To track respiratory motion during CyberKnife stereotactic radiosurgery in the lung, several (three to five) cylindrical
gold fiducials are implanted near the planned target volume (PTV). Since these fiducials remain in the human body after
treatment, we hypothesize that tracking fiducial movement over time may correlate with the tumor response to treatment
and pulmonary fibrosis, thereby serving as an indicator of treatment success. In this paper, we investigate fiducial
migration in 24 patients through examination of computed tomography (CT) volume images at four time points: pre-treatment,
three, six, and twelve month post-treatment. We developed a MATLAB based GUI environment to display
the images, identify the fiducials, and compute our performance measure. After we semi-automatically segmented and
detected fiducial locations in CT images of the same patient over time, we identified them according to their
configuration and introduced a relative performance measure (ACD: average center distance) to detect their migration.
We found that the migration tended to result in a movement towards the fiducial center of the radiated tissue area
(indicating tumor regression) and may potentially be linked to the patient prognosis.