Combined ¹⁸F-FDG and Fluoride Approach in PET/CT Imaging: Is There a Clinical Future?

TO THE EDITOR: The April 2009 issue of the journal contains a paper on the combined use of ¹⁸F-FDG and ¹⁸F-fluoride for dual detection of soft-tissue and skeletal metastatic disease (1). There are 2 comments that I feel should be made about this paper.

First, the authors state “Combining ¹⁸F and ¹⁸F-FDG in a single PET/CT scan for cancer detection has not been reported to date.” Although it is technically true that this is the first paper to be published that has reported combined ¹⁸F-FDG and ¹⁸F-fluoride imaging with a combined PET/CT scanner, it is slightly disingenuous not to mention until well into the “Discussion” section of the manuscript that these agents have been used in combination with PET-only scanners for more than a decade. The impression given is that it is a novel concept to combine the 2 agents into a single study. It is not.

Second, the authors use a “bone mask” with which to “separate” skeletal uptake, assumed to be attributable to the ¹⁸F-fluoride, from the soft-tissue uptake attributable to ¹⁸F-FDG. They state “We successfully separated the metabolic skeletal uptake and allowed interpretation of the ¹⁸F and ¹⁸F-FDG tissue distribution, even though the 2 tracers were administered at the same time.” This is nonsense. The authors acknowledge that “bone marrow–stimulating therapy” and soft tissue abutting bone, and therefore being included in the bone mask, may confound this separation by including some ¹⁸F-FDG in the skeletal images. However, it is well established that osteoblastic and osteolytic lesions display different uptake patterns with ¹⁸F-FDG, with lytic lesions in bone demonstrating high ¹⁸F-FDG uptake (2). Indeed, Cook and Fogelman have previously reported the combined use of ¹⁸F-FDG and ¹⁸F-fluoride in numerous publications and texts (3,4). Further, from a purely scientific point of view, their assertion could be substantiated only if they performed both separate and simultaneous PET scans with the 2 tracers to see how many lesions were seen in the skeleton with ¹⁸F-FDG.

It is disappointing to see such a flawed and naïve piece of work published in what is an otherwise excellent journal.