Spatial distribution of the imaging dose and characterization of the scatter radiation contribution in CyberKnife radiosurgery

Purpose: The imaging dose for intra-and extra-cranial CyberKnife radiosurgery applications was calculated and the scattered radiation reaching the digital detectors was quantified and analyzed with regard to its origin.Methods: The image guidance subsystem of the CyberKnife was modeled based on vendor-provided information. The emitted X-ray energy spectrum for 120 kV was estimated using the SpekPy software tool. Monte Carlo (MC) image acquisition simulations were performed to calculate the total, primary and scattered photon fluences reaching each detector as a function of the imaged object dimensions. MC calculations of the imaging dose were performed for intra-and extra-cranial applications assuming 120 kV and 10 mAs acquisition settings.Results: The amount of scattered radiation reaching each detector was found to depend on the dimensions of the imaged anatomical region, contributing more than 40 % to the total photon fluence for regions more than 20 cm thick. More than 20 % of this scattered radiation originates from the contralateral imaging field. A maximum organ dose of 1.5 mGy at the nasal bones and an average dose of 0.37 mGy to the eye lenses per image pair acquisition was calculated for head applications. An entrance imaging dose of 0.4 mGy was calculated for extracranial applications.Conclusions: Scattered radiation reaching each detector in the skull and spine tracking applications can be reduced by acquiring the pair of radiographs sequentially instead of simultaneously. A dose of 3.7 cGy to the eye lenses is estimated assuming 100 image pair exposures required for treatment completion.

Automated summary

In this study, the imaging dose and scattered radiation in CyberKnife radiosurgery applications were calculated and analyzed. The amount of scattered radiation reaching each detector was found to depend on the size of the imaged anatomical region. For head applications, the maximum organ dose at the nasal bones was 1.5 mGy, and the average dose to the eye lenses was 0.37 mGy. For extracranial applications, the entrance imaging dose was 0.4 mGy. Sequentially acquiring radiograph pairs instead of simultaneous acquisition can reduce scattered radiation. An estimated dose of 3.7 cGy to the eye lenses is assumed for 100 image pair exposures required for treatment completion.