Electronic portal imaging device in the real-time correction of setup error in intensity modulated radiation therapy for nasopharyngeal carcinoma

Abstract

Objective To study the real-time correction of setup error in intensity modulated radia-tion therapy (IMRT) for nasopharyngeal carcinoma (NPC) using an electronic portal imaging device (EP-ID), and to reduce systematic and random setup errors. Methods Thirty NPC patients treated by IMRT were enrolled into this study. During irradiation, three metal markers which embedded in the occlusal splint were used to monitor the setup error. Anterior and lateral isocentric electronic portal images were acquired before radiation and compared to digitally reconstructed radiographs. If the setup error in x-axis (right-left direction ) , y-axis ( superior-inferior direction) or z-axis ( anterior-posterior direction) exceeded 2 mm, real-time correction was performed before radiation and then the setup error after the correction were recorded. Results Before correction, 88.3% of all translational setup errors were less than 3 mm, and 99.1% less than 5 mm. Real-time correction was given to 16 patients who had translational setup errors more than 2 mm in one or two directions. The standard deviation of overall systematic and random setup errors after correction were obviously lower than those before correction. The M PTV value before correction was about 3 mm in the three directions and was only about 1.5 mm after correction. Conclusions In IMRT for NPC, Real-time correction of setup error using EPID and oeclusal splint method can improve the setup accuracy by reducing the systematic and random setup errors, and reduce the margin from CTV to PTV. Key words: Nasopharyngeal neoplasms/intensity modulation radiotherapy; Electronic portal ima-ging device; Setup error; Correction method