Comparing the position of GTVs defined using RIR and DIR PET images. The displacement along the z-axis is shown as a function of the displacement in the transverse plane for the head and neck (left) and lung (right) cancer subjects. One point falls beyond the range of the histogram for a head and neck subject, where displacements of 4.5 mm in the transverse plane and 0.5 mm along the longitudinal axis are observed. Ten different symbols are used for the 10 different subjects. The primary tumors are indicated by larger symbols in blue, whereas secondary nodes are drawn using smaller matching symbol in red.