was used to calculate optimal scanning patterns to generate dose distributions with interesting properties for fast IMRT. To fully utilize the dose distributional advantages with scanned beams, it is necessary to use narrow high energy beams from a thin bremsstrahlung target and a powerful purging magnet capable of deflecting the transmitted electron beam away from the generated photons onto a dedicated electron collector. During the present measurements the scanning system, purging magnet, and electron collimator in the treatment head of the MM50 racetrack accelerator was used with 3-6 mm thick bremsstrahlung targets of beryllium. The dose distributions were measured with diodes in water and with EDR2 film in PMMA. Monte Carlo simulations with GEANT4 were used to study the influence of the electrons transmitted through the target on the photon pencil beam kernel.
The full width at half-maximum (FWHM) of the scanned photon beam was 34 mm measured at isocenter, below 9.5 cm of water, 1 m from the 3 mm Be bremsstrahlung target. To generate a homogeneous dose distribution in a 10 x 10 cm2 field, the authors used a spot matrix of 100 equal intensity beam spots resulting in a uniformity of collimated 80%-20% penumbra of 9 mm at a primary electron energy of 50 MeV. For the more complex cardioid shaped dose distribution, they used 270 spots, which at a pulse repetition frequency of 200 Hz is completed every 1.36 s.
The present measurements indicate that the use of narrow scanned photon beams is a flexible and fast method to deliver advanced intensity modulated beams. Fast scanned photon IMRT should, therefore, be a very interesting modality in the delivery of biologically optimized radiation therapy with the possibility for in vivo treatment verification with PET-CT imaging.
自 20 世纪 80 年代初期首次发表关于强度调制放射治疗（IMRT）的论文以来，几乎所有的努力都集中在相当耗时的动态或分段多叶准直上。随着窄快扫描光子束的应用，射束成形的灵活性和准确性得到了提高，尤其是与快速半影修剪多叶准直器结合使用时。此前，已经使用全范围靶标进行了实验，生成了宽 Bremsstrahlung 束，并结合多叶准直器或材料补偿器使用。在本出版物中，首次展示了使用快速窄高能（50MV）扫描光子束的测量结果，表明即使使用了一些非最优的硬件，也能获得有趣的性能提升。
采用逆治疗计划来计算最优的扫描模式，以生成具有快速 IMRT 特性的剂量分布。为了充分利用扫描束的剂量分布优势，需要使用来自薄 Bremsstrahlung 靶和强大的净化磁铁的窄高能束，该磁铁能够将透射电子束从生成的光子上偏转开，射向专用电子收集器。在本测量中，使用 MM50 赛道加速器治疗头中的扫描系统、净化磁铁和电子准直器，以及 3-6mm 厚的铍 Bremsstrahlung 靶。剂量分布用水中的二极管和 PMMA 中的 EDR2 胶片进行测量。使用 GEANT4 进行蒙特卡罗模拟，研究透射靶的电子对光子铅笔束核的影响。
在离 3mmBe Bremsstrahlung 靶 1 米、水深低于 9.5cm 处，测量得到扫描光子束的半高全宽（FWHM）为 34mm。为了在 10x10cm2 射野中生成均匀的剂量分布，作者使用了 100 个强度相等的光斑矩阵，在初级电子能量为 50MeV 时，得到了 9mm 的准直 80%-20%半影均匀度。对于更为复杂的心形剂量分布，使用了 270 个光斑，在 200Hz 的脉冲重复频率下，每 1.36s 完成一次照射。
本测量表明，使用窄扫描光子束是一种灵活快速的方法，可以提供先进的强度调制射束。因此，快速扫描光子 IMRT 应该是一种非常有趣的治疗方式，可以实现生物学优化放射治疗，并有可能通过 PET-CT 成像进行体内治疗验证。
