Association between 18F-FDG PET/CT and MRI appearance of spinal leptomeningeal disease before and after treatment at a tertiary referral center.

Objective: Leptomeningeal disease (LMD), the presence of metastasis in the subarachnoid space, has devastating implications if left untreated. The gold standard for LMD diagnosis is cytologic analysis of cerebrospinal fluid (CSF); MRI is also used to evaluate suspected LMD. The purpose of this study was to compare the appearance of LMD in the spinal canal on ¹⁸F-FDG PET/CT imaging with the appearance of LMD on MRI and with CSF cytology.

Methods: In twenty-one patients with cytologically-proven spinal LMD, findings on ¹⁸F-FDG PET/CT, MRI, and CSF cytology at diagnosis of LMD and after the initiation of treatment for LMD were retrospectively reviewed.

Results: At diagnosis of LMD, abnormal ¹⁸F-FDG avidity was demonstrated in the spinal canal in six patients, and the anatomic distribution of ¹⁸F-FDG activity corresponded to the sites of LMD on MRI. All six of these patients were then treated with intrathecal chemotherapy. Follow-up ¹⁸F-FDG PET/CT and MRI were obtained in four of the six cases. In all four cases, normalization of ¹⁸F-FDG activity in the spinal canal and reduction of enhancement on MRI corresponded to the cytologic response to treatment, as determined by CSF analysis.

Conclusion: ¹⁸F-FDG avidity in the spinal canal greater than the normal contents of the canal can suggest spinal LMD. This abnormal avidity may be detected before the diagnosis of LMD has been established with MRI or CSF cytology. The spinal canal should be routinely evaluated on ¹⁸F-FDG PET/CT in patients with suspected LMD so that appropriate treatment is initiated.