Five-year evaluation of linear accelerator-based SRS platform isocentricity

Linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) has become a mainstay in the management of intracranial tumors. However, the high fractional doses and sharp gradients used in SRS place heavy demands on geometric accuracy. Image guidance systems such as ExacTrac (ETX, Brainlab AG, Munich, Germany) have been developed to facilitate position verification at nonzero table angles. Though convenient, potential loss of mechanical rigidity between the imaging and treatment systems can be cause for concern, as the ETX system is not mounted to the rotating gantry. In this retrospective study, we analyzed 518 Winston-Lutz (WL) tests performed in the last 5 years with ETX alignment on our Elekta Versa HD (Elekta AB, Stockholm, Sweden) linear accelerator to determine the achievable limits of precision and stability over time for our LINAC-based SRS platform. Results demonstrated remarkable stability over time. 3D and directional misalignments never exceeded 1.0 mm over the study period; however, table rotation was shown to be the most significant source of positional uncertainty. Gantry sag, as measured by gun-to-target misalignments at the gantry-0 and gantry-180-degree positions, was consistent, measuring 1.23 ± 0.18 mm over the study period. Measured accuracy was well within acceptable tolerances for cranial SRS treatment delivery. Notably, the use of the ETX system for intrafraction repositioning effectively eliminates couch walkout, the most significant source of uncertainty identified in this study. Our results thus corroborate safe SRS treatment delivery on our Versa HD with ExacTrac image guidance.