SU‐FF‐J‐43: Correlation Between External Abdominal and Internal Liver Fiducial Motion in 4D‐CT

Purpose: to determine how well the anterior‐posterior location of a marker, placed on a supine patient's abdominal surface, correlates with the location of a radio‐opaque fiducial implanted within the liver. We investigate which respiratory phases exhibit the best correlation between the marker and the fiducial. Methods and Materials: data was obtained from five patients, each having a fiducial (either an implanted gold pellet or the tip of a stent) in the liver. Each patient received a cine‐mode 4D‐CT scan of the liver, with the position of an external marker used to associate each cine CT image with a specific respiratory phase. The trace of the anterior‐posterior motion of the marker was normalized under the assumption that, among ten phases of a full respiratory cycle (0%, 10%, 20% … 90%), the marker displacements correlated linearly with the fiducial's superior‐inferior displacements. Each cycle of the normalized marker trace was then superimposed upon a plot of the fiducial's displacement; the latter was measured within the 4D‐CT data set for each of the ten phases. Results: comparisons of the marker trace and the fiducial coordinate from 4D‐CT indicated that, in general, the marker motion correlated reasonably well with internal liver motion during the process of exhalation (from end‐inspiration to end‐expiration), with optimum correlation during end‐expiration (40% to 60% phase). The correlation tends to be poorer during inhalation, extending from 70% to 90% phase. Conclusions: a tight correlation near end‐expiration suggests that using an external‐motion‐based respiratory trace, as a guide for gated treatments for liver cancer, should enable reliable and reproducible coverage of the target volume. Poorer correlation during inhalation suggests that the marker's anterior‐posterior motion may not adequately characterize the internal motion during those phases, and that gating should be avoided during this stage of the respiratory cycle.