Department of Bioengineering, Stanford University, Stanford, California 94305, USA.
Med Phys. 2010 Feb;37(2):590-9. doi: 10.1118/1.3276738.
Methods used for small animal radiation treatment have yet to achieve the same dose targeting as in clinical radiation therapy. Toward understanding how to better plan small animal radiation using a system recently developed for this purpose, the authors characterized dose distributions produced from conformal radiotherapy of small animals in a microCT scanner equipped with a variable-aperture collimator.
Dose distributions delivered to a cylindrical solid water phantom were simulated using a Monte Carlo algorithm. Phase-space files for 120 kVp x-ray beams and collimator widths of 1-10 mm at isocenter were generated using BEAMnrc software, and dose distributions for evenly spaced beams numbered from 5 to 80 were generated in DOSXYZnrc for a variety of targets, including centered spherical targets in a range of sizes, spherical targets offset from centered by various distances, and various ellipsoidal targets. Dose distributions were analyzed using dose volume histograms. The dose delivered to a mouse bearing a spontaneous lung tumor was also simulated, and dose volume histograms were generated for the tumor, heart, left lung, right lung, and spinal cord.
Results indicated that for centered, symmetric targets, the number of beams required to achieve a smooth dose volume histogram decreased with increased target size. Dose distributions for noncentered, symmetric targets did not exhibit any significant loss of conformality with increasing offset from the phantom center, indicating sufficient beam penetration through the phantom for targeting superficial targets from all angles. Even with variable collimator widths, targeting of asymmetric targets was found to have less conformality than that of spherical targets. Irradiation of a mouse lung tumor with multiple beam widths was found to effectively deliver dose to the tumor volume while minimizing dose to other critical structures.
Overall, this method of generating and analyzing dose distributions provides a quantitative method for developing practical guidelines for small animal radiotherapy treatment planning. Future work should address methods to improve conformality in asymmetric targets.
用于小动物放射治疗的方法尚未达到与临床放射治疗相同的剂量靶向。为了了解如何更好地计划小动物放射治疗，作者使用为此目的开发的系统来描述配备可变孔径准直器的微 CT 扫描仪中小动物适形放疗产生的剂量分布。
使用蒙特卡罗算法模拟圆柱形实心水模体的剂量分布。使用 BEAMnrc 软件生成 120 kVp X 射线束的相空间文件和等中心处 1-10mm 的准直器宽度，并在 DOSXYZnrc 中为各种目标生成均匀间隔的光束数量从 5 到 80 的剂量分布，包括大小范围的中心球形目标、从中心偏移的各种距离的球形目标以及各种椭圆形目标。使用剂量体积直方图分析剂量分布。还模拟了携带自发性肺肿瘤的小鼠的剂量分布，并为肿瘤、心脏、左肺、右肺和脊髓生成了剂量体积直方图。
结果表明，对于中心对称的目标，随着目标尺寸的增加，实现平滑剂量体积直方图所需的光束数量减少。对于非中心对称的目标，随着从体模中心的偏移增加，剂量分布没有表现出任何明显的适形性损失，这表明光束穿透体模足以从各个角度靶向浅层目标。即使使用可变准直器宽度，也发现非对称目标的靶向适形性不如球形目标。用多种光束宽度照射小鼠肺肿瘤，发现可以有效地将剂量输送到肿瘤体积，同时将其他关键结构的剂量最小化。
总的来说，这种生成和分析剂量分布的方法为制定小动物放射治疗计划的实用指南提供了一种定量方法。未来的工作应该解决改善非对称目标适形性的方法。
