Stereotactic Cones in a 6MV Flattening Filter-free Beam: A Dosimetric Comparison of Varian Cones on TrueBeam versus CyberKnife Cones

Purpose: Dosimetric comparison of conical collimator (CC) supplied by Varian Medical System on a TrueBeam (TB) for 6MV-flattening filter-free (FFF) beams versus CC on CyberKnife-G4 (CK) by Accuray Inc. Methods: 5 cones with nominal diameters of 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm were considered in our study. Percentage depth dose (PDD), off-axis ratio (OAR), tissue maximum ratio (TMR), and output factor (OF) were presented and compared. Results: PDD comparisons between TB and CK cones show good agreement across the range of cones; the mean difference of the % dose values, for all cones, was − 0.9% ±1.2% across the five considered depths along the curve. The agreement between CK and TB cones is poorer for TMR; the discrepancies between CK and TB values increase with depth and lightly decrease with increased cone size. OAR profiles are in agreement, although CK cones tend to overestimate the dose between 80% and 5% dose; consequently, the FHWM (full width at half maximum) for CK cones is slightly larger. Except for the 5-mm cone with a difference percentage of −3.7% between CK and Varian cones, CK cones show the largest output factors, with a maximum difference percentage was 1.9% for the 7.5-mm cone. Conclusion: CK and TB cones show similar dosimetric characteristics. The observed differences suggest that the 6MV-FFF beams from TB cones would be slightly “softer” than the 6MV-FFF beams from CK cones; Varian cones may potentially provide better sparing of organs at risk.