Antolak J, Hogstrom K
Mayo Clinic, Rochester, MN.
Louisiana State University, Baton Rouge, LA.
Med Phys. 2012 Jun;39(6Part28):3979. doi: 10.1118/1.4736232.
Delivery constraints on monitor units (MUs) are not currently considered during optimization of intensity modulated proton therapy (IMPT) planning in our treatment planning system (TPS). This may result in differences between the delivered and ideally optimized dose distributions. The purpose of this study is to develop a deliverable optimization algorithm incorporating MU constraints (DOMU) in IMPT treatment planning and evaluate its clinical applicability.
A novel objective formula was incorporated with the limited-memory Broyden-Fletcher-Goldfarb-Shannon (L-BFGS) algorithm to directly optimize the deliverable IMPT plan with minimum MU constraint (0.005 MUs per spot). Treatment plans for a homogeneous phantom were generated for different spot spacings, using optimization without MU constraints and post-processing (OMPP) and DOMU. A treatment plan for a head-neck cancer patient prescribed to 63 Gy was also designed with DOMU using different spot spacings. For all plans, the dose-volume-histograms (DVHs) were analyzed to evaluate the benefits of the DOMU.
For the regular shaped homogeneous phantom, only the plan generated by OMPP using 5mm spacing showed pronounced dose distortion compared to ideally optimized dose. Plan using 7mm spacing still produced accepted target coverage. For the patient study, plans generated by OMPP using both 5mm and 7mm spacing displayed significant dose distortion and unacceptable target coverage. The dose covering 99% of the target volume was 60.2 Gy and 61.7 Gy for 5mm and 7mm spot spacing plans generated by DOMU, an increase of 18.8 Gy and 18.4 Gy compared with plans generated by OMPP. The parotid sparing of DOMU generated plans in 5 mm spot spacing was significantly better than that in 7 mm spot spacing indicating the importance of using smaller spacing to improve the IMPT quality.
The DOMU algorithm, which incorporates the minimum MU constraints into the optimization, can reduce the dos distortion caused by post-processing process and achieve the optimum delivered dose.
在我们的治疗计划系统（TPS）中，强度调制质子治疗（IMPT）计划优化过程中目前未考虑监测单位（MU）的递送限制。这可能导致实际递送的剂量分布与理想优化的剂量分布之间存在差异。本研究的目的是开发一种在IMPT治疗计划中纳入MU约束的可递送优化算法（DOMU），并评估其临床适用性。
将一个新的目标公式与有限内存布罗伊登-弗莱彻-戈德法布-香农（L-BFGS）算法相结合，以直接优化具有最小MU约束（每个射束点0.005 MU）的可递送IMPT计划。针对不同的射束点间距，使用无MU约束优化和后处理（OMPP）以及DOMU生成均匀体模的治疗计划。还使用不同的射束点间距，通过DOMU为一名处方剂量为63 Gy的头颈癌患者设计了治疗计划。对于所有计划，分析剂量体积直方图（DVH）以评估DOMU的益处。
对于规则形状的均匀体模，与理想优化剂量相比，仅使用5mm间距通过OMPP生成的计划显示出明显的剂量畸变。使用7mm间距的计划仍能实现可接受的靶区覆盖。对于患者研究，使用5mm和7mm间距通过OMPP生成的计划均显示出明显的剂量畸变和不可接受的靶区覆盖。DOMU生成的5mm和7mm射束点间距计划中，覆盖99%靶区体积的剂量分别为60.2 Gy和61.7 Gy，与OMPP生成的计划相比分别增加了18.8 Gy和18.4 Gy。DOMU生成的5mm射束点间距计划对腮腺的保护明显优于7mm射束点间距计划，这表明使用较小间距对提高IMPT质量的重要性。
将最小MU约束纳入优化的DOMU算法可减少后处理过程引起的剂量畸变，并实现最佳的实际递送剂量。
