Cold vertebrae on F-18 FDG PET: Causes and characteristics

Background: Photon deficiency in the vertebrae on FDG PET can arise as a result of varying etiologies, and a proper interpretation of this incidental observation is essential for an accurate diagnosis. Materials and Methods: Cases with "cold" vertebrae on FDG PET resulting from different causes were selected and analyzed from a population of patients with cancer who underwent conventional whole-body FDG PET for staging, disease viability assessment, or treatment monitoring purposes. The patterns were studied and correlated with the appropriate clinical setting. Results: Three distinct causes were observed. Postexternal radiotherapy photopenia involving multiple vertebrae corresponding to the radiotherapy portals represented the most common setting for such findings followed by the destruction of the vertebral marrow cavity in the center by metastatic tumor cells. The third situation was vertebral hemangioma, in which relative photon deficiency in the concerned vertebra was incidentally observed amid generalized marrow uptake after colony-stimulating factor therapy. The characteristics and features of "cold" vertebrae in prototype cases selected from each etiology are described with illustrations. Conclusion: Pattern recognition coupled with further correlation of this observation is the key to the correct diagnosis of the underlying etiology, which can at times have a bearing on subsequent management. With the growing use of FDG PET, this observation is likely to be encountered with increasing frequencies and possibly newer etiopathologies.