Room: Exhibit Hall
Purpose: A specialized Hidden Target Test (HTT) was developed to measure the targeting accuracy of a patient positioning system used for single-isocentre multiple target Stereotactic Radiosurgery (SRS) treatment deliveries.
Methods: The HTT measurements were made using an anthropomorphic head phantom, into which six radio-opaque markers were inserted simulating treatment targets within the head. The treatment procedure consisted of fixation mask construction, acquisition of reference CT images, treatment planning and delivery. The treatment plan was generated in Eclipse and contained static fields defined by a multi-leaf collimator (MLC) with sizes ranging from 7.5 x 7.5 mm2 to 15.0 x 15.0 mm2 with each field exposing a target marker. The plan was delivered using a TrueBeam STx linac equipped with a six degree of freedom (6DoF) couch and On-board Imaging System (OBI) capable of acquiring cone beam CT (CBCT) and 2D kV projection images. Acquired images were registered with a reference planning dataset to correct translational and rotational misalignments of phantom position to isocentre. The positional accuracy of the system was evaluated by comparing the portal images of the multi-port treatment fields acquired during delivery with the corresponding digitally reconstructed radiographs calculated from the planning CT.
Results: For the CBCT setup, deviations of the six target markers from their expected locations ranged between 0.3 mm to 0.8 mm, with mean deviation of 0.5 Â± 0.2 mm. For the setup with orthogonal 2D x-ray pairs, deviations ranged between 0.2 mm to 1.2 mm, with mean deviation of 0.7 Â± 0.2 mm.
Conclusion: This work indicates that a positional accuracy of < 1.0 mm can be achieved for cranial off-axis targets when corrections are made using CBCT imaging in combination with the 6DoF couch.