Determination of Setup Margin for Nasopharyngeal Carcinoma by Using Electroinc Portal Imaging Device

Abstract

Introduction: This study aimed to assess three-dimensional (3D) setup mistakes and provide optimal margins for planned target volume (PTV) coverage in head and neck radiation. Methods: Ten patients participated in the trial, receiving IMRT in conjunction with weekly electronic portal imaging (EPI). A total of 170 portal pictures were analyzed. The systematic (S) and random (s) errors in the population of patients with head and neck cancer were assessed using portal images in the caudocranial longitudinal (CC) and left-right lateral (LR) orientations, measured within the anterior-posterior (AP) field. The clinical-to-planning target volume (CTV-PTV) margins were determined in accordance with ICRU Report 62 guidelines and van Herk’s formulae. Results: The group systematic errors and random errors were 0.19 and 0.26 cm , respectively in the anteroposterior direction; the group systematic errors and random errors were 0.15 cm and 0.24 cm in SI direction, respectively; the group systematic errors and random errors were 0.13 cm and 0.25 cm in LR direction, respectively; According to the classical van-Herk formula MPTV =2.5Σ +0.7σ, we figured out the ideal PTV margins (MPTV) based on the setup errors and 0.65 cm, 0.55 cm, and 0.5 cm were required in the AP, SI, and RL directions. Discussion and conclusions: we can conclude that a 6-mm extension of CTV to PTV margin, as the optimal margin and can be reduced with increasing frequency of online verification (daily imaging) or for patients where the prescribed does not exceed tolerance doses for organs at risk.