Comparative assessment of kV imaging air kerma for intrafraction prostate fiducial monitoring using three clinical systems

PURPOSE: To compare the accumulated kerma-in-air associated with intrafraction fiducial-based prostate localization in stereotactic body radiotherapy (SBRT) across three clinical platforms: Varian TrueBeam with AutoBeam Hold (ABH), Varian TrueBeam with Brainlab ExacTrac Dynamic (ETD), and Accuray Radixact with Synchrony. MATERIALS AND METHODS: An anthropomorphic pelvis phantom containing four gold fiducial markers was used. For each system, the lowest kilovoltage exposure parameters allowing stable automatic fiducial localization were identified. The corresponding kerma-in-air per image was then measured at isocenter using a calibrated dosimetry system. Per-image values were extrapolated to per-fraction and full-course accumulated kerma-in-air estimates based on clinically representative intrafraction monitoring frequencies for prostate SBRT. RESULTS: The mean kerma-in-air per image was 1.47 mGy for TrueBeam ABH, 0.034 mGy for ETD, and 0.15 mGy for Radixact Synchrony. When extrapolated to a five-fraction SBRT regimen, cumulative kerma-in-air was 557.1 mGy for ABH, 13.7 mGy for ETD, and 179.6 mGy for Synchrony. Relative to ETD, ABH delivered approximately 41 times higher accumulated kerma-in-air, and Synchrony approximately 13 times higher values. CONCLUSION: Under harmonized conditions and equivalent fiducial localization performance, ETD resulted in the lowest accumulated intrafraction kerma-in-air, Synchrony with intermediate values, and ABH with the highest contribution. These differences reflect intrinsic imaging geometry and acquisition design. Although kerma-in-air does not directly represent absorbed patient or organ dose, the results emphasize that imaging exposure should be considered when selecting intrafraction monitoring strategies for prostate SBRT.