GRPR and neoplasm: Figure 3 shows that PC-3 xenografts, unlike NCI-H69 xenografts, could be clearly delineated in both nuclear and optical images at 24 h p.i. (provided that the tumor was located in the field of view). Target specificity was demonstrated by the low signal detected in the GRPR-negative tumor. The signal in the abdominal regions in the coronal SPECT/CT images reflects the excretion of the probes, and the signal in the cardiac region in the axial SPECT/CT image and the increased background signal in the optical image of [111In]In-15 again highlight the prolonged blood circulation of this probe.