Lack of correlation between external fiducial positions and internal tumor positions during breath-hold CT

For thoracic tumors, if four-dimensional computed tomography (4DCT) is unavailable, the internal margin can be estimated by use of breath-hold (BH) CT scans acquired at end inspiration (EI) and end expiration (EE). By use of external surrogates for tumor position, BH accuracy is estimated by minimizing the difference between respiratory extrema BH and mean equivalent-phase free breathing (FB) positions. We tested the assumption that an external surrogate for BH accuracy correlates with internal tumor positional accuracy during BH CT.

In 16 lung cancer patients, 4DCT images, as well as BH CT images at EI and EE, were acquired. Absolute differences between BH and mean equivalent-phase (FB) positions were calculated for both external fiducials and gross tumor volume (GTV) centroids as metrics of external and internal BH accuracy, respectively, and the results were correlated.

At EI, the absolute difference between mean FB and BH fiducial displacement correlated poorly with the absolute difference between FB and BH GTV centroid positions on CT images (R(2) = 0.11). Similarly, at EE, the absolute difference between mean FB and BH fiducial displacements correlated poorly with the absolute difference between FB and BH GTV centroid positions on CT images (R(2) = 0.18).

External surrogates for tumor position are not an accurate metric of BH accuracy for lung cancer patients. This implies that care should be taken when using such an approach because an incorrect internal margin could be generated.