Department of Radiation Oncology, Osaka University Graduate School of Medicine, 565-0871, Osaka, Japan.
Medical Radiation Physics, Department of Physics, Stockholm University, S-171 76, Stockholm, Sweden.
Med Phys. 2018 Mar;45(3):1210-1221. doi: 10.1002/mp.12749. Epub 2018 Jan 25.
Radiotherapy using grids containing cm-wide beam elements has been carried out sporadically for more than a century. During the past two decades, preclinical research on radiotherapy with grids containing small beam elements, 25 μm-0.7 mm wide, has been performed. Grid therapy with larger beam elements is technically easier to implement, but the normal tissue tolerance to the treatment is decreasing. In this work, a new approach in grid therapy, based on irradiations with grids containing narrow carbon-ion beam elements was evaluated dosimetrically. The aim formulated for the suggested treatment was to obtain a uniform target dose combined with well-defined grids in the irradiated normal tissue. The gain, obtained by crossfiring the carbon-ion beam grids over a simulated target volume, was quantitatively evaluated.
The dose distributions produced by narrow rectangular carbon-ion beams in a water phantom were simulated with the PHITS Monte Carlo code. The beam-element height was set to 2.0 cm in the simulations, while the widths varied from 0.5 to 10.0 mm. A spread-out Bragg peak (SOBP) was then created for each beam element in the grid, to cover the target volume with dose in the depth direction. The dose distributions produced by the beam-grid irradiations were thereafter constructed by adding the dose profiles simulated for single beam elements. The variation of the valley-to-peak dose ratio (VPDR) with depth in water was thereafter evaluated. The separation of the beam elements inside the grids were determined for different irradiation geometries with a selection criterion.
The simulated carbon-ion beams remained narrow down to the depths of the Bragg peaks. With the formulated selection criterion, a beam-element separation which was close to the beam-element width was found optimal for grids containing 3.0-mm-wide beam elements, while a separation which was considerably larger than the beam-element width was found advantageous for grids containing 0.5-mm-wide beam elements. With the single-grid irradiation setup, the VPDRs were close to 1.0 already at a distance of several cm from the target. The valley doses given to the normal tissue at 0.5 cm distance from the target volume could be limited to less than 10% of the mean target dose if a crossfiring setup with four interlaced grids was used.
The dose distributions produced by grids containing 0.5- and 3.0-mm wide beam elements had characteristics which could be useful for grid therapy. Grids containing mm-wide carbon-ion beam elements could be advantageous due to the technical ease with which these beams can be produced and delivered, despite the reduced threshold doses observed for early and late responding normal tissue for beams of millimeter width, compared to submillimetric beams. The treatment simulations showed that nearly homogeneous dose distributions could be created inside the target volumes, combined with low valley doses in the normal tissue located close to the target volume, if the carbon-ion beam grids were crossfired in an interlaced manner with optimally selected beam-element separations. The formulated selection criterion was found useful for the quantitative evaluation of the dose distributions produced by the different irradiation setups.
使用含有厘米宽射束单元的栅格进行放射治疗已经断断续续地进行了一个多世纪。在过去的二十年中，已经进行了含有小射束单元（25μm-0.7mm 宽）的栅格放射治疗的临床前研究。技术上更容易实现含有较大射束单元的栅格治疗，但治疗中正常组织的耐受度正在降低。在这项工作中，基于含有窄碳离子射束单元的栅格照射的新栅格治疗方法在剂量学上进行了评估。所提出的治疗方案的目标是在照射的正常组织中获得均匀的靶剂量和明确的栅格。通过在模拟靶区上交叉照射碳离子栅格获得的增益进行了定量评估。
使用 PHITS 蒙特卡罗代码模拟了水模体中窄矩形碳离子束产生的剂量分布。在模拟中，射束单元的高度设置为 2.0cm，而宽度从 0.5 到 10.0mm 不等。然后，在栅格中的每个射束单元中创建一个扩展布拉格峰（SOBP），以便在深度方向上用剂量覆盖靶区。此后，通过添加为单个射束单元模拟的剂量分布来构建由射束栅格照射产生的剂量分布。此后，评估了水深处谷峰剂量比（VPDR）随深度的变化。使用选择标准确定了不同照射几何形状下栅格内射束单元的分离。
模拟的碳离子束在布拉格峰的深度仍然保持狭窄。使用规定的选择标准，对于含有 3.0mm 宽射束单元的栅格，发现接近射束单元宽度的分离是最佳的，而对于含有 0.5mm 宽射束单元的栅格，发现明显大于射束单元宽度的分离是有利的。对于单个栅格照射设置，在距离靶区几个厘米处，VPDR 已经接近 1.0。如果使用四个交错栅格的交叉照射设置，则可以将靶区 0.5cm 距离处的正常组织的谷剂量限制在靶区平均剂量的 10%以下。
含有 0.5 和 3.0mm 宽射束单元的栅格产生的剂量分布具有可用于栅格治疗的特征。由于毫米宽度的射束可以更容易地产生和输送，因此含有毫米宽度碳离子射束单元的栅格可能具有优势，尽管与亚毫米宽度的射束相比，观察到早期和晚期反应正常组织的阈值剂量降低。治疗模拟表明，如果以最优选择的射束单元分离方式交错照射碳离子栅格，则可以在靶区内部创建几乎均匀的剂量分布，并在靠近靶区的正常组织中产生低谷剂量。所提出的选择标准对于评估不同照射设置产生的剂量分布是有用的。
