Background: The use of IMRT treatment delivery has caused dramatic change in aspects of the treatment delivery process. The importance of correcting systematic setup uncertainty has led to use of sophisticated offline or online repositioning scheme. Daily Image Guided Radiation Therapy provides detailed information about the positioning and make the adaptive treatment changes possible and allow for individualisation of the margin and setup techniques. The aim of the study was to compare the setup errors as detected by Electronic Portal Imaging Device (EPID) and ExacTrac systems. Materials and Methods: 30 patients were taken up for the study, 20 cases of head and neck cancer and 10 cases of pelvic cancer. Electronic portal images were taken in the anterior and lateral views during the first 5 fractions of treatment and day 1 of every subsequent week till completion of treatment. Each portal image was compared with ExacTrac and the setup error was calculated. Results: In the head and neck patients the mean systematic error were found to be 0.84, 1.21 and 0.77 in the lateral, longitudinal and vertical coordinates by ExacTrac. By EPID the mean systematic error was 0.12, 0.17 and 0.16 in lateral, longitudinal and vertical directions. The random errors by ExacTrac were 1.58, 1.24, 1.58 in the lateral, longitudinal and vertical directions respectively, and by EPID it was 0.25, 0.24 and 0.26 respectively. The differences were statistically significant. Stroom’s margin recipe was used to calculate the PTV margin which was about 0.5 mm in all the directions by EPID and 2.9 mm by ExacTrac. By van Herk margin recipe the PTV margin was 0.6 mm by EPID and 3.4 mm by ExacTrac. Similarly in pelvic cancer the systematic and random errors were higher by ExacTrac when compared to EPID. The PTV margins were about 0.8 mm and 3.8 mm by EPID and ExacTrac using Stroom’s margin recipe and 0.8 mm and 4.2 mm respectively by van Herk Margin recipe. Conclusion: The setup errors estimated by Exac Trac are significantly higher than that with EPID. CTV to PTV margin of 4 mm in head and neck cancers and 5 mm in pelvic cancers may be considered to account for most of the setup errors.
