EVALUATION OF SETUP ERRORS IN INTENSITY MODULATED RADIOTHERAPY (IMRT) FOR NASOPHARYNGEAL CARCINOMA USING AN ELECTRONIC PORTAL IMAGING DEVICE (EPID)

Abstract

Background& Aims: Accurate patient positioning is crucial in Intensity-Modulated Radiation Therapy (IMRT) for nasopharyngeal carcinoma (NPC) to ensure effective dose delivery. This study aims to quantify setup errors using an Electronic Portal Imaging Device (EPID) and to compute appropriate planning target volume (PTV) margins using established margin recipes Materials and Methods: A retrospective analysis was conducted on 175 NPC patients treated with IMRT at CENAR, Quetta, from January 2022 to November 2024. A total of 2100 portal images were evaluated using anteriorposterior and lateral views. Translational setup deviations in vertical, longitudinal, and lateral directions were measured. Systematic (Σ) and random (σ) errors were calculated using the Royal College of Radiologists guidelines. PTV margins were computed using Van Herk’s, Stroom’s, and Landberg’s formulas. Results: Mean setup errors were -0.01 cm (lateral), 0.03 cm (longitudinal), and 0.04 cm (vertical). The largest systematic error was 0.10 cm (lateral), and the largest random error was 0.04 cm (longitudinal). Based on Van Herk’s formula, the required PTV margin in all directions was 0.2 cm. These findings align with previously published studies, supporting the validity of the adopted protocols. Conclusion: Setup errors in NPC IMRT, though minimal, can affect dose coverage if unaccounted for. EPID is a reliable tool for detecting and minimising these errors. The recommended PTV margin of 0.2 cm ensures adequate target coverage and should be incorporated into clinical planning at this centre. Further studies are suggested to assess rotational and internal anatomical variations. KEYWORDS: Nasopharyngeal Carcinoma, Intensity Modulated Radiotherapy, Setup Error, Electronic Portal Imaging Device, Systematic Error, Random Error