Validation of an integrated patient positioning system: Exactrac and iViewGT on Synergy Platform

Purpose: Evaluation of the newly integrated system for its validation and designing a quality assurance frame work to assess its geometrical, radiological and mechanical accuracy. Methods: Isocentric accuracy of two independent imaging modalities, kV based ExacTrac and MV based iViewGT was evaluated using Winston-Lutz test. A pelvic humanoid phantom was used for the radiological end-to-end test for its clinical utilization. Image quality for the systems was evaluated using Las Vegas Phantom and ETR-1 plate. The kV system was also assessed for kVp accuracy, kVp dose linearity, mAs-dose linearity and timer linearity and its accuracy. The system was tested for total filtration and output consistency. Tests for uniformity and noise measurement of kVp accuracy and its reproducibility, linearity test between applied kVp and the x-ray dose, linearity Test between applied mAs and the x-ray dose were also done. Results: Winston-Lutz test gave the isocentric deviation of 0.058 ± 0.015 mm with the average lateral deviation as 0.028 ± 0.021 mm, average longitudinal deviation as 0.032 ± 0.015 mm and average vertical deviation as 0.030 ± 0.016 mm. With the phantom test, the minimum measured displacement of Exactrac positioning was 0.2 ± 0.3 mm, 0.0 ± 0.2 mm and 0.1 ± 0.3 mm in longitudinal, lateral and vertical directions respectively. In image quality test, visible smallest visible hole size seen by both Exactrac and EPID imaging system was 5 mm and can resolve 1.5 lp/mm or better. The image uniformity was found to be 132.9 ± 3.06 pixels for MV images and 139 ± 4.41 pixels for kV images with the associated noise of ≤1% both for 120 kV-20 mAs and 4 MV beam energy of ExacTrac and iViewGT respectively. The uniformity and noise test, measured pixel intensity values for various points on MV and kV images separately were found to agree within ± 1% with respect to the central axis pixel value. The kVp accuracy and its reproducibility were tested for kV imager only. The deviation of kVp was found to be than ± 1% and its precision was seen to be even lesser than ± 0.1%. Linearity test between applied kVp and the x-ray dose and applied mAs and x-ray dose were tested only for the ExacTrac. Both the coefficient of linearity for kVp as well as mAs was found to be < 0.1. Conclusion: It is feasible to install ExacTrac imaging system with an Elekta linear accelerator. Both the imaging systems were found to be compatible in terms of image quality test and isocentric accuracy and can be used for the patient imaging in the same Linear accelerator.