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

Abstract

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 (BH_insp, BH_exp) CT images have been suggested as an alternative. In such cases, an external fiducial on the abdomen is often 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 (FB_insp, FB_exp). In this study we retrospectively determined the accuracy of these matches. As part of routine radiation therapy planning, FB CT images were acquired, followed by BH_insp and BH_exp 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 BH_insp and BH_exp CT acquisition and compared these displacements with corresponding mean FB extrema positions (FB_insp and FB_exp, respectively) using a student’s t-test. In 72% of patients, mean positions at BH_insp differed significantly from mean positions at FB_insp (p < 0.05). In 92% of patients, mean positions at BH_exp differed significantly from mean positions at FB_exp (p < 0.05). Our findings indicate that relying solely on external markers for accurate BH CT imaging in order to facilitate in the definition of the ITV may be subject to significant error.