Quantifying the setup uncertainty of a stereotactic murine micro-image guided radiation therapy system.

objective: Investigate the reproducibility of murine cranial positioning using solely a stereotactic stage, and quantify the potential improvements from the on-board image guidance of the X-RAD SmART irradiator.

methods: For intermouse reproducibility, athymic nude mice (N = 5, ×4 groups) were cranially fixed on a stereotactic stage. Each mouse was imaged via cone-beam CT (CBCT). A virtual isocenter target was placed in the brain, the stage shifted to that target, and the couch positions recorded. The mouse was removed from the stage and this process repeated twice (N=60 measurements). The first acquired CBCT coordinates (within each group of five mice) were used to define "stereotactic couch coordinates." CBCT shifts were calculated to quantify the accuracy of setup based on couch coordinates alone. For intramouse reproducibility, C57BL/6 mice (N=4) were imaged daily for 7 days. Each mouse had individual stereotactic coordinates defined from their first day of CBCT localization, and positional shifts required on the six subsequent days of imaging were quantified (N = 24 measurements).

results: The mean vector shift between stereotactic setup and CBCT alignment for inter and intramouse analysis was 0.78 ± 0.27 mm and 0.82 ± 0.34 mm, respectively.

conclusion: Cranial irradiation that can permit positional uncertainties on the order of a millimeter can rely solely on stereotactic coordinates derived from a single daily CBCT. Irradiations of subregions requiring submillimeter accuracy require daily image guidance for each mouse. ADVANCES IN KNOWLEDGE:: This is the first investigation of stereotactic reproducibility using the X-RAD SmART and it suggests a method for increased efficiency in high-throughput experiments.