BACKGROUND: Here we provide an analysis on the set-up and positioning accuracy of SABR for skull base malignancies to evaluate the use of site- or axis-specific margins to reduce field size. METHODS: Data was prospectively collected on 63 patients with 304 fractions of SABR for recurrent/previously-irradiated skull base tumors. Using our custom cushion-mask-bite-block immobilization system combined with ExacTrac X-ray and cone-beam CT (CBCT), set-up, residual, CBCT-positioning agreement, and intrafractional errors were measured. The resulting planning target volume (PTV) margins were estimated across four skull base subsites: anterior (group 1), central (2), postero-lateral (3), and skull base-associated sites (e.g. nasopharynx/retropharyngeal, cervical vertebrae 1-2, occiput) (4). RESULTS: On initial set-up, 66% of treatment courses required shifts of >2 mm or >2°, necessitating 4.9 mm PTV margins without image-guidance. After correction, only 6/304 treatment sessions had residual errors >1 mm. CBCT-ExacTrac agreement was ≤1 mm in 89.1% of treatments and ≤1.5 mm in all but one session. Group 4 showed a higher rate of >1 mm or >1° CBCT-positioning differences compared to other groups (24.5% vs. 7.8%; p = 0.0001) and the greatest variations occurred in the cranio-caudal translational and the pitch rotational axes. Overall calculated PTV margins (based on intrafractional error) were 1.5 mm across subsites except for group 4 which required 2.0 mm margins. CONCLUSIONS: The use of 2.0 mm PTV margins for skull base SABR appears feasible using ExacTrac x-ray as the sole imaging modality for most subsites. However, PTVs were not uniformly equal and the use of a site-specific non-uniform margin reduction to optimize critical organ dose-sparing may be feasible for select cases. These findings warrant clinical investigation.
BACKGROUND: Here we provide an analysis on the set-up and positioning accuracy of SABR for skull base malignancies to evaluate the use of site- or axis-specific margins to reduce field size. METHODS: Data was prospectively collected on 63 patients with 304 fractions of SABR for recurrent/previously-irradiated skull base tumors. Using our custom cushion-mask-bite-block immobilization system combined with ExacTrac X-ray and cone-beam CT (CBCT), set-up, residual, CBCT-positioning agreement, and intrafractional errors were measured. The resulting planning target volume (PTV) margins were estimated across four skull base subsites: anterior (group 1), central (2), postero-lateral (3), and skull base-associated sites (e.g. nasopharynx/retropharyngeal, cervical vertebrae 1-2, occiput) (4). RESULTS: On initial set-up, 66% of treatment courses required shifts of >2 mm or >2°, necessitating 4.9 mm PTV margins without image-guidance. After correction, only 6/304 treatment sessions had residual errors >1 mm. CBCT-ExacTrac agreement was ≤1 mm in 89.1% of treatments and ≤1.5 mm in all but one session. Group 4 showed a higher rate of >1 mm or >1° CBCT-positioning differences compared to other groups (24.5% vs. 7.8%; p = 0.0001) and the greatest variations occurred in the cranio-caudal translational and the pitch rotational axes. Overall calculated PTV margins (based on intrafractional error) were 1.5 mm across subsites except for group 4 which required 2.0 mm margins. CONCLUSIONS: The use of 2.0 mm PTV margins for skull base SABR appears feasible using ExacTrac x-ray as the sole imaging modality for most subsites. However, PTVs were not uniformly equal and the use of a site-specific non-uniform margin reduction to optimize critical organ dose-sparing may be feasible for select cases. These findings warrant clinical investigation.