Intrafraction variations in linac-based image-guided radiosurgery of intracranial lesions.

PURPOSE: This study investigated image-guided patient positioning during frameless, mask-based, single-fraction stereotactic radiosurgery of intracranial lesions and intrafractional translational and rotational variations in patient positions.

PATIENTS AND METHODS: A non-invasive head and neck thermoplastic mask was used for immobilization. The Exactrac/Novalis Body system (BrainLAB AG, Germany) was used for kV X-ray imaging guided positioning. Intrafraction displacement data, obtained by imaging after each new table position, were evaluated.

RESULTS: There were 269 radiosurgery treatments performed on 190 patients and a total of 967 setups within different angles. The first measured error after each table rotation (mean 2.6) was evaluated (698 measurements). Intrafraction translational errors were (1 standard deviation [SD]) on average 0.8, 0.8, and 0.7mm for the left-right, superior-inferior, and anterior-posterior directions, respectively, with a mean 3D-vector of 1.0mm (SD 0.9mm) and a range from -5mm to +5mm. On average, 12%, 3%, and 1% of the translational deviations exceeded 1, 2, and 3mm, respectively, in the three directions.

CONCLUSION: The range of intrafraction patient motion in frameless image-guided stereotactic radiosurgery is often not fully mapped by pre- and post-treatment imaging. In the current study, intrafraction motion was assessed by performing measurements at several time points during the course of stereotactic radiosurgery. It was determined that 12% of the intrafraction values in the three dimensions are above 1mm, the usual safety margin applied in stereotactic radiosurgery.