Purpose: The purpose of this work is to assess the variation in performance of various commercially available dosimetry diodes for quantitative small field dosimetry, specifically by intercomparing measurements of SRS cone factors.
Methods: Measurements were made in 6 MV photon beams with fixed SRS cones for two accelerator-based SRS systems: a Varian Clinac iX (Varian/Zmed cones) at 600 MU/min and a CyberKnife model G4 at 800 MU/min. Measurements were made at 1.5 cm depth in water using the IBA Dosimetry "blue phantom" 3D scanning system, controlled by OMNIPRO-ACCEPT software. Source-to-detector distance was 100 cm for the Clinac, 80 cm for the CyberKnife. Two normalization methods were used for the Clinac, one directly referenced to diode measurements in a 10 cm x 10 cm square field and the other indirectly by "daisy-chaining" diode measurements to ion chamber measurement in the 10 cm x 10 cm reference field through an intermediate 4 cm x 4 cm square field. CyberKnife factors were referenced directly to measurements in the 60 mm reference field. Seven commercial diodes were evaluated: PTW TN60008, TN60012, TN60016, TN60017; IBA Dosimetry SFD; Sun Nuclear EDGE; Exradin SD1 (first generation prototype).
Results: With the exception of the SFD, all the evaluated devices yielded surprisingly consistent results. Standard deviations of Clinac factors for four diodes (SD1, EDGE, TN60008, and TN60012) ranged from approximately 0.50% at 30 mm to 2.0% at 5 mm cones size when referenced directly to the 10 cm x 10 cm measurement. The daisy-chaining strategy reduced the standard deviation to approximately 0.30% at 30 mm and 1.9% at 5 mm. Standard deviations for the same four diodes in the CyberKnife beam ranged up to approximately 1.0% at 5 mm.
Conclusions: The inter-detector variation is small and appears to be systematic with detector packaging, more inherent filtration producing flatter curves for both the relatively hard Clinac beam and the softer CyberKnife beam. The daisy-chain strategy appears to be of limited value for most of the diodes, but does bring the SFD results into significantly better agreement with the others.