Evaluation of positional accuracy of epid using imrt graticule phantom in extended source to imager distance setups: formalism of qa

Purpose: The aim of the present study was to evaluate the utility of IMRT graticule phantom to check the positional accuracy of EPID (amorphous silicon flat panel detector, retractable arm) and to develop a quality assurance program for geometrical verification.
Method: The radiographic images of graticule phantom were acquired using computed tomography (CT) and beam shapes for desired dose distribution was generated using computer based treatment planning system ( Ecllipse, version 8.6, Varian, Palo, alto, CA). A known shift of 0.5 cm, 1.0 cm and 1.5 cm were introduced in longitudinal, lateral and vertical directions, respectively w.r.t. treatment couch of medical linear accelerator. The EPID images were taken for each shift at different source to imager distance (SID) and beam orientations.
Results: The maximum and minimum shift between the expected and observed value in all the direction were found to be 3 mm and zero respectively. In longitudinal and vertical directions, maximum error of 2 mm was obtained for SID 179.9 cm and 149.9 cm, respectively, while in lateral direction the 2 mm maximum error was obtained for imager distance 149.9 cm and 179.9 cm. However, the maximum error of 3 mm was found to be most frequent in the longitudinal and vertical directions for SID 149.9 cm and 179.9 cm, respectively.
Conclusion: The methodology used in the present study is very effective to check the mechanical characteristic and consistency of the retractable arm EPID and can be used routinely in radiotherapy units. The effect of EPID sag due to gravity was not significant for detection of shift in patient’s position.