**2. 18F-FDG PET**/**CT in MM**

PET/CT is a whole-body imaging technique combining the functional information of PET with the morphological assessment provided by CT. 18F-FDG, the workhorse of PET imaging, is a biomarker of intracellular glucose metabolism. The tracer is actively transported into cells by the glucose transporter proteins (GLUT), which are expressed at a high degree in tumor cells due to their enhanced glucose demands. 18F-FDG, as a glucose analogue, is taken up by the neoplastic cells, undergoes phosphorylation and then gets trapped intracellularly, since 18F-FDG is not a substrate for further metabolic processing by either phosphohexose isomerase or glucose-6-phosphate dehydrogenase [6].

18F-FDG PET/CT has become nowadays a standard imaging technique in several tumor entities. Due to its ability in providing whole-body evaluations in a single session, the modality can assess the extent of oncological disease in a satisfying manner. In MM in particular, PET/CT can detect with a high sensitivity and specificity both medullary and extramedullary lesions [7]. Another important advantage of PET is the potential of quantification of tracer uptake by means of the index standardized uptake value (SUV), which reflects the amount of tracer activity in a particular region of interest. This quantification of tracer uptake aids in objective interpretation of PET/CT scans in addition to obtaining cross-sectional imaging and assessing 18F-FDG uptake visually, particularly in terms of patient follow-up. Furthermore—and most importantly—18F-FDG PET/CT can assess the metabolic burden and activity of MM in different stages of the disease due to its ability in differentiating between metabolically active and inactive lesions, with significant implications in treatment response assessment [5,7].
