Sub-millimeter spine position monitoring for stereotactic body radiotherapy using offline digital tomosynthesis

PURPOSE: Spine stereotactic radiotherapy (SBRT) requires intrafraction motion <1-2mm. We evaluated the accuracy and precision of digital tomosynthesis (DTS) in combination with triangulation for spine position tracking.

MATERIALS/METHODS: Single-slice DTS images were generated from kV cone beam CT (CBCT) projection images. They were registered to reference DTS images reconstructed from the planning CT-scan to determine 2D shifts between actual patient position and treatment plan position. 3D spine position was obtained by triangulation of each registration with a previous registration, for every 1° of data. For 7 patients who underwent spine SBRT, the standard deviation (SD) of DTS+triangulation over one entire dataset was evaluated for different DTS angles (2-16°) and triangulation angles (1-46°). For 32 CBCT datasets, acquired before or after treatment of the 7 patients, using 4° DTS and 18° triangulation angle, SDs were determined and average positions were compared to clinically performed CBCT registrations.

RESULTS: Mean SDs were 0.29±0.10mm for lateral (range 0.1-0.55mm), 0.14±0.08 for longitudinal (0.05-0.39) and 0.24±0.10 for the vertical direction (0.10-0.57). Lateral and vertical SDs for thoracic spine were higher than for lumbar spine. Differences between clinical CBCT registration and DTS+triangulation were 0.1±0.26, 0.02±0.33 and -0.07±0.21mm.

CONCLUSION: The combination of DTS and triangulation allows for monitoring spine position with sub-mm accuracy and precision.