Synergistic role of simultaneous PET-MRS in tumor metabolism imaging

Objectives Both PET and MR spectroscopy (MRS) have been used clinically for evaluating tumor metabolism. Although MRS allows simultaneous measurement of multiple metabolites, most clinical applications are limited to a small volume of known lesion with single- or multi- voxel(s). In contrast, FDG-PET highlights potential metabolic abnormalities throughout the body, but may yield false positive results associated with inflammation, etc. In a case in which FDG-PET is indeterminate for residual tumor vs. post-treatment inflammation, this ambiguity can be solved by PET-guided MRS on an integrated PET-MR system.

Methods A patient with sarcoma of the right thigh underwent PET-MR scan pre- and post- preoperative radiotherapy. The MRI and 18F-FDG-PET scans were performed simultaneously followed by MRS. MRS was acquired with volume of interest positioned a residual hyper-metabolic region detected by PET. Pathology results were obtained after surgery for confirmation of imaging findings.

Results Post-radiotherapy MRI showed a residual T2-hyperintense mass with significantly reduced FDG-uptake, compatible with response to radiotherapy. However, a small region of high FDG uptake was detected at the tumor margin. MRS of this region had similar metabolite profile as normal tissue, and was thus considered false positive. This was confirmed by pathology. NMR spectra of tumor core had higher Choline level, and thus higher membrane turnover, as compared to spectra of normal tissue. Radiation-induced cell-death prevents glucose transportation across cell membranes, resulting in no FDG uptake in treated tumor. However, radiation damage may take weeks to break down cell membranes which are reflected by the high Choline level on MRS.

Conclusions While PET can quickly identify regions of hyper-metabolism, NMR spectra provide additional metabolic information in those suspicious regions. PET-guided MRS has potential to provide more accurate preoperative assessment and treatment planning in sarcoma patients with equivocal FDG-PET findings.