Volumetric modulated arc therapy for nasopharyngeal carcinoma patients with poor compliance: setup error and expansion margin of target volume

Abstract

Objective To investigate the setup error for nasopharyngeal carcinoma (NPC) patients with poor compliance using kV cone-beam computed tomography, and to calculate the expansion margin from the clinical target volume (CTV) to planning target volume (PTV). Methods In 45 NPC patients from 2013 to 2015, the setup error, 95% confidence interval (CI)-1 for random error, and PTV-1 value were calculated. Moreover, in 16 NPC patients with poor compliance based on five verifications (random error not within 95% CI-1), the setup error, 95% CI-2 for random error, and PTV-2 value were calculated. For the 16 special patients, PTV-1 and PTV-1 combined with PTV-2 were used to develop the plan-1 and plan-2, respectively. The dosimetric difference between plan-1 and plan-2 was evaluated. Results Both PTV-1 and PTV-2 had the largest expansion margin in the y direction. The CTV of plan-1 could not meet the requirement of the prescription dose after the setup error was introduced. Compared with plan-1, the V95% and D95 values for the CTV of plan-2 were increased by 6.26% and 4.43%, respectively. The D01 value was significantly larger in plan-2 than in plan-1(P=0.005), which, however, met the clinical requirement. Conclusions In patients with poor compliance, the dose to target volume can be effectively elevated and the normal tissue can be spared from damage when PTV-1 combined with PTV-2 is selected as expansion margin. Key words: Nasopharyngeal neoplasmsm/volumetric modulated arc therapy; Setup error; Target volume margin; Dose distribution