Clinical Significance of Incidental Focal 18F-FDG Uptake in the Spinal Cord of Patients with Cancer

Correlation of 18F-FDG PET/CT uptake with severity of MRI findings and epidural steroid injection sites in patients with symptomatic degenerative disease of the lumbar spine: a retrospective study

PURPOSE

We aimed to retrospectively correlate 18F-fluorodeoxyglucose (18F-FDG) metabolic activity with lumbar spine magnetic resonance imaging (MRI) findings and epidural steroid injection sites in patients with symptomatic degenerative disease of the lumbar spine.

METHODS

A database search was conducted for patients receiving epidural injections <12 months after a positron emission tomography/computed tomography (PET/CT). Maximum standard uptake values (SUVmax) were measured at the facet joints, neural foramina, and spinal canal. Severity of facet arthrosis, disc degeneration, neuroforaminal, and canal stenosis was determined on MRI using previously described grading scales. Spearman rank coefficient assessed association between PET/CT FDG uptake and severity of MRI findings. The SUVmax was also compared with injection sites.

RESULTS

Twenty-five patients were included, comprising MRI (n=19) and injection (n=22 patients; 18 interlaminar, 8 transforaminal) groups. Injections were performed an average of 2.6 months after PET/CT. The greatest SUVmax occurred at the L5-S1 spinal canal (mean SUVmax = 2.25). A statistically significant, positive correlation between uptake and grade of spinal canal stenosis was seen only at L4-L5 (r=0.60, p = 0.007). No other significant association was found with spinal canal or neuroforaminal stenosis, or grade of facet joint or disc degeneration. All patients reported symptomatic improvement after injections with mean pain score improvement of 4.4 on a 10-point scale (SD, 2.9). There was moderate agreement between sites of epidural injection and highest SUVmax (κ= 0.591, p < 0.001).

CONCLUSION

18F-FDG metabolic activity on PET/CT corresponds with MRI findings about the lumbar spinal column, but there is no significant correlation between severity of MRI findings and radiotracer uptake. Given the moderate agreement between metabolic activity and levels of symptomatic spinal stenosis, further studies are warranted to fully evaluate the diagnostic potential of FDG PET/CT as a surrogate for guiding epidural injections.

Physiological distribution of 18F-FDG in the spinal cord: A systematic review

Context: The importance of physiologic distribution of ¹⁸F-FDG in the spinal cord.Objective: The recognition of the physiologic distribution of ¹⁸F-FDG in the spinal cord is pivotal for accurate PET/CT imaging interpretation, especially in oncologic patients. Therefore, we performed a systematic review to investigate the normal distribution of ¹⁸F-FDG throughout the spinal cord.Methods: Data sources: We carried out a comprehensive search of the literature on the physiologic patterns of ¹⁸F-FDG distribution in the spinal cord. PubMed and Scopus databases were searched using the following keywords: "spinal cord" AND "FDG". Data extraction: Findings of the selected articles were described.Results: Thirteen studies comprising 24,125 patients entered the systematic review. These investigations showed discrepancies in location, size, number, and intensity of ¹⁸F-FDG uptake throughout the spinal cord. However, cumulative results showed that ¹⁸F-FDG uptake was higher in the lower thoracic portion of spinal cord (T11-T12). Moreover, a decreasing trend in ¹⁸F-FDG uptake was observed from cervical to lumbar levels. Low maximal standardized uptake values, female sex, and higher body weight seem to be related to the physiological spinal cord ¹⁸F-FDG uptake.Conclusions: On ¹⁸F-FDG PET/CT imaging, focal hypermetabolism of the spinal cord at the level of lower thoracic and lower cervical vertebrae should be considered physiological until proven otherwise.

[18F]FDG uptake of the normal spinal cord in PET/MR imaging: comparison with PET/CT imaging

BACKGROUND

The lack of visualization of the spinal cord hinders the evaluation of [¹⁸F]Fluoro-deoxy-glucose (FDG) uptake of the spinal cord in PET/CT. By exploiting the capability of MRI to precisely outline the spinal cord, we performed a retrospective study aimed to define normal pattern of spinal cord [¹⁸F]FDG uptake in PET/MRI.

METHODS

Forty-one patients with lymphoma without clinical or MRI signs of spinal cord or bone marrow involvement underwent simultaneous PET and MRI acquisition using Siemens Biograph mMR after injection of 3.5 MBq/kg body weight of [¹⁸F]FDG for staging purposes. Using a custom-made software, we placed ROIs of 3 and 9 mm in diameter in the spinal cord, lumbar CSF, and vertebral marrow that were identified on MRI at 5 levels (C2, C5, T6, T12, and L3). The SUVmax, SUVmean, and the SUVmax and SUVmean normalized (NSUVmax and NSUVmean) to the liver were measured. For comparison, the same ROIs were placed in PET-CT images obtained immediately before the PET-MRI acquisition following the same tracer injection.

RESULTS

On PET/MRI using the 3 mm ROI, the following average (all level excluding L3) spinal cord median (1st and 3rd quartile) values were measured: SUVmean, 1.68 (1.39 and 1.83); SUVmax, 1.92 (1.60 and 2.14); NSUVmean, 1.18 (0.93 and 1.36); and NSUVmax, 1.27 (1.01 and 1.33). Using the 9 mm ROI, the corresponding values were SUVmean, 1.41 (1.25-1.55); SUVmax, 2.41 (2.08 and 2.61); NSUVmean, 0.93 (0.79 and 1.04); and NSUVmax, 1.28 (1.02 and 1.39). Using the 3 mm ROI, the highest values of PET-MRI SUVmax, SUVmean, NSUVmax, and NSUVmean were consistently observed at C5 and the lowest at T6. Using a 9 mm ROI, the highest values were consistently observed at C5 and the lowest at T12 or T6. The spinal cord [¹⁸F]FDG-uptake values correlated with the bone marrow uptake at the same level, especially in case of NSUVmax. Comparison with PET-CT data revealed that the average SUVmax and SUVmean of the spinal cord were similar in PET-MRI and PET-CT. However, the average NSUVmax and NSUVmean of the spinal cord were higher (range 21-47%) in PET-MRI than in PET-CT.

CONCLUSIONS

Using a whole-body protocol, we defined the maximum and mean [¹⁸F]FDG uptake of the normal spinal cord in PET/MRI. While the observed values show the expected longitudinal distribution, they appear to be higher than those measured in PET/CT. Normalization of the SUVmax and SUVmean of the spinal cord to the liver radiotracer uptake could help in multi-institutional comparisons and studies.