Role of FDG-PET in the clinical management of paraneoplastic neurological syndrome: detection of the underlying malignancy and the brain PET-MRI correlates.

The role of 2-deoxy-2-[F-18]fluoro-D-glucose-positron emission tomography (FDG-PET) imaging in the clinical management of paraneoplastic neurological syndrome (PNS) is in evolution. The initial results are promising, and the potential of this modality in this setting has been demonstrated in the literature. In the setting of PNS, FDG-PET imaging can be of value from the following standpoints: (1) detection of the occult malignant focus and (2) objective assessment of the presence and extent of the functional abnormality in the brain and correlation of the imaging findings with the clinical features and disease activity. For this communication, we have not considered case reports described in the literature, while tabulating the results of the published studies. However, we have referred to important observations made in such reports. In part, this effort has been made because of the relative paucity of the existing literature on this issue. The studies investigating the role of FDG-PET imaging in detecting the sites of malignancy can be divided into two broad categories: (A) studies that incorporated FDG-PET at the initial work up of PNS along with other modalities and (B) studies where FDG-PET was applied after conventional modalities had failed to detect the offending malignancy. While majority of the studies have been of the second category, both categories have demonstrated the incremental role of FDG-PET in the detection of the primary cancer in this setting. Also, it is evident that serial studies at certain intervals should be carried out if the initial FDG-PET scan is negative. The literature describing the central nervous system findings with PET in PNS are primarily restricted to paraneoplastic limbic encephalitis (PLE) and paraneoplastic cerebellar degeneration (PCD). FDG-PET usually shows hypermetabolism in one or both temporal lobes in the setting of paraneoplastic limbic encephalitis, and the magnetic resonance imaging (MRI) findings correlate with the FDG-PET scan results in only a fraction of cases. The functional-anatomic discordance between PET and MRI in PLE needs to be examined further, which might open up new insights into the disease process and might generate further subgroups within this entity. Both modalities complement each other in PLE, and frequently, abnormalities noted on FDG-PET images can provide additional clinical information which is of great value in further patient management. In the setting of paraneoplastic cerebellar degeneration, FDG-PET generally reveals cerebellar hypometabolism similar to the cerebellar atrophy demonstrated by MRI, and FDG-PET tends to show the abnormality more often than MRI.