Segment Edit and Segment Weight Optimization: Two Techniques for Intensity Modulated Radiation Therapy and Their Application to the Planning for Nasopharyngeal Carcinoma

The purpose of this study was to evaluate the two functions: segment weight optimization (SWO) and segment edit (SE) in the latest XiO 4.7 radiation treatment planning system and their effect on the planning of intensity modulated radiation therapy (IMRT) for Nasopharyngeal Carcinoma (NPC). SWO first appeared in XiO 4.5 and SE in XiO 4.7. Twelve patients with NPC were selected and there were three plans for each patient: the common step-and-shoot IMRT plan (C-IMRT); S-IMRT was based on the result of C-IMRT and the plan was further optimized with SWO; F-IMRT was based on S-IMRT and the segments were edited for lowering the dose received by normal tissues. The paired plans were analyzed by comparing the total number of segments, monitor units, the homogeneity index and conformity index of the target volumes and the dose delivered to organs at risk (OAR) including spinal cord, brain stem, optic nerves, chiasm, parotids and larynx. The study exhibited that the total number of segments and monitor units of S-IMRT and F-IMRT were around 25.3%, 3.4% less than those of C-IMRT respectively. The HI and CI indexes of target volumes among three kinds of plans did not show the significant difference. The doses received by spinal cord, brain stem, parotids, larynx were decreased at S-IMRT and F-IMRT as compared to C-IMRT; the highest doses delivered to chiasm and optic nerves were S-IMRT, the next C-IMRT, the lowest F-IMRT. This study showed that the SWO function could substantially reduce the total number of segments of step-and-shoot IMRT plans and the SE function had the incredible ability to lower the dose received by normal tissues.