Physiological FDG avid uptake in the lower thoracic spinal cord
Presentation
A patient with metastatic squamous cell cancer of the vulva had an FDG PET scan for further staging.
Patient Data
FDG F-18 PET-CT
The PET scan shows avid FDG uptake in the vulva, compatible with known primary malignancy.
FDG-avid metastatic lymphadenopathy at the bilateral groins, external iliac nodal stations, and right pelvic sidewall.
There is possible metastatic adenopathy in the bilateral right greater than left axillae versus reactive nodes.
Nonspecific increased FDG uptake within the gastroesophageal junction.
The focal area of intense FDG uptake in the spinal canal at the T11–T12 level is indeterminate in the absence of other CNS diseases.
MRI thoracic spine
There is physiologic intradural vascularity. The post-contrast images show no abnormal nodular enhancement along the surfaces of the spinal cord or within the spinal cord substance. There is no evidence for intramedullary signal abnormality to suggest spinal cord edema. The spinal cord is normal in this study.
Elsewhere within the thoracic spine, there is no evidence of a tumor within the marrow or the prevertebral or paraspinal soft tissues. There is no evidence of a CSF spread/leptomeningeal tumor within the thoracic spine. The focus of increased FDG uptake within the thecal sac region on the PET scan shows no MRI correlation.
The above findings indicate that there is no evidence of a tumor within the thoracic spine.
Case Discussion
PET-CT is a useful modality when evaluating malignancy and treatment response. With the increasing popularity of PET-CT, clinicians have noticed 18F-FDG uptake in the spinal cord, which has been documented as a potential physiologic phenomenon 1. It has been hypothesized that the increased FDG uptake in the spinal cords of patients with non-central nervous system malignancy can explain the physiologic processes of glucose metabolism in the spinal cord 1.
Increased FDG uptake in the spinal cord of patients with underlying malignancy raises suspicion of spinal cord metastases 2. However, physiologic uptake in the C4 and T11-T12 segments has been consistently documented 2. It has been hypothesized that the increased uptake in the cervical and thoracolumbar regions can be linked to the increased cross-sectional area related to the cervical and lumbar enlargements that provide innervation to most of the upper and lower limbs 2. Another hypothesized explanation for the increased FDG uptake is the delayed FDG clearance from the artery of Adamkiewcz 3.
In retrospective studies, it has been reported that incidental focal spinal cord uptake in patients with malignancy occurred at a 0.3% incidence rate 4. In the subset of patients with spinal cord uptake, T12-L1 was the most common site (85% of uptake patients) 4. Follow-up PET-CT (15 months) showed a disappearance of uptake in 52% of patients, unchanged uptake in 38% of patients, and MRI follow-up showed no abnormalities that could suggest malignancy or metastasis 4.
Thus, FDG uptake in the cervical and thoracolumbar regions may be a physiologic process but is generally not well understood. In patients with underlying malignancies, metastasis to the spinal cord should be considered since it is a serious complication. However, the literature notes that specific FDG uptake in the thoracolumbar junction region, in the absence of CNS malignancy, is almost always physiological.
In our patient, there was increased FDG uptake in the spinal cord at the T11–T12 level. A follow-up MRI showed no abnormality in the spinal cord that could suggest metastasis or other processes to explain the PET finding. There was low clinical suspicion that the thoracolumbar spinal cord FDG uptake was metastatic, but thoracic spinal MRI was done out of an abundance of caution because of the intensity of the FDG uptake and the presence of other metastatic disease on the PET. It was concluded that the T11-T12 FDG uptake was benign. A lumbar puncture for CSF sampling to exclude malignant cells was not performed.
Future radiologists should be aware that increased FDG uptake on PET-CT at the T11-L1 levels is almost certainly benign in the absence of CNS malignancy. Knowing this can obviate the need to do a lumbar puncture for CSF sampling and further imaging of the neuroaxis.
This case was edited and provided by Ashwin Hampole, MD.
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