Multitracer PET imaging of bone metastases from paraganglioma: peripheral halo of uptake on 18F-FLT PET mismatching with central uptake of 18F-FDOPA, 18F-fluorodopamine, and 18F-FDG

Abstract

A 70-year-old man with a history of a right adrenal paraganglioma (24 cm) surgically removed 2 years before was referred for lower back pain, palpitations, and increased plasma catecholamine levels suspicious for recurrent paraganglioma. The patient underwent a multi-tracer PET imaging evaluation consisting of four different radiopharmaceuticals, each performed on different days in the same week: 18F-Fdopamine and 18F-FDOPA to specifically detect the presence of paraganglioma [1], 18F-FDG to assess glycolytic activity, and 18F-fluorothymidine (18F-FLT) to assess cell proliferation [2]. The scans revealed retroperitoneal masses, bilateral lung lesions, and bone lesions, with high and variable avidity for 18F-Fdopamine (A), 18F-FDOPA (B), and 18F-FDG (C). The inter-lesion and inter-tracer tracer avidity variability illustrates the metastatic metabolic heterogeneity. All lesions had low 18F-FLT uptake (D), suggesting a low rate of cellular proliferation. However, the bone lesions (green arrows) showed an interesting pattern on 18F-FLT PET. High 18F-FLT uptake is known to occur in normal bone, due to hematopoietic cells in the bone marrow compartment that are continually replacing circulating peripheral blood cells and are highly proliferative [3]. In this patient, bone lesions detected and delineated by T2-weighted MRI (E) and CT (F) showed low cellular proliferation, contrasting with their high catecholamine metabolism and high glycolytic activity. High non-specific peri-tumoral 18F-FLT uptake was seen (higher than the normal bone), most likely corresponding to the reactive proliferation of hematopoietic cells around the lesion. This should elicit caution when interpreting focal uptake (higher than the constitutively high bone marrow background uptake) of 18F-FLT in bone.