Session Item

Optical tracking in combination with preoperative imaging provides the opportunity to navigate an x-ray source in intraoperative radiotherapy (IORT). The knowledge of the x-ray source isocenter represents the basis for treatment planning. Tracking the isocenter with an optical tracking system and a rigid body with optical markers requires a previous calibration to define the offset from the rigid body to the isocenter of the source. The aim of this project is to create a navigation workflow, which yields an optimized x-ray source positioning and allows for treatment planning for kypho-IORT.

Two rigid bodies with four optical markers each were used. One was connected to the floor stand of an Intrabeam system (Carl Zeiss Meditec AG) to track the x-ray source and the other one was fixed to a needle applicator to be used as calibrator with a known offset between its markers and the source isocenter. Measurements were taken with a Polaris Vega system (Northern Digital Inc.). The calibration was performed, recording movement in 750 frames. In order to simulate a treatment navigation, three 3D printed vertebras were used and seven metallic fiducials were attached for registration purposes. Next, another needle applicator was connected to the floor stand and placed close to the vertebras and a cone beam CT (CBCT) was performed. The fiducials’ positions previously recorded in the optical tracking system and determined in the CBCT scan were used to register both systems. Finally, the isocenter coordinates in the CBCT scan and in the optical system were determined and evaluated regarding their geometrical deviation.

The calibration estimating the offset between rigid body of the floor stand and isocenter showed a mean geometrical error of 0.24 ± 0.13 mm and 0.65 mm as maximum error. Five trials of the navigation were evaluated and the results are shown in Table 1. For the registration from the optical system to CBCT scan, the fiducial registration error (FRE) was between 0.87 mm and 1.04 mm. Comparing the isocenter coordinate from the CBCT and the optical tracking system, the geometrical deviation was between 0.78 mm and 1.58 mm.

Table 1: FRE and error of the isocenter coordinate for each trial

In this project, we showed the feasibility to use an optical tracking system to navigate an x-ray source for kypho-IORT. The influences of different equipment used during a surgery in an operating room and of the patient ifself on the precison of the navigation need to be evaluated in further studies.