Influence Of Multi-leaf Collimator Angle On Dose Distribution Of Volumetric Modulated Arc Therapy For Nasopharyngeal Carcinoma Patients

Compared with other radiation therapy techniques,Volumetric modulated arc therapy(VMAT)has the advantage of less MU,less treatment time,better dose conformity and uniformity for nasopharyngeal carcinoma patients.We normally choose dual arc for better dose distribution.Since the commercial plan treatment system doesn’t support Multi-leaf Collimator(MLC)angle optimization,the planner has to select the MLC angle manually.The whole procession of VMAT optimization based on this angle,final dose distribution may be impacted due to the specific angle of MLC,it is particularly important to choose the proper MLC angle.This study selects 10 patients with nasopharyngeal carcinoma from Cancer Hospital Chinese Academy of Medical Sciences during the period Jan2015 to Jan2016.Pinnacle 9.10 treatment planning system is used to design dual arc VMAT plan,reoptimized the plan via selecting four angle groups of MLC with other conditions unchanged,compare the target and organ at risk(OAR)’s dose distribution,Dose-volume Histogram(DVH),MU,treatment time difference among those plans.The four angle groups are(0°0°),(10°350°),(15°345°),(30°330°).The results showed that compared with(0°0°)plan,other MLC angle plans has lower heterogeneous Index(HI)of PTV and higher Conformal Index(CI)。Considering the Organ at risk(OAR),the plan with rotated MLC angle has lower max dose of cord and lens but higher dose of Normal Tissue(NT).There is no significant difference in other organs and there is no statistical difference between MU and treatment time.Among the three rotated MLC angle group plans,they have same dose index with PTV and PGTV(PTV CI:0.81、HI:0.31,PGTV CI:0.83 HI:0.07),plan of(10°350°)has better result in GTVnd’s dose index(CI:0.66、HI:0.06).Plan of(10°350°)has the lowest cord dose(32.21Gy).Plan of(15°345°)has lowest lens dose(left lens 7.29 Gy,right lens 8.45Gy).Due to the plan quality score,plan of(10°,350°)has the best plan quality(SD:0.12)and plan of(0°,0°)has the worst plan quality(SD:1.2).NPC has the characteristic of large volume,complicated target and lots of OAR around.Different MLC angle’s VMAT plan will impact the dose distribution differently.The medical physicist could choose the proper MLC angle according to the clinical need and strategy.