Analytic study on the effects of the number of MLC segments and the least segment area on step-and-shoot head-and-neck IMRT planning using direct machine parameter optimization

In this study, we present the concurrent effects of the number of segments (NS) and the least segment area (LSA) for step-and-shoot head-and-neck intensity-modulated radiation therapy (IMRT) planning using the direct machine parameter optimization (DMPO), on which basis we suggest the optimal NS and LSA ranges. We selected three head-and-neck patients who had received IMRT via the simultaneous integrated boost (SIB) technique and classified them as easy, intermediate, and difficult cases. We formulated a benchmark plan and made 11 additional plans by re-optimizing the benchmark by varying the NS and the LSA for each case. Clinical and physical plan-quality evaluation parameters were considered separately: the conformality index (CI), the homogeneity index (HI) and the maximum or mean doses for the organs-at-risk were the clinical factors, and these were summarized as plan-quality parameter, Q. The modulation index (MI), the total monitor units (MUs), and the final composite cost function F were employed as parameters in the evaluation of the physical aspects. A 2-way analysis of variance (2-way ANOVA) was used to determine the effects of the NS and the LSA concurrently. Pearson's correlations among the total MU, MI, F, and Q were examined as well. Overall plan-efficiency factor e{open} was defined to estimate the optimal NS and LSA by considering the plan's quality and the beam delivery efficiency together. Plans with simple targets or a small number of beams (NB) were affected by the LSA whereas plans with complex targets or large NB were affected by the NS. Moreover, smaller NS and smaller LSA were advantageous for simple plans whereas larger NS and smaller LSA were beneficial for complex plans. When we consider the plan's quality and the beam delivery efficiency, {NS = 60-80, LSA = 8-12 cm²} are the proper ranges for head-and-neck IMRT planning with DMPO; however, the combination may differ based on the complexity of a given plan.