Field Size Evaluation with a High Resolution 2D Diode Array for Variable-Aperture Collimators Used in Robotic Radiosurgery

Robotic radiosurgery/Radiotherapy is increasingly adopted in clinics, and quality assurance (QA) of CyberKnife’s variable-aperture Iris^TM collimators requires sub-millimeter precision. Conventional film-based QA for the 12 Iris^TM cone sizes (ranging from 5 to 60 mm) is both time consuming (120 minutes for all or 30 minutes for 3 cone sizes) and highly experience dependent. To improve the efficiency, a high-resolution 2D diode detector array, sampling every 2.5 mm, was evaluated for Iris^TM aperture size QA. This study focused on a spatial frequency analysis, a dose profile reconstruction, and a sensitivity study to beam size variances. Dose profiles of the 12 cones scanned with a high-resolution diode in a water tank were utilized as the gold standard for comparison. Spatial Fourier transform of these profiles were analyzed to explore applicable sampling frequency. Next, the dose profiles were artificially sampled with a 2.5 mm gap, and then interpolated using a Python-based cubic B-spline. Finally, sensitivity of the diode array system to various field sizes was measured by changing source to detector distance. We found, utilizing the diodes system, QA time was reduced to less than 10 minutes. Spatial frequency of the dose profile showed little contribution beyond 0.2 mm^-1, so a Nyquist sampling of 0.4 mm^-1 is appropriate for dose verification, corresponding to a 2.5 mm gap. Dose profiles were reconstructed using Cubic B-spline with good agreements to nominal for cones 7.5 mm and larger. The measured Iris^TM size using the SRS MapCheck had a standard error of ±0.12 mm. Primarily, the 2D Diode array with a spatial resolution 0.4 mm^-1 is appropriate for dose verification for these cones above 7.5 mm, and its application would substantially improve Iris^TM QA efficiency.