Ahmed Shahbaz, Brown Derek, Ahmed Saad B S, Kakakhel Muhammad B, Muhammad Wazir, Hussain Amjad
Pakistan Institute of Engineering and Applied Sciences (PIEAS).
J Appl Clin Med Phys. 2016 Mar 8;17(2):112-122. doi: 10.1120/jacmp.v17i2.5951.
A revised translating bed total body irradiation (TBI) technique is developed for shielding organs at risk (lungs) to tolerance dose limits, and optimizing dose distribution in three dimensions (3D) using an asymmetrically-adjusted, dynamic multileaf collimator. We present a dosimetric comparison of this technique with a previously developed symmetric MLC-based TBI technique. An anthropomor-phic RANDO phantom is CT scanned with 3 mm slice thickness. Radiological depths (RD) are calculated on individual CT slices along the divergent ray lines. Asymmetric MLC apertures are defined every 9 mm over the phantom length in the craniocaudal direction. Individual asymmetric MLC leaf positions are optimized based on RD values of all slices for uniform dose distributions. Dose calculations are performed in the Eclipse treatment planning system over these optimized MLC apertures. Dose uniformity along midline of the RANDO phantom is within the confidence limit (CL) of 2.1% (with a confidence probability p = 0.065). The issue of over- and underdose at the interfaces that is observed when symmetric MLC apertures are used is reduced from more than ± 4% to less than ± 1.5% with asymmetric MLC apertures. Lungs are shielded by 20%, 30%, and 40% of the prescribed dose by adjusting the MLC apertures. Dose-volume histogram analysis confirms that the revised technique provides effective lung shielding, as well as a homogeneous dose coverage to the whole body. The asymmetric technique also reduces hot and cold spots at lung-tissue interfaces compared to previous symmetric MLC-based TBI technique. MLC-based shielding of OARs eliminates the need to fabricate and setup cumbersome patient-specific physical blocks.
开发了一种改进的平移式全身照射（TBI）技术，用于将危及器官（肺部）的剂量屏蔽至耐受剂量限值，并使用不对称调整的动态多叶准直器优化三维（3D）剂量分布。我们将该技术与先前开发的基于对称多叶准直器的TBI技术进行了剂量学比较。使用3毫米层厚对一个拟人化的RANDO体模进行CT扫描。沿发散射线在各个CT切片上计算放射学深度（RD）。在体模的头脚方向上每隔9毫米定义不对称的多叶准直器孔径。根据所有切片的RD值优化各个不对称多叶准直器叶片的位置，以实现均匀的剂量分布。在Eclipse治疗计划系统中对这些优化后的多叶准直器孔径进行剂量计算。RANDO体模中线处的剂量均匀性在2.1%的置信限（CL）内（置信概率p = 0.065）。使用对称多叶准直器孔径时在界面处观察到的剂量过高和过低问题，使用不对称多叶准直器孔径时从超过±4%降低到了小于±1.5%。通过调整多叶准直器孔径，肺部可被屏蔽掉20%、30%和40%的处方剂量。剂量体积直方图分析证实，改进后的技术可有效屏蔽肺部，同时为全身提供均匀的剂量覆盖。与先前基于对称多叶准直器的TBI技术相比，不对称技术还减少了肺组织界面处的热点和冷点。基于多叶准直器的危及器官屏蔽消除了制作和设置繁琐的患者个体化物理挡块的需求。
