SU-E-J-69: Development of an Ambiguity-Free Optical Tracking System for SBRT.

PURPOSE Commercial optical tracking system such as BrainLab ExacTrac© suffers from ambiguous markers which can cause significant delay in patient treatment. We developed an ambiguity-free optical tracking system for high precision SBRT. METHODS The optical tracking system consists of a pair of camera sensors, 4∼8 infra-red (IR) reflective markers affixed on patient skin and in-house developed software. We developed an effective algorithm to automatically eliminate ambiguous markers. A recursive backtracking algorithm was developed to build one-to-one correspondence between reference markers and optical markers. The algorithm was able to deal with missing, misplaced, or occluded markers by introducing 'dummy' markers. Isocenter displacements were calculated in six-degree of freedom through rigid-body registration between reference markers and optical markers. The system accuracy was evaluated with a pelvis phantom against cone beam CT (CBCT) and ExacTrac. The impact of number of markers on tracking accuracy was also evaluated. The performance of automatic ambiguity elimination was assessed with synthetic and clinic marker patterns. RESULTS Difference of isocenter displacement reported by our system and commercial systems (CBCT, ExacTrac) were within 0.5 mm/0.1o. Impact of number of markers on tracking accuracy in our phantom test was negligible with isocenter displacement varying within 0.2 mm/0.2o when number of markers varied from 4 to 8. The system successfully identified and eliminated ambiguous markers in both synthetic and clinic marker patterns. CONCLUSIONS An accurate optical tracking system free of ambiguous markers was developed for patient positioning and monitoring in high precision SBRT. The developed algorithms to eliminate ambiguous markers and to handle missing, misplaced or occluded markers were effective and efficient.