Markovic M, Stathakis S, Mavroidis P, Gutierrez A, Esquivel C, Papanikolau N
Dept. Of Radiological Studies, University of Texas Helath Science Center, San Antonio, TX.
Dept. of Medical Physics, Larissa University Hospital, Larissa, Greece.
Med Phys. 2012 Jun;39(6Part19):3844. doi: 10.1118/1.4735690.
The aim of this study is to compare the plan results that are obtained by using different calculation grid sizes ranging from 0.15 to 0.50 cm, and the same dose calculation algorithm (Superposition), in Intensity Modulated Radiotherapy (IMRT) for different treatment sites. Results are then used to study the suitability of dose grid size with respect to site.
For each of the calculation grid sizes, three different sites; namely, Lung, Prostate, and Head and Neck were analyzed. Treatment plans were created using 6MV photon beam quality and IMRT technique on the CMS XiO (Computerized Medical System, St.Louis, MO) treatment planning system. Dose volume histograms were generated for each of the cases and statistical analysis performed included mean relative difference and Homogeneity Index for target structures. Comparison was done first by using 0.30 cm calculation grid as a golden standard and keeping the same number of monitor units (MUs) per beam for each grid size, then the second part involved renormalizing plans to have the same target coverage (100% of the prescription dose covering at least 95% of the target volume) for each grid size used. Future study plans include treatment plans delivery on Varian 21 EX linear accelerator with Millennium (120) MLC and their verification with the Sun Nuclear Mapcheck 2D array. To increase the diode array resolution, 2D array will be shifted in 1 mm increments in x and y direction. Measured fields will be merged using Sun Nuclear Files Combined function and compared with intensity maps exported from the CMS XiO treatment planning system calculated with minimum segment size of 1 cm.
The maximum percentage of variation recorded between calculation grid sizes used was in the case of the Head and Neck treatments. For the lung and prostate cases there was little variation in the results based on the calculation grid size chosen, specifically between 0.30, 0.20 and 0.15 cm. However head and neck and prostate cases with nodal involvement showed significant variation in the dosimetric results based on the grid size chosen. Overall results vary from case to case and also depend on the plan complexity. For larger treatment areas calculating with the grid size smaller than 0.30 cm may be impossible as time needed for calculation rises exponentially with the field size involved.
IMRT places a higher requirement on dose grid resolution than conventional radiation therapy. While 0.30-0.40 cm grid was assumed adequate for conformal treatment planning, smaller dose grid is required at least in the areas of high dose. In the cases where steep dose gradients exist smaller grid size should be used while calculating and evaluating treatment plans, as the choice of the calculation grid size may in certain cases even influence clinical results.
本研究旨在比较在不同治疗部位的调强放射治疗（IMRT）中，使用从0.15至0.50厘米不等的不同计算网格尺寸以及相同剂量计算算法（叠加法）所获得的计划结果。然后将结果用于研究剂量网格尺寸相对于部位的适用性。
对于每个计算网格尺寸，分析了三个不同部位，即肺部、前列腺以及头颈部。在CMS XiO（计算机医学系统，密苏里州圣路易斯）治疗计划系统上，使用6MV光子束质量和IMRT技术创建治疗计划。为每个病例生成剂量体积直方图，进行的统计分析包括靶区结构的平均相对差异和均匀性指数。首先以0.30厘米计算网格作为黄金标准进行比较，并且为每个网格尺寸的每束射线保持相同数量的监测单位（MU），然后第二部分涉及对计划进行重新归一化，以使每个使用的网格尺寸具有相同的靶区覆盖度（处方剂量的100%覆盖至少95%的靶区体积）。未来的研究计划包括在配备Millennium（120）多叶准直器的Varian 21 EX直线加速器上进行治疗计划投照，并使用Sun Nuclear Mapcheck 2D阵列进行验证。为了提高二极管阵列分辨率，2D阵列将在x和y方向上以1毫米的增量移动。使用Sun Nuclear Files Combined功能合并测量野，并与从CMS XiO治疗计划系统导出的、最小分割尺寸为1厘米计算得到的强度图进行比较。
在所使用的计算网格尺寸之间记录到的最大变化百分比出现在头颈部治疗病例中。对于肺部和前列腺病例，基于所选的计算网格尺寸，结果几乎没有变化，特别是在0.30厘米、0.20厘米和0.15厘米之间。然而，有淋巴结受累的头颈部和前列腺病例，基于所选网格尺寸，剂量学结果显示出显著变化。总体结果因病例而异，并且还取决于计划的复杂性。对于较大治疗区域，使用小于0.30厘米的网格尺寸进行计算可能是不可能的，因为计算所需时间会随着所涉及的射野大小呈指数增长。
IMRT对剂量网格分辨率的要求高于传统放射治疗。虽然0.30 - 0.40厘米的网格被认为足以进行适形治疗计划，但至少在高剂量区域需要更小的剂量网格。在存在陡峭剂量梯度的情况下，在计算和评估治疗计划时应使用更小的网格尺寸，因为计算网格尺寸的选择在某些情况下甚至可能影响临床结果。
