Investigating the spatial accuracy of CBCT-guided cranial radiosurgery: A phantom end-to-end test study

PurposeTo evaluate the spatial accuracy of a frameless cone-beam computed tomography (CBCT)-guided cranial radiosurgery (SRS) using an end-to-end (E2E) phantom test methodology.Methods and materialsFive clinical SRS plans were mapped to an acrylic phantom containing a radiochromic film. The resulting phantom-based plans (E2E plans) were delivered four times. The phantom was setup on the treatment table with intentional misalignments, and CBCT-imaging was used to align it prior to E2E plan delivery. Comparisons (global gamma analysis) of the planned and delivered dose to the film were performed using a commercial triple-channel film dosimetry software. The necessary distance-to-agreement to achieve a 95% (DTA95) gamma passing rate for a fixed 3% dose difference provided an estimate of the spatial accuracy of CBCT-guided SRS. Systematic (∑) and random (σ) error components, as well as 95% confidence levels were derived for the DTA95 metric.ResultsThe overall systematic spatial accuracy averaged over all tests was 1.4 mm (SD: 0.2 mm), with a corresponding 95% confidence level of 1.8 mm. The systematic (Σ) and random (σ) spatial components of the accuracy derived from the E2E tests were 0.2 mm and 0.8 mm, respectively.ConclusionsThe E2E methodology used in this study allowed an estimation of the spatial accuracy of our CBCT-guided SRS procedure. Subsequently, a PTV margin of 2.0 mm is currently used in our department.