The study find that PET/CT is extensively used in the assessment of patients with lymphoma; as Fluoro-18-deoxyglucose (FDG) is routinely employ to detect areas of active disease. Although FDG is a sensitive tracer; its distribution is not tumour-specific, since activated macrophages and other inflammatory cells may present increased glucose metabolism; and is report in aseptic inflammations (healing, burns) or septic bacterial, viral, or mycotic infections.
Increased glucose metabolism
This is especially relevant in the setting of suspected recurrence or in follow-up; since several authors reported a high rate of false-positive (FP) results. For instance; Rhodes et al. described an FDG-positive predictive value (PPV) of 53% in the follow-up of paediatric lymphoma patients. Other authors reported significant FDG FPs in surveillance; with a PPV as low as 23% and 19% in a setting of adult diffuse large B cell lymphoma (DLBCL), while another group observed a PPV approaching 21% in aggressive non-Hodgkin lymphoma (NHL) .
A prospective study investigating the value of serial FDG-PET over time; in 421 lymphoma patients (160 Hodgkin lymphoma HL, 183 aggressive NHL, and 78 indolent follicular NHL) who achieved the first complete remission, demonstrated that, in the particular signs of FDG-PET positivity; histological confirmation by biopsy is mandatory because at least one-third of such patients can present benign conditions.
Attractive biological target
A more select series suggested that a mediastinal FDG positivity not be consider sufficient for diagnostic purposes in view of its lack of discrimination (actually not confirm by histology in approximately 40%) . Being aware of FDG limitations; F18–3′-fluoro-3′ deoxythymidine (FLT) is develop as a PET proliferation tracer, an attractive biological target in cancer imaging being a distinctive feature of tumour cells and a key component of tumour development and growth.
Thymidine is a native nucleoside which is utilized by proliferating cells for DNA replication during the S-phase of the cell cycle. FLT is handle by cytosolic thymidine kinase 1 (TK1), the salvage pathway key enzyme; whose activity is cell cycle-dependent and threeto-four times higher in malignant cells than in benign cells. Thus, FLT uptake is not related to glucose metabolism, and reflects essentially the activity of cytosolic TK1 .
Cytosolic thymidine kinase
Preclinical studies have shown a clear relationship between FLT maximum standard uptake value (SUVmax) and cancer proliferative activity measured by biochemical indices of proliferation [proliferating cell nuclear antigen immunostaining; Ki-67/MIB-1 or S-phase fraction). Given the high uptake of FLT in normal bone marrow and the high frequency of marrow involvement in lymphoma; and given its lower tumour-to-normal-tissue contrast than FDG outside the brain, the clinical usefulness of FLT-PET in staging lymphoma is limit.