Disher B, Hajdok G, Gaede S, Battista J J
Department of Medical Biophysics, Western University.
Physics and Engineering Department, London Regional Cancer Program, London, Ontario.
Med Phys. 2012 Jul;39(7Part2):4622. doi: 10.1118/1.4740101.
Stereotactic body radiation therapy(SBRT), a technique that uses tightly conformed Megavoltage(MV) x-ray fields, improves local control of lung cancer. However, small MV x-ray fields can cause lateral electron disequilibrium(LED), which reduces the dose within lung. These effects are difficult to predict and are presently a cause of alarm for the radiotherapy community. Previously, we developed The Relative Depth Dose Factor(RDDF), which is an indicator of the extent of LED (RDDF < 1). We propose a positive application of LED for lung sparing in SBRT: LED can be exploited to irradiate a small tumor while greatly reducing the dose in surrounding lung tissue. The Monte Carlo code, DOSXYZnrc, was employed to calculate dose within a cylindrical lung phantom. The phantom's diameter and height were set to 25 cm, and consisted of water and lung (density = 0.25g/cm ) shells surrounding a small water tumor (volume = 0.8 cm ). Two 180° 6MV arcs were focused onto the tumor with field sizes of 1×1cm (RDDF∼0.5) and 3×3cm (RDDF∼1). Analyzing dose results, the 1×1cm arc reduced dose within lung and water tissues by 70% and 80% compared to the 3×3cm arc. Although, central tumor dose was also reduced by 15% using the 1×1cm arc, these reductions can be offset by escalating the prescription dose appropriately. Using the RDDF as a guideline, it's possible to design a SBRT treatment plan that reduces lung dose while maintaining relatively high tumor dose levels. Clinical application requires an accurate dose algorithm and may lower SBRT dose-induced toxicity levels in patients.
立体定向体部放射治疗（SBRT）是一种使用紧密适形的兆伏级（MV）X射线束的技术，可提高肺癌的局部控制率。然而，小MV X射线束会导致侧向电子不平衡（LED），从而降低肺内剂量。这些效应难以预测，目前是放疗界担忧的一个原因。此前，我们开发了相对深度剂量因子（RDDF），它是LED程度的一个指标（RDDF < 1）。我们提出在SBRT中对LED进行积极应用以实现肺组织保护：可以利用LED照射小肿瘤，同时大幅降低周围肺组织的剂量。使用蒙特卡罗代码DOSXYZnrc计算圆柱形肺模体内的剂量。模体的直径和高度设定为25 cm，由水和肺（密度 = 0.25g/cm³）外壳围绕一个小的水肿瘤（体积 = 0.8 cm³）组成。两个180°的6MV弧形束聚焦于肿瘤，射野大小分别为1×1cm（RDDF约为0.5）和3×3cm（RDDF约为1）。分析剂量结果，与3×3cm的弧形束相比，1×1cm的弧形束使肺组织和水组织内的剂量分别降低了70%和80%。虽然使用1×1cm的弧形束时中心肿瘤剂量也降低了15%，但这些降低可通过适当提高处方剂量来抵消。以RDDF为指导原则，有可能设计出一种SBRT治疗计划，在保持较高肿瘤剂量水平的同时降低肺剂量。临床应用需要精确的剂量算法，并且可能降低患者中SBRT剂量诱导的毒性水平。
