Increased 18F-2- Fluorodeoxysorbitol (18F-FDS) Activity in a Pituitary Spindle Cell Carcinoma: Reply

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To the Editor:
I would like to thank Dr Javadi for their letter about our article, “Increased 18F-FDS Activity in a Pituitary Spindle Cell.” We are grateful for issues raised in the letter. However, we cannot agree with the mechanism of 18F-FDS uptake that was proposed.
Efforts have been paid on looking for imaging agents identifying tumors and inflammation. Weinstein et al1 reported that they confirmed a higher 18F-FDS uptake in infection than in brain tumor when using the U87MG human brain tumor model and Escherichia coli brain infection model in mice. By performing dynamic 18F-FDS PET imaging over 120 minutes, they found that the signal from brain tumors dissipated at 60 to 120 minutes after the tracer injection, whereas signals from the E. coli brain infection were still high. According to these results, FDS may differentiate between inflammation and tumors.
Li et al2 were the first to synthesize 18F-FDS by reducing FDG for the molecular imaging of brain tumors.2 In his work, 18F-FDS had good tumor uptake at 5 to 60 minutes after the tracer injection in U87MG and GL-26 orthotopic tumor model in mice. But 18F-FDS was cleared more in this GL-26 tumor model because a 60-minute dynamic scan was carried out.
As mentioned previously, Weinstein et al1 and Li et al2 characterize the imaging in U87MG brain tumor model within 60 and 120 minutes after the tracer injection, respectively. After approval by the institutional review board, dynamic PET scans were first obtained to identify FDS distribution in normal human body at 5 to 120 minutes after the tracer injection.3 During the scan, the FDS uptake in the cerebral cortex was the lowest, which was consistent with that of animal models. The uptake in the pituitary was slightly higher than that in the cerebral cortex at 5 minutes, whereas it was much lower in bones. The uptake in the internal carotid beside the pituitary was high. The uptake in pituitary was close to that in the cerebral cortex at 120 minutes. Based on the distribution of 18F-FDS in the human body, the low-level background uptake in the pituitary and the cerebral cortex facilitates a clear lesion imaging.
In our work, we used 18F-FDG and 18F-FDS PET to differentiate malignancy from inflammation in a 33-year-old woman with a pituitary spindle cell carcinoma.4 The scans were obtained at 5 minutes after the tracer injection. The metabolic activity at the lesion was obviously higher than that at the pituitary of the normal human. Besides, as MRI and pathology revealed that the entire pituitary became tumor tissues and no normal pituitary tissue was found, the high uptake in the tumor lesion at 5 minutes was considered to correlate with tumor tissue metabolism.
To exclude the possibility of the blood pool effect, scans were performed in a case with primary intracranial diffuse large B-cell lymphoma at 130 minutes. The result showed a high FDS uptake in tumor, which contradicts the results that the tumor lesion activity eliminated at 120 minutes in the animal experiment by Weinstein et al.1 We consider that the difference in results may be attributed to the possibility that the decay of FDS signal in specific tumor in human differs from that in the animal. In the future, clinical test will be performed to test this idea.
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