SU-E-J-272: Long-Term Evaluation and Cross-Checking of Two Geometric Calibration Systems On Linacs




Geometric or mechanical accuracy of kV and MV imaging systems of two Varian Truebeam linacs have been monitored by two geomertirc calibration systems, Varian IsoCal geometric calibration system and home-developed gQA system. Results of both systems are cross-checked and the long term geometric stabilities of linacs are evaluated.


Two geometric calibration methodologies, the built-in IsoCal system (Varian Medical Systems) and the home-developed gQA system, have been used to assess kV and MV imaging systems and their coincidence periodically on two Truebeam linacs. Both systems analyze kV or MV projection images of special designed phantoms to retrieve geometric parameters of the imaging systems. The iso-centers are then calculated based on results of multiple projections from different angles. Calibration results from both systems are compared for cross-checking.


Special tests were performed by misaligning QA phantoms on purpose and both systems could detect the known deviations. The errors are within 1 mm. To date, 32 sessions of side-by-side calibrations has been performed by both systems on two Truebeam linacs and the study still continues. All the disagreements of iso-centers between two calibrations systems are less than 1 mm with 0.1 mm standard deviations. The most of large disagreements occurred in vertical direction (AP direction) with an averaged disagreement of 0.45 mm. The average disagreements of iso-centers are 0.09 mm in other directions. The calibrations have been lasted for about one year. The deviations of geometric parameters have been small and consistent for both Truebeam linacs.


Both geometric calibration systems, IsoCal and gQA, are capable of detecting geometric deviations of kV and MV imaging systems of linacs. The long term evaluation also shows that the geometric accuracies of both linacs are very consistent.

This project has been supported by a Varian Research Grant.