Respiration-
induced tumor motion is an important geometrical uncertainty in esophageal cancer radiation therapy. The aim of this study was to quantify this motion using fiducial markers and four-dimensional computed tomography (4DCT).
Materials and methods
Twenty esophageal cancer patients underwent endoscopy-guided marker implantation in the tumor volume and 4DCT acquisition. The 4DCT data were sorted into 10 breathing phases and the end-of-inhalation phase was selected as reference. We quantified for each visible marker (n = 60) the motion in each phase and derived the peak-to-peak motion magnitude throughout the breathing cycle. The motion was quantified and analyzed for four different regions and in three orthogonal directions.
Results
The median(interquartile range) of the peak-to-peak magnitudes of the respiration-induced marker motion (left–right/anterior–posterior/cranial–caudal) was 1.5(0.5)/1.6(0.5)/2.9(1.4) mm for the proximal esophagus (n = 6), 1.5(1.4)/1.4(1.3)/3.7(2.6) mm for the middle esophagus (n = 12), 2.6(1.3)/3.3(1.8)/5.4(2.9) mm for the distal esophagus (n = 25), and 3.7(2.1)/5.3(1.8)/8.2(3.1) mm for the proximal stomach (n = 17).
Conclusions
The variations in the results between the three directions, four regions, and patients suggest the need of individualized region-dependent anisotropic internal margins. Therefore, we recommend using markers with 4DCT to patient-specifically adapt the internal target volume (ITV). Without 4DCT, 3DCTs at the end-of-inhalation and end-of-exhalation phases could be alternatively applied for ITV individualization.
