A Dosimetric Study on the Impact of Collimator Angles in Nasopharynx Carcinoma using VMAT Planning

Abstract

Objective: This study aims to rule out the best collimator angle using Monte- Carlo algorithm in Nasopharynx Carcinoma (NPC) with adequate tumour coverage and less OAR. Methods: Twenty-nine NPC patients were selected retrospectively. VMAT with dual arc were made with same constraints but with different collimator angles for each patient. Total often plans were created for each patient with a collimator angles of 0;5;10;15;20;25;30;35;40;45 in Monaco Treatment Planning System(TPS) using Monte-Carlo algorithm. Plans were evaluated based on Dosimetric parameters such as target coverage, organ at risk (OAR), conformity index(CI) and homogeneity index(HI). Mean dose (Dmean), and Maximum dose (Dmax) of OARs were also analysed. Data were further evaluated by using SPSS software. Findings: The VMAT plans with collimator angles 15◦-30◦ demonstrated better target coverage with a range of (92-94) %0.7195 for the high-dose PTV and significant reduction in monitor units (MU) by an average of 75 with a CI (0.95-0.96)0.0068. Moreover, significant reduction in Dmax of Brainstem and Right optic nerve were obtained with increasing angles. Also,5◦-10◦ showed least PTV coverage and 45◦ got highest MUs compared to 0◦. Novelty : This study shows that collimator angles 15◦-30◦ is superior compared with other angles in Ca Nasopharynx VMAT planning by Elekta-HD Versa Linac using Monte-Carlo algorithm. Keywords: Ca Nasopharynx; Collimator angle; Montecarlo algorithm; Elekta HD Versa Linac; VMAT