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

Abstract

PurposeTo 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 methodsAn 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.ResultsThe 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.ConclusionUnder 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.