2420 - Assessment of Diode Detectors for SRS Beam Profile Measurements

Purpose/Objective(s): AAPM TG-178 requires annual verification of beam profiles for the Gamma Knife (GK) Icon unit used in stereotactic radiosurgery, traditionally performed using film dosimetry. While film provides high spatial resolution, it is labor-intensive and subject to processing variability. This study evaluates high-resolution diode detectors as a potentially more efficient and reproducible alternative for three-dimensional (3D) dose profile measurements.

Materials/Methods: Three PTW diode detectors (TN60008, TN60016, and TN60019; PTW, Freiburg, Germany) were used to measure three-dimensional dose profiles for the 4, 8, and 16 mm GK Icon collimators. Each detector was mounted in a custom-designed 3D-printed holder to ensure reproducible positioning. Profiles were acquired along the X, Y, and Z axes with a 1 mm step size. Each spatial direction was scanned independently – the measurement axis spanned 70–130 mm, while the remaining two axes fixed at 100 mm to maintain consistent geometry.

Measured profiles were normalized to the mean value in the vicinity of the peak dose region. The full width at half maximum (FWHM) and the 80%–20% penumbra widths were derived from the diode-measured profiles and compared with film-based measurements obtained during annual calibration, as well as with the nominal values reported in TG-178.

Results: Dose profiles measured via diode demonstrated strong internal consistency, with correlation coefficients exceeding 99% for the majority of scans. For measurements obtained with the TN60016 detector, coefficients remained consistently above 98%. FWHM values for the TN60008 and TN60019 agreed within 1%; although TN60008 values were up to 10% lower, all deviations fell within the 1 mm tolerance specified by TG-178.

Film-based FWHM values were generally comparable to diode results, though several measurements were 5%–10% higher, likely reflecting film blurring effects. Compared with TG-178 nominal values, all diode-measured FWHM values were within the 1 mm tolerance except for the Z-profile of the 8 mm collimator, where the measured value was approximately 1.1 mm lower than the nominal 9.8 mm.

For 80%–20% penumbra widths, all measurements except one met the 2 mm TG-178 agreement threshold, indicating that diode detectors reliably measure beam edge gradients for GK Icon collimators. Slightly larger deviations were observed primarily with the TN60016 detector in the Y-direction of the 4 mm collimator.

Conclusion:

High-resolution diode detectors provide accurate and reproducible 3D beam profile measurements for Gamma Knife Icon collimators, with strong agreement to both film-based results and TG-178 reference data. Minor discrepancies in the Z-profile of the 8 mm collimator warrant further investigation, particularly since diode-measured profiles show less blurring than film. Future work will focus on automating data acquisition and analysis to streamline annual quality assurance.