*2.5. Limitations of 18F-FDG PET*/*CT*

Limitations of 18F-FDG PET/CT include its limited availability in comparison to conventional radiological modalities as well as its higher cost. Moreover, the poor sensitivity for the detection of diffuse bone marrow infiltration or skull lesions, due to masking of their activity by the underlying physiological tracer uptake in the brain, is an important drawback. In a report of 227 MM patients the incidence of PET false-negativity was 11% in these patients, a finding attributed to the significantly lower expression of the gene coding for hexokinase-2, which catalyzes the first step of glycolysis [32]. However, this explanation warrants further validation [33]. Further, 18F-FDG, as a glucose analog, is generally restricted in oncological imaging by both false positive (inflammation, post-surgical areas, recent use of chemotherapy, fractures, etc.) and false negative results (hyperglycemia, recent administration of high-dose steroids, etc.). Finally, issues are raised due to the lack of established criteria for image interpretation of 18F-FDG PET/CT scans in MM, resulting in poor interobserver reproducibility in interpreting results. In an attempt to standardize the interpretation of 18F-FDG PET/CT, the Bologna group has recently proposed the Italian Myeloma criteria for PET Use (IMPeTUs) based on the standard Deauville five-point system [34]. These descriptive criteria take into account the number and site of focal lesions, the presence of EMD, as well as the diffuse bone marrow involvement. The first results from the application of IMPeTUs seem to improve the interobserver reproducibility in scan interpretation; however, this needs to be confirmed in further studies.
