Objective: To evaluate a new commercial PTW-60019 microDiamond™ (PTW, Freiburg, Germany) synthetic singlecrystal diamond detector for relative dosimetry measurements on a clinical CyberKnife™ VSI (Accuray Inc., Sunnyvale, CA) system. Methods: Relative output factors (ROFs) were measured for collimator diameters from 5 to 60 mm, and compared with diode [PTW-60017, PTW-60018 and IBA Dosimetry (Schwarzenbruck, Germany) SFD] and ionization chamber (PTW-31014 PinPoint® and PTW-31010 Semiflex) measurements. Beam profiles were measured at a range of depths, and collimator sizes, with the detector stem oriented both parallel and perpendicular to the central axis (CAX). Percentage depth-dose (PDD) curves were obtained for the 60-mm collimator and compared with natural Diamond Detector (PTW-60003) and ionization chamber curves to evaluate energy dependence. Results: Penumbral broadening was noted on profile measurements made with the microDiamond oriented with the stem parallel to the CAX, in comparison with diodes. Oriented perpendicular to the CAX, the profile penumbra was sharper, but stem effects could not be ruled out. The PDD measurements were within 0.5% of ionization chamber measurements, indicating insignificant dose-rate dependence. The ROF for the microDiamond fell between diode and ionization chamber results. Published Monte Carlo-derived CyberKnifespecific factors were applied to the PTW-60017, PTW-60018 and PTW-31014 ROFs, and the microDiamond factors agreed within 2.0% of the mean of these. Conclusion: Over a range of small field relative dosimetry measurements, the microDiamond detector shows excellent spatial resolution, dose-rate independence and water equivalence. Advances in knowledge: The microDiamond is a suitable tool for commissioning stereotactic systems.The CyberKnife™ (Accuray Inc., Sunnyvale, CA) system allows submillimetre positional accuracy for stereotactic radiosurgery (SRS) and stereotactic ablative body radiotherapy treatments. It is operated without a flattening filter, with small field sizes (5-to 60-mm-diameter circular beams) and at a dose rate of the order of 10 Gy min 21, as defined at the depth of maximum dose (15 mm deep) for a 60-mm collimator at an 800-mm source-axis distance. This places an increased demand on the accuracy of dosimetric measurements used for commissioning and quality assurance. The absence of a flattening filter in radiotherapy machines is becoming more widespread, which in turn is leading to an increase in clinical dose rates. It is therefore important to consider dose-rate effects of detectors used for commissioning both stereotactic and conventional fields.Although standard detectors are recognized to be unsuitable for small field applications, owing to dose averaging and perturbation effects, SRS-specific detectors have also been shown to have limitations. Silicon diode detectors exhibit an over-response to low-energy scatter. The relative contribution of this scatter increases with field size, and, for regular field sizes, shielding has been used to compen...

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