Chen Mingli, Chao Edward, Lu Weiguo
21st Century Oncology, Madison, WI 53719, USA.
Med Dosim. 2013 Autumn;38(3):280-6. doi: 10.1016/j.meddos.2013.02.009. Epub 2013 Apr 1.
Megavoltage computed tomography (MVCT) is used as image guidance for patient setup in almost every tomotherapy treatment. Frequent use of ionizing radiation for image guidance has raised concern of imaging dose. The purpose of this work is to quantify and characterize tomotherapy MVCT dosimetry. Our dose calculation was based on a commissioned dose engine, and the calculation result was compared with film measurement. We studied dose profiles, center dose, maximal dose, surface dose, and mean dose on homogeneous cylindrical water phantoms of various diameters for various scanning parameters, including 3 different jaw openings (of nominal value J4, J1, and J0.1) and couch speeds (fine, normal, and coarse). The comparison between calculation and film measurement showed good agreement. In particular, the thread pattern on the film of the helical delivery matched very well with calculation. For the J1 jaw and coarse imaging mode, the maximum difference between calculation and measurement was about 6% of the center dose. Calculation on various sizes of synthesized phantoms showed that the center dose decreases almost linearly as the phantom diameter increases, and that the fine mode (couch speed of 4mm/rotation) received twice the dose of the normal mode (couch speed of 8mm/rotation) and 3 times that of the coarse mode (couch speed of 12mm/rotation) as expected. The maximal dose ranged from 100% to ∼200% of the center dose, with increasing ratios for larger phantoms, smaller jaws, and faster couch speed. For all jaw settings and couch speeds, the mean dose and average surface dose vary from 95% to 125% of the center dose with increasing ratios for larger phantoms. We present a quantitative dosimetric characterization of the tomotherapy MVCT in terms of scanning parameters, phantom size, center dose, maximal dose, surface dose, and mean dose. The results can provide an overall picture of dose distribution and a reference data set that enables estimation of CT dose index for the tomotherapy MVCT.
在几乎所有的断层放射治疗中，兆伏级计算机断层扫描（MVCT）都被用作患者摆位的图像引导。频繁使用电离辐射进行图像引导引发了对成像剂量的担忧。这项工作的目的是对断层放射治疗MVCT剂量学进行量化和特征描述。我们的剂量计算基于一个已调试的剂量引擎，并将计算结果与胶片测量结果进行比较。我们研究了在各种直径的均匀圆柱形水模体上，针对不同扫描参数（包括3种不同的准直器开口尺寸（标称值J4、J1和J0.1）以及治疗床速度（精细、正常和粗糙））的剂量分布、中心剂量、最大剂量、表面剂量和平均剂量。计算结果与胶片测量结果之间的比较显示出良好的一致性。特别是，螺旋照射时胶片上的条纹图案与计算结果非常吻合。对于J1准直器开口和粗糙成像模式，计算结果与测量结果之间的最大差异约为中心剂量的6%。对各种尺寸的合成模体进行计算表明，中心剂量几乎随着模体直径的增加呈线性下降，并且正如预期的那样，精细模式（治疗床速度为4毫米/旋转）所接受的剂量是正常模式（治疗床速度为8毫米/旋转）的两倍，是粗糙模式（治疗床速度为12毫米/旋转）的三倍。最大剂量范围为中心剂量的100%至约200%，随着模体尺寸增大、准直器开口变小以及治疗床速度加快，该比例增加。对于所有的准直器设置和治疗床速度，平均剂量和平均表面剂量在中心剂量的95%至125%之间变化，随着模体尺寸增大，该比例增加。我们从扫描参数、模体尺寸、中心剂量、最大剂量、表面剂量和平均剂量等方面对断层放射治疗MVCT进行了定量剂量学特征描述。这些结果可以提供剂量分布的总体情况以及一个参考数据集，从而能够估算断层放射治疗MVCT的CT剂量指数。
