Purpose: This study aims to show how fiducial bracketing affects lung tumor tracking.
Methods: Patients receiving CyberKnife treatment have their tumor tracked with a correlation model between internal fiducials and external LEDS. Frequently not all fiducials are capable of being tracked. The tracking is dependent upon which markers are trackable and selected. The placement and selection of fiducials determines the bracketing around the tumor. To determine what bracketing occurred, the center of mass (COM) of each fiducial and the GTV was found. The COM was calculated using a MATLAB program with post-processing. The COMs were analyzed to see the fiducial’s position relative to the GTV. This study had 27 patients with 20 upper lung cases with 79 fiducials and 7 lower lung cases with 23 fiducials. Fiducials were deemed good tracking if the discrepancy in distance to the GTV’s COM was less than 2 mm. The centroid closest to the GTV’s COM and the centroid that followed the GTV the best were found.
Results: There were significantly more fiducials inferior (52) than superior (27) and distal (49) than proximal (30) for upper lung tumors. Lower lung fiducials placed posterior were more often good tracking than anterior (p=0.05). The desired centroids for upper lung patients had bracketing in the AP direction (10/20) most. Those centroids for lower lung patients had bracketing in the SI direction most for the closest centroid (4/7) and the centroid that matched tumor motion (6/7).
Conclusion: Directional bracketing is important for accurate lung tumor tracking. The requirements for tracking are different for upper and lower lung patients, thus marker placement and selection recommendations should be separate.
Radiation Therapy, Lung, Target Localization
TH- External Beam- Photons: Motion management - intrafraction