Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, USA.
Med Phys. 2010 Mar;37(3):1210-9. doi: 10.1118/1.3314073.
To investigate the effect of monitor unit (MU) constraints on the dose distribution created by intensity modulated proton therapy (IMPT) treatment planning using single-field optimization (SFO).
Ninety-four energies between 72.5 and 221.8 MeV are available for scanning beam IMPT delivery at our institution. The minimum and maximum MUs for delivering each pencil beam (spot) are 0.005 and 0.04, respectively. These MU constraints are not considered during optimization by the treatment planning system; spots are converted to deliverable MUs during postprocessing. Treatment plans for delivering uniform doses to rectangular volumes with and without MU constraints were generated for different target doses, spot spacings, spread-out Bragg peak (SOBP) widths, and ranges in a homogeneous phantom. Four prostate cancer patients were planned with and without MU constraints using different spot spacings. Rounding errors were analyzed using an in-house software tool.
From the phantom study, the authors have found that both the number of spots that have rounding errors and the magnitude of the distortion of the dose distribution from the ideally optimized distribution increases as the field dose, spot spacing, and range decrease and as the SOBP width increases. From our study of patient plans, it is clear that as the spot spacing decreases the rounding error increases, and the dose coverage of the target volume becomes unacceptable for very small spot spacings.
Constraints on deliverable MU for each spot could create a significant distortion from the ideally optimized dose distributions for IMPT fields using SFO. To eliminate this problem, the treatment planning system should incorporate the MU constraints in the optimization process and the delivery system should reliably delivery smaller minimum MUs.
研究在使用单野优化（SFO）的强度调制质子治疗（IMPT）计划中，监视器单位（MU）限制对剂量分布的影响。
在我们的机构中，可用于扫描束 IMPT 输送的能量为 72.5 至 221.8 MeV 之间的 94 种。每个铅笔束（点）的最小和最大 MU 分别为 0.005 和 0.04。在治疗计划系统的优化过程中不考虑这些 MU 限制；在后期处理中，将点转换为可输送的 MU。在均匀剂量输送到具有和不具有 MU 限制的矩形体积的情况下，为不同的靶剂量、点间距、扩展布拉格峰（SOBP）宽度和均匀体中的射程生成了治疗计划。使用不同的点间距，对 4 名前列腺癌患者进行了有和没有 MU 限制的计划。使用内部软件工具分析了舍入误差。
从体模研究中，作者发现，具有舍入误差的点的数量以及剂量分布从理想优化分布的失真程度都随着场剂量、点间距、射程减小以及 SOBP 宽度增加而增加。从我们对患者计划的研究中可以清楚地看出，随着点间距的减小，舍入误差增加，并且对于非常小的点间距，靶体积的剂量覆盖变得不可接受。
对于使用 SFO 的 IMPT 场，每个点的可输送 MU 限制可能会导致与理想优化剂量分布产生显著失真。为了消除这个问题，治疗计划系统应该在优化过程中包含 MU 限制，并且输送系统应该可靠地输送更小的最小 MU。
