SU-E-T-740: The Optimization in Segment Numbers for Nasopharyngeal Carcinoma (NPC) Treated by Intensity Modulated.

PURPOSE: In premise of uninfluenced to dose distribution of tumor target and organ at risk(OAR) in Nasopharyngeal carcinoma (NPC),the number of segment was changed to increase efficacy and optimize treatment method by designing different plan of intensity modulated radiotherapy(IMRT).

METHODS: 10 cases of NPC were confirmed in pathology and treated with step and shoot IMRT. The target volumes were defined: GVT-T (the primary tumor); GVT-N (the metastasis lymph nodes in neck) ; CTV-T and CTV-N (the clinical targets of GVT-T and GTV-N) ; and PTV (incidental lymph nodal region). Dose of GTV/CTV/PTV was 70/64/54Gy for 30 fractions.Every patient was designed 7 treatment plans of IMRT by Pinnacle 8.0m planning system,each plan was used with 9 beams of uniform distribution and fixing incidence direction(200°,240°,280°,320°,0°,40°,80°,120°and 160°respectively),and designed for delivery on Elekta Synergy linear accelerator.All plans were optimized with the direct machine parameter optimization(DMPO) algorithm.Minimum MU in each segment was defined at 4MU,and minimal segment area was 4cm(2).Number of maximum segment field was defined at 120,110,100,90,80,70,60, respectively.Coverage and conformity of GTV/CTV/PTV,sparing of OARs(spinal cord, brain, parotids, lens and optic nerves) were compared.

RESULTS: In this group,mean volume of GTV(GTV-T+GTV-N),CTV(CTV-T+CTV-N)and PTV were 91.39±71.13cm3,177.27±92.16cm3 and 1028.05±167.03cm3,.respectively. Compared with the number of segment decreased from 120 to 60, there were no difference in MU(p>0.05),and the dose distribution of targets(GTV/CTV/PTV),OARs and CI were not different too(p>0.05).But when the segment number was <60, CI of PTV was changed compared with others(p<0.05).

CONCLUSION: The minimal segment number of IMRT plan designed by Pinnacle 8.0m planning system in NPC should be decreased reasonably and the limit of minimal segment number was recommended by 70.