Session Item

In frameless radiosurgery, in-room x-ray images and automated software routines are used to register the planned dose distributions to the treated lesions with submillimeter accuracy. These x-ray images are associated with an imaging dose to the patient. The purpose of this work is to calculate the dose distributions from the image guidance x-ray based system of the CyberKnife robotic radiosurgery system (Accuray^TM Inc., Sunnyvale, USA). Doses to the eye lenses and thyroid radiosensitive organs are reported for typical intracranial treatments. In addition, beam hardening filters of variable thickness were studied to minimize imaging dose. 

The CyberKnife system employs two ceiling mounted x-ray kV tubes (40 - 150 kV) and two in-floor aSi detectors. The imaging field is confined to (17x17) cm² at isocenter using trapezoidal collimators in order to cover the active surface of the detectors. The image guidance system of the CyberKnife was modeled using the C++ class library (egs++) of the EGSnrc Monte Carlo software package. The geometrical characteristics of the developed model were based on the information shared by the vendor. The total tube filtration was calculated based on Halve Value Layer (HVL) measurements with the XR solid state detector (IBA Dosimetry, Germany) positioned at the system isocenter. The x-ray spectrums used for sampling the energy of the emitted photons were calculated using the SpekPy software toolkit. Absorbed dose was calculated on digital patient models created using corresponding Computed Tomography (CT) images and appropriate software tools of the EGSnrc package. To minimize the imaging dose, additional simulations using Tin beam hardening filters of variable thickness were studied.

The absorbed imaging dose distributions in an intracranial CyberKnife application is presented in Figure 1 using nominal imaging parameters of 120 kVp and 10 mAs. The dose to the eye lenses from both x-rays was found equal to 0.8 mGy per acquisition. The dose to the thyroid which is outside the imaging field of view for intracranial applications was found equal to 0.01 mGy per acquisition.  When Tin filter was used to harden the imaging photon energy spectrum, the dose was found to decrease reaching up to 20% for the lenses and for Tin filter thickness of 1mm.

The absorbed dose to the eye lenses from the image guidance in typical CyberKnife intracranial applications (i.e., using 120 kVp and 10 mAs x-ray image acquisition parameters) was found equal to 0.8 mGy per acquisition. The presence of Tin filter was found to decrease the imaging dose to the lenses by up to 20%. However, simulation findings should be verified by corresponding measurements using optimized imaging parameters.