Comparison of breath‐hold and free‐breathing positions of an external fiducial by analysis of respiratory traces

An internal target volume (ITV) accounting for respiratory‐induced tumor motion is best obtained using 4DCT. However, when 4DCT is not available, inspiratory/expiratory breath‐hold [Formula: see text] CT images have been suggested as an alternative. In such cases, an external fiducial on the abdomen can be used as a substitute for tumor motion and CT images are acquired when the marker position matches – as judged by the therapist/physicist ‐ its positions at previously determined free‐breathing (FB) respiratory extrema [Formula: see text]. In this study we retrospectively determined the accuracy of these matches. Free breathing 4DCT images were acquired, followed by [Formula: see text] and [Formula: see text] CT images for 25 patients with non‐small‐cell lung cancer. Respiration was monitored using a commercial external fiducial system, which generates positional information while CT studies are conducted. Software was written for statistically analyzing the displacement of the external fiducial during [Formula: see text] and [Formula: see text] CT acquisition and comparing these displacements with corresponding mean FB extrema positions [Formula: see text] using a Student's t‐test. In 72% of patients, mean positions at [Formula: see text] differed significantly from mean positions at [Formula: see text] [Formula: see text]. In 92% of patients, mean positions at [Formula: see text] differed significantly from mean positions at [Formula: see text] [Formula: see text] , although this difference was smaller than 0.5 cm in many cases [Formula: see text]. Our findings indicate that relying solely on abdominal external markers for accurate BH CT imaging in order to accurately estimate FB extrema positions may be subject to significant error. PACS numbers 87.53.bd, 87.57.C‐, 87.59.Fm, 87.55.Gh