Immunohistochemistry. Two tumor lesions dissected from the abdomen of a BALB/c nude mouse that received 68Ga-IMP288 after pretargeting with TF2 (a). One lesion (a: left lesion) showed normal vital tumor cells on microscopic hematoxylin and eosin [HE]- and CEA-stained images (b: HE, × 5; c: HE, × 20; and d: CEA, × 20) and high specific tumor uptake of 68Ga-IMP288 (17.7% ID/g). On the contrary, another lesion in the same animal (a: right lesion) showed much lower tumor activity concentration (0.49% ID/g) in biodistribution and a much lower signal on the PET/CT images. This result was explained by the HE sections and CEA-stained images showing > 90% of non-vital tumor tissue (necrosis and infiltrated lymphocytes), lacking CEA expression (e: HE, × 5; f: HE, × 20; and g: CEA, × 20).