Purpose: The quality assurance of CyberKnife systems requires monthly beam profile measurement, to verify beam shape, symmetry, and field size. These QAs are typically performed using Gafchromic films due to their high spatial resolution. However, the film has drawbacks such as film polarization, nonuniformity, and relatively slow develop time. The objective of this study was to simplify these QA procedures and offer real-time reading using a custom-made 3D scanning tool.
Methods: A diode was attached to an optical stage configured in linear XYZ translation, and stepper motor actuators with positioning accuracy <5μm were mounted on all axis. The diode motion was programmed along crossline and inline directions in 0.2 mm step, in step-by-step scanning mode. The beam profiles were measured in air for diode and compared against films irradiated with no buildup material. All films were scanned in one image, and film-derived readings were corrected by a batch-specific calibration.
Results: The beam profiles measured with diode showed less than 3% difference from the film references with a field size of 5 mm and 7.5 mm for fixed cone, and variable aperture Iris collimator. The dose map from film showed uniformity variations and noise, where diode profile showed fewer signal fluctuations. For diode, the FWHM calculated from the profile showed better agreement to nominal field size. In addition, the differences in FWHM between diode and film for the same field were typically within 0.2 mm range.
Conclusion: We have designed a diode scanning tool with three-dimensional high-spatial resolution and accuracy, the program capability in acceleration/deceleration, and motion path. The scanning diode setup was shown to be within acceptable tolerance compared to film and enables potential monthly dosimetric verification for small radiation fields.
Quality Assurance, Small Fields, Diodes