Inter fractional variability of breathing phase definition as determined by fiducial location

Reconstruction of four-dimensional (4D) imaging typically requires an externally measurable surrogate to represent the real-time relative phase of respiration. A common method is to use a reflective marker on the external surface of the patient which moves with respiration and can be tracked in real time. The location of the marker is often chosen to maximize the observable motion, though this location may not be at the region of interest. We evaluate the importance of infrared (IR) marker placement location on breathing phase definition for the purpose of respiratory gating and 4D computed tomography (CT) image reconstruction. Data were collected for ten patients enrolled on an approved IRB protocol. Real-time position data were collected during CT imaging and daily treatments for two external IR reflective markers: one placed near the xyphoid and another at the approximate location of the treatment isocenter. Motion traces from the markers were compared using cross-correlation coefficient and by estimating the relative respiratory phase, based on either marker, as would be used for 4D-CT reconstruction. Cross-correlation analysis revealed differences in the motion waveform, as well as phase differences, both of which were variable between patients as well as day to day for the same patient. Estimated relative phases from each marker were compared by the percentage amount of time the estimated phase for each marker was different, binned based on increments of 10% of a full cycle. For all collected data combined, the frequency with which breathing phase mismatch led to different bin allocation in steps of 10% was as follows: T0%-10% = 65.1%, T10%-20% = 25.3%, T20%-30% = 7.8%, T30%-40% = 1.5% and T40%-50% = 0.4%. Based on ten images per cycle, this indicates that 4D reconstructions would be influenced, depending on which marker was used, by at least 1 bin 34.9% of the time. This number was noticeably higher for some patients; the maximum was 71% of the time for one patient of ten. In conclusion, the respiratory amplitude and relative phase depend significantly on the location of the IR marker used to monitor respiration. For some patients the xyphoid and isocentric markers may be completely out of phase. More importantly, this relationship varies day to day, suggesting that a single marker may be inadequate for the purposes of respiratory gating.