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Intramedullary Spinal Cord Metastases: Conspicuity on PET and Correlation with MRI Features 2013

Category General Spine Petrice Mustard, Medical Student, Mayo Clinic Felix E. Diehn, Neuroradiologist/M.D., Mayo Clinic Jeffrey Rykken, Neuroradiologist, University of Minnesota Kara M. Schwartz, Neuroradiologist, Mayo Clinic Laurence J. Eckel, Neuroradiologist/M.D., Mayo Clinic John T. Wald, Neuroradiologist, Mayo Clinic Timothy J. Kaufmann, Neuroradiologist, Mayo Clinic Christopher P. Wood, Neuroradiologist, Mayo Clinic Christopher H. Hunt, Neuroradiologist, Mayo Clinic Purpose Intramedullary spinal cord metastases (ISCMs) are rare. The literature describing PET findings is limited to case reports.  A retrospective study was performed to evaluate the conspicuity of ISCMs on PET in a large series of patients with ISCMs. Materials & Methods A search of the radiologic and clinical databases at our institution had previously yielded a group of patients with ISCMs, in whom pre-treatment MRI had identified the ISCMs. For inclusion in the current study, this group was refined to those patients in whom a PET was available for electronic review. Patients in whom the PET was performed more than 60 days prior or 14 days after the pretreatment diagnostic MRI were excluded. Two radiologists and a medical student retrospectively reviewed the PET examinations, first blinded and then unblinded to the PET report and all MRI findings. Multiple characteristics were analyzed on PET for ISCMs which were identified, including: lesion location, superior-inferior length (mm), morphology of FDG uptake (fusiform vs. round), and maximum standardized uptake value (SUV). These features were compared to MRI lesion characteristics previously analyzed by two radiologists, including: lesion location, superior-inferior enhancement length (mm), extent of enhancement and T2 signal abnormality (number of vertebral segments), and ratio of extent of T2 signal abnormality to contrast enhancement.  One-way t-test assuming unequal variances was performed to assess correlation of the following features with visibility of ISCMs on PET: 1) number of days between MRI and PET, 2) enhancement length, 3) enhancement extent, 4) T2 signal abnormality extent, and 5) ratio of extent of T2 signal abnormality to contrast enhancement. The original PET interpretations were reviewed to assess if each ISCM had been prospectively reported. Results Among 49 patients harboring 70 ISCMs, 17 (35%) had a total of 22 PET examinations performed. 12 PET exams from 7 (41%) of these 17 patients were excluded because the PET was more than 60 days prior or 14 days after the pre-treatment MRI.  The final study sample was 10 PET exams (9 PET-CT and 1 PET only) in 10 patients with 13 ISCMs. The primary malignancies were lung cancer (8 ISCMs) and melanoma (5 ISCMs). 9 (90%) patients had solitary ISCM, while 1 (10%) patient with lung cancer harbored multiple ISCMs.  The mean interval between MRI and PET was 11 days (range: 49 days prior to 13 days after), and this interval did not correlate with ISCM visibility on PET. In 7 (70%) patients retrospective blinded review of the PET demonstrated convincing evidence of 10 (77%) ISCMs. The FDG uptake for each of these 10 ISCMs corresponded anatomically to the level of contrast enhancement on MRI, with round morphology in 8 (80%), mean length of 25.3 mm, and mean SUV maximum of 6.7. Three (23%) ISCMs in 3 (30%) patients could not be seen on PET, despite unblinding to the MRI results. Three MRI features correlated with ISCM visibility on PET: 1) larger lesion enhancement size: mean size of lesions visible vs. not visible on PET 32.1 mm vs. 6.0 mm, respectively (p=0.038); 2) larger extent of T2 signal abnormality: mean of 5.6 vs. 1.0 segments (p = 0.0081); and 3) larger ratio of extent of T2 signal abnormality to contrast enhancement: 3.8 vs. 1.0 (p = 0.0069).  ISCM was reported prospectively in only 2 (20%) of the patients, accounting for 5 (38%) of the ISCMs. In 4 patients, ISCM could be seen on blinded retrospective review despite not being described on the prospective PET report. One of these 4 patients had their PET performed 49 days prior to the MRI which eventually diagnosed the ISCM. Conclusion The majority of ISCMs can be detected on PET when this is performed near the time of diagnostic MRI. However, ISCMs may not be prospectively reported on PET.  Smaller lesions are less likely to be visible on PET. Despite their relative rarity, the spinal cord should be specifically and carefully assessed on PET for evidence of ISCM in order to provide timely and accurate diagnosis. References Poggi MM, Patronas N, Buttman JA, Hewitt SM, Fuller B. Intramedullary spinal cord metastasis from renal cell carcinoma: detection by positron emission tomography. Clin Nucl Med. 2001 Oct;26(10):837-9.