PurposeTo study the influence of Monaco 5.4 treatment planning system (TPS) on the dosimetry of radiotherapy for nasopharynx carcinoma (NPC) under the condition of different segment shape optimization (SSO) times.MethodsFifteen patients with T3-4N0-2M0 stage NPC were enrolled, and each case was designed with SSO of 3, 5, 7 and 10 times respectively. The dose results of the target area and the major organs at risk (OARs) were statistically analyzed by DVH statistics; moreover, the isodose lines of 70Gy, 60Gy and 54Gy were intercepted at the same plane in the transverse, coronal and sagittal views and the segment shapes were compared at the angle of 30°, 120°, 240° and 330° in beam eye view (BEV); In addition, optimization time (OT), delivery time (DT), segments# and monitor unit (MU#) were obtained and analyzed by optimization console; the plans were verified and analyzed by using ArcCheck phantom.ResultsFor target area D2, the results of the SSO7 group and the SSO10 group were similar and both better than those of SSO3 and SSO5 groups, and the D2 results of the SSO3 group were notable higher than those of the other three groups; for the major OARs, the results of the maximum dose of spinal cord, brain stem, and lens and the mean dose and V30 of parotid glands showed the same trend. It showed that SSO7 and SSO10 share similar dose results, too which are notable better than the similar dose results shared by SSO3 and SSO5; in the dose deprogram distribution of 70Gy, 60Gy and 54Gy, partial 70Gy dose spillover occurred in both groups SSO3 and SSO5 and it was more obvious in group SSO3. While there was a no significant dose spillover in group SSO7 and group SSO10; in the sub-field alignment comparison under the same angle, the alignment became more complicated and the sub-fields were smaller as the number of SSO increased; the results of segment#, MU# and plan delivery time between different SSO groups were slightly different, while the plan optimization time changed significantly. The difference between group SSO3 and group SSO10 was more than 500s; the results were compared in ArcCheck, there was no significant difference between the groups.ConclusionsThe user-defined SSO function of Monaco 5.4 TPS effectively balances the relationship between plan design efficiency and plan quality. When SSO is 7, it is better value for efficiency and quality in clinical radiotherapy for NPC.