Positioning Errors of the Conventional Method in Nasopharyngeal Carcinoma Radiotherapy: A Clinical Study of an Optical Patient Position Guidance System

Purpose: Positioning errors represented a source of uncertainty in nasopharyngeal carcinoma (NPC) radiotherapy. The purpose of this study was to evaluate the conventional method's positioning accuracy through Optical Positioning System (OPS), a novel approach of patient positioning. Materials and Methods: 50 patients with nasopharyngeal carcinoma were selected without any other criteria for investigation. All patients were positioned using a conventional method; meanwhile, OPS measured the positioning error for each patient and provided a dataset comprised of positioning errors in left to right (LR), inferior to superior (IS) and anterior to posterior (AP) directions. When the positioning error exceeded 1.0 mm in any direction, the CT scans and simulator films would be compared by several experienced clinicians and radiation therapists to investigate the error sources. Results: In all 50 clinical datasets obtained, there were 6 sets with positioning errors of less than 1.0 mm in all three directions. The remaining 44 sets needed to be analyzed for error sources; among them, the mean 3-D positioning error was up to 2.1 mm; and the mean errors in LR, IS and AP directions were 1.4 mm, 1.0 mm and 0.8 mm, respectively. Conclusion: This study indicates that the manual operation is mainly responsible for the positioning error of the conventional method. The optical patient position guidance system, OPS, can perform high-precision positioning by decreasing manual operations.