Real-time nonstandard-shaped gold fiducial marker tracking on x-ray fluoroscopic images for prostate radiotherapy.

Objective.The prostate moves in accordance with the movement of surrounding organs. Tumor position can change by ≥3 mm during radiotherapy. Given the difficulties of visualizing the prostate fluoroscopically, fiducial markers are generally implanted into the prostate to monitor its motion during treatment. Recently, internally motion guidance methods of the prostate using a 99.5% gold/0.5% iron flexible notched wire fiducial marker (Gold Anchor® , Naslund Medical AB, Huddinge, Sweden), which requires a 22 gauge needle, has been used. However, because the notched wire can retain its linear shape, acquire a spiral shape, or roll into an irregular ball, detecting it on fluoroscopic images in real-time incurs higher computation costs.Approach.We developed a fiducial tracking algorithm to achieve real-time computation. The marker is detected on the first image frame using a shape filter that employs inter-class variance for the marker likelihood calculated by the filter, focusing on the large difference in densities between the marker and its surroundings. After the second frame, the marker is tracked by adding to the shape filter the similarity to the template cropped from the area around the marker position detected in the first frame. We retrospectively evaluated the algorithm's marker tracking accuracy for ten prostate cases, analyzing two fractions in each case.Main results.Tracking positional accuracy averaged over all patients was 0.13 ± 0.04 mm (mean ± standard deviation, Euclidean distance) and 0.25 ± 0.09 mm (95th percentile). Computation time was 2.82 ± 0.20 ms/frame averaged over all frames.Significance.Our algorithm successfully and stably tracked irregularly-shaped markers in real time.