Abstract: Background and purpose: Mistreatment in intensity-modulated radiation therapy (IMRT) may cause major harms. However, pre-treatment plan verification approaches are not able to detect on-treatment errors. The aim of this study was to develop an in vivo dose verification method to quickly identify gross errors during radiation delivery and prevent larger or smaller delivered dose from adversely affecting patient. Methods: For each patient who was treated in Fudan University Shanghai Cancer Center, a cone beam computed tomography (CBCT) scan was acquired before the start of the first fraction treatment. The CBCT scans were implemented image registration with the planning CT. The electronic portal imaging device (EPID) images were captured during the first treatment delivery and used as the baseline measurement for the rest of the treatment. Dose comparisons were made using gamma analysis to verify whether the dose delivered to the patient as planned. Tests were developed to simulate incorrect site and setup error to evaluate the accuracy of this method. Results: This study showed that our method successfully detected simulated gross errors due to incorrect site and non-parallel beam direction shifts, however, it was insensitive to the translational shifts at the parallel beam direction. The inter-fraction setup was easily observed in clinical application for 30 nasopharyngeal carcinoma patients. Conclusion: Real-time error detection method for online IMRT treatment has been developed, and could detect gross errors immediately after an individual field is delivered.

Key words: In vivo dosimetry, Electronic portal imaging device, Incorrect site, Setup error