Chung Heeteak, Jin Hosang, Palta Jatinder, Suh Tae-Suk, Kim Siyong
Department of Nuclear and Radiological Engineering, University of Florida, Gainesville, 32611-8300, USA.
Phys Med Biol. 2006 Oct 7;51(19):4841-56. doi: 10.1088/0031-9155/51/19/008. Epub 2006 Sep 14.
Ever since the advent and development of treatment planning systems, the uncertainty associated with calculation grid size has been an issue. Even to this day, with highly sophisticated 3D conformal and intensity-modulated radiation therapy (IMRT) treatment planning systems (TPS), dose uncertainty due to grid size is still a concern. A phantom simulating head and neck treatment was prepared from two semi-cylindrical solid water slabs and a radiochromic film was inserted between the two slabs for measurement. Plans were generated for a 5,400 cGy prescribed dose using Philips Pinnacle(3) TPS for two targets, one shallow ( approximately 0.5 cm depth) and one deep ( approximately 6 cm depth). Calculation grid sizes of 1.5, 2, 3 and 4 mm were considered. Three clinical cases were also evaluated. The dose differences for the varying grid sizes (2 mm, 3 mm and 4 mm from 1.5 mm) in the phantom study were 126 cGy (2.3% of the 5,400 cGy dose prescription), 248.2 cGy (4.6% of the 5,400 cGy dose prescription) and 301.8 cGy (5.6% of the 5,400 cGy dose prescription), respectively for the shallow target case. It was found that the dose could be varied to about 100 cGy (1.9% of the 5,400 cGy dose prescription), 148.9 cGy (2.8% of the 5,400 cGy dose prescription) and 202.9 cGy (3.8% of the 5,400 cGy dose prescription) for 2 mm, 3 mm and 4 mm grid sizes, respectively, simply by shifting the calculation grid origin. Dose difference with a different range of the relative dose gradient was evaluated and we found that the relative dose difference increased with an increase in the range of the relative dose gradient. When comparing varying calculation grid sizes and measurements, the variation of the dose difference histogram was insignificant, but a local effect was observed in the dose difference map. Similar results were observed in the case of the deep target and the three clinical cases also showed results comparable to those from the phantom study.
自治疗计划系统问世和发展以来，与计算网格大小相关的不确定性一直是个问题。即便时至今日，对于高度复杂的三维适形和调强放射治疗（IMRT）治疗计划系统（TPS），因网格大小导致的剂量不确定性仍是一个令人担忧的问题。用两块半圆柱形固体水模体制作了一个模拟头颈部治疗的模体，并在两块模体之间插入一张放射变色胶片用于测量。使用飞利浦Pinnacle(3)TPS针对两个靶区生成了处方剂量为5400 cGy的计划，一个靶区较浅（深度约0.5 cm），另一个靶区较深（深度约6 cm）。考虑了1.5、2、3和4 mm的计算网格大小。还评估了三个临床病例。在模体研究中，对于不同网格大小（相对于1.5 mm的2 mm、3 mm和4 mm），浅靶区病例的剂量差异分别为126 cGy（占5400 cGy剂量处方的2.3%）、248.2 cGy（占5400 cGy剂量处方的4.6%）和301.8 cGy（占5400 cGy剂量处方的5.6%）。结果发现，仅通过移动计算网格原点，对于2 mm、3 mm和4 mm网格大小，剂量分别可变化至约100 cGy（占5400 cGy剂量处方的1.9%）、148.9 cGy（占5400 cGy剂量处方的2.8%）和202.9 cGy（占5400 cGy剂量处方的3.8%）。评估了不同相对剂量梯度范围下的剂量差异，发现相对剂量差异随相对剂量梯度范围的增加而增大。在比较不同计算网格大小与测量结果时，剂量差异直方图的变化不显著，但在剂量差异图中观察到了局部效应。在深靶区病例中也观察到了类似结果，并且三个临床病例的结果与模体研究的结果相当。
