Multi-site evaluation of the Razor stereotactic diode for CyberKnife small field relative dosimetry

The aims of this study were: (i) to validate in a multi-site context the suitability of the IBA Razor silicon diode detector for CyberKnife relative dosimetry. (ii) to fit the multi-center experimental data into a function relating the field output factors to the effective field size (EFS).

Ratio of detector readings in clinical and reference field (OF_det) and beam profiles were acquired on five CyberKnife units for fixed collimator diameters (range 5-60 mm), using both Razor and PTW 60017 diodes. Measured OF_det were corrected using published MonteCarlo correction factors to get field output factors Ω_Qclin,Qmsr^fclin,fmsr. Profiles were analyzed in terms of penumbra and EFS. Ω_Qclin,Qmsr^fclin,fmsr obtained in four centers were fitted as a function of EFS, while the data of the 5th center were used to validate the fitting curve.

Differences between Razor and PTW60017 Ω_Qclin,Qmsr^fclin,fmsr were within 1.5% over all centers down to 7.5 mm aperture and within 3.5% for the 5 mm diameter. The fit showed a coefficient of determination R2 = 0.997. The mean deviation of measured points from the predictive curve was within 0.5%. Data of the 5th center showed a mean deviation of 0.4% from the curve, with maximum differences within 2.5% for the 7.5 mm aperture.

The results confirmed the suitability of Razor detector for CyberKnife dosimetry by comparison to the PTW 60017 diode which has been well characterized and is in widespread use. The proposed mathematical relation between Ω_Qclin,Qmsr^fclin,fmsr and EFS is a robust predictive model applicable to different CyberKnife systems and detectors.