TITLE:
Field Size Evaluation with a High Resolution 2D Diode Array for Variable-Aperture Collimators Used in Robotic Radiosurgery
AUTHORS:
Xiaofeng Zhu, Wei Nie, James David George, Qianyi Xu, Dandan Zheng, Jiajin Fan
KEYWORDS:
CyberKnife, Diode Array, QA, FFT
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.12 No.3,
August
10,
2023
ABSTRACT: Robotic radiosurgery/Radiotherapy is increasingly adopted in clinics, and
quality assurance (QA) of CyberKnife’s variable-aperture IrisTM collimators requires sub-millimeter precision. Conventional film-based QA for
the 12 IrisTM 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 IrisTM 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 IrisTM 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 IrisTM QA efficiency.