Evaluation of dosimetric margins in prostate IMRT treatment plans
Evaluation of dosimetric margins in prostate IMRT treatment plans
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DOI：
10.1118/1.2826558
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发表时间：
2008-02-01
期刊：
影响因子：
3.8
通讯作者：
Siebers, J. V.
中科院分区：
文献类型：
--
作者：
Gordon, J. J.;Siebers, J. V.
这项工作引入了一个新概念——剂量裕度分布 (DMD)——并用它来解释一组前列腺 IMRT 治疗计划对患者设置错误的敏感性。之前的工作模拟了设置误差对 27 个前列腺 IMRT 治疗计划的影响，发现这些计划可以容忍比 van Herk 裕度公式预测的更大的设置误差。造成这种分歧的推测原因是范赫克假设的错误，即计划的剂量分布完全符合目标结构。为了解决分歧，这项工作采用了相同的 27 个计划来评估以下之间存在的实际裕度分布：(i) 临床目标体积 (CTV) 和计划目标体积 (PTV) 以及 (ii) CTV 和 PTV 最小剂量等剂量表面。针对前列腺和淋巴结目标评估这些分布。分布 (ii) 是 DMD。给定方向上的剂量裕度决定了 CTV 由于该方向上的设置错误而剂量不足的概率。对超过 4 pi sr 进行平均可得出 CTV 覆盖的总体概率。前列腺和淋巴结 PTV 的最小剂量从剂量体积直方图中获得。创建相应的等剂量表面并将其转换为感兴趣区域 (ROI)。 CTV、PTV 和等剂量 ROI 保存为网格文件，然后导入到计算几何应用程序中，该应用程序计算 614 个离散方向上的网格之间的距离（即边缘），覆盖 4 pi sr，增量为 10 度。测量的前列腺 CTV 至 PTV 边缘接近治疗计划方案中指定的 0.5 cm 标称值。然而，根据方向的不同，前列腺剂量裕度范围为 0.5 至 3 厘米，反映了计划剂量分布与前列腺 PTV 的不完全一致性。对于淋巴结 CTV，治疗计划中采用的标称 CTV 与 PTV 的间隙再次为 0.5 cm。然而，由于规划协议，节点 PTV 在多个位置遵循节点 CTV 的表面，确保节点 PTV 内部没有节点 CTV 的刚体运动空间。因此，测量的节点 CTV 至 PTV 裕度为零，而节点剂量裕度范围为 0.2 至 2.8 cm。发现前列腺和淋巴结目标覆盖范围与测量的 DMD 密切相关，从而解决了与我们之前结果的明显不一致。主要结论是，存在设置误差时的目标覆盖范围应使用 DMD 进行评估，而不是使用 CTV 到 PTV 的裕度分布。 DMD 是一个有用的规划指标，它概括了 ICRU 合格指数。 DMD 可能因光束数量、光束排列、TPS 和治疗部位而异。 (C) 2008 年美国医学物理学家协会。
This work introduces a new concept-the dosimetric margin distribution (DMD)-and uses it to explain the sensitivity of a group of prostate IMRT treatment plans to patient setup errors. Prior work simulated the effect of setup errors on 27 prostate IMRT treatment plans and found the plans could tolerate larger setup errors than predicted by the van Herk margin formula. The conjectured reason for this disagreement was a breakdown in van Herk's assumption that the planned dose distribution conforms perfectly to target structures. To resolve the disagreement, this work employed the same 27 plans to evaluate the actual margin distributions that exist between: (i) the clinical target volume (CTV) and planning target volume (PTV) and (ii) the CTV and PTV minimum dose isodose surface. These distributions were evaluated for both prostate and nodal targets. Distribution (ii) is the DMD. The dosimetric margin in a given direction determines the probability that the CTV will be underdosed due to setup errors in that direction. Averaging over 4 pi sr gives the overall probability of CTV coverage. Minimum doses for prostate and nodal PTVs were obtained from dose volume histograms. Corresponding isodose surfaces were created and converted to regions of interest (ROIs). CTV, PTV, and isodose ROIs were saved as mesh files and then imported into a computational geometry application which calculated distances between meshes (i.e., margins) in 614 discrete directions covering 4 pi sr in 10 deg increments. Measured prostate CTV-to-PTV margins were close to the nominal value of 0.5 cm specified in the treatment planning protocol. However, depending on direction, prostate dosimetric margins ranged from 0.5 to 3 cm, reflecting the imperfect conformance of the planned dose distribution to the prostate PTV. For the nodal CTV, the nominal CTV-to-PTV margin employed in treatment planning was again 0.5 cm. However, due to the planning protocol, the nodal PTV follows the surface of the nodal CTV in several places, ensuring that there is no room for rigid body motion of the nodal CTV inside the nodal PTV. Measured nodal CTV-to-PTV margins were therefore zero, while nodal dosimetric margins ranged from 0.2 to 2.8 cm. Prostate and nodal target coverage were found to be well correlated with the measured DMDs, thereby resolving the apparent disagreement with our prior results. The principal conclusion is that target coverage in the presence of setup errors should be evaluated using the DMD, rather than the CTV-to-PTV margin distribution. The DMD is a useful planning metric, which generalizes the ICRU conformity index. DMDs could vary with number of beams, beam arrangements, TPS, and treatment site. (C) 2008 American Association of Physicists in Medicine.