Investigation of Discrepancies between Baseline MRI and PET-CT Studies in Patients with Newly Diagnosed Multiple Myeloma.

Introduction: Magnetic Resonance Imaging (MRI) and ¹⁸F-labeled fluorodeoxyglucose (FDG) positron emission tomography - computed tomography (PET-CT) imaging are both useful for diagnosis, staging, and restaging of multiple myeloma (MM), yet they are based on different physical principles and do not give equivalent results. It is widely known that FDG uptake can be transiently inhibited in a variety of malignancies due to recent treatment, whether transient or durable. We investigated patients from Total Therapies 2 and 3 who had baseline PET-CT exams without focal lesions (FL) at time of diagnosis that had 1 or more baseline MRI-defined FL to determine reasons for the discrepancies between imaging studies.

Results: Thirty-three (33) TT2 and TT3 patients with baseline MRI-FL but without baseline PET-FL were identified. A detailed review of the clinical records and imaging studies was performed to identify potential reasons for the discrepancies between the imaging studies. One (3%) was a database error with no discrepancy. One patient (3%) had MRI-FL limited to the calvarium, an area difficult to visualize on PET-CT because of normal intense uptake in the brain. Four (12%) had “masking” of the PET FL (the PET-FL were obscured against the background by intense uptake in the surrounding marrow from severe diffuse tumor infiltration), 7 (21%) had FL seen by MRI that were below the PET resolution (sub-5mm), and 10 (30%) had PET suppression from treatment prior to arrival at our institution (glucocorticoids, such as dexamethasone or prednisone most commonly, but other medication-related causes included VDTPACE and bisphosphonates). In ten patients (30%) the causes for the discrepant imaging results were not determined, though indolent disease with low glucose metabolism resulting in poor FDG uptake relative to background is a possible explanation.

Conclusion: We conclude that FDG PET-CT and MRI imaging at baseline are important and complementary examinations that do not provide equivalent results. The PET-CT can be transiently and rapidly suppressed by pretreatment with a variety of medications that can inhibit glucose metabolism. Additionally, FDG PET-CT imaging can underestimate the number of FL if there is severe, diffuse metabolically active tumor infiltration (“masking”) obscuring the margins of the PET-defined FL, if the FL are very small (sub-5mm, below FDG PET-CT resolution), if the FL are located in “blind spots” for the PET-CT scan (such as near the brain), or if the PET-CT FL are low in metabolic activity relative to normal tissue (poor FDG uptake). In some cases, no apparent reason was determined. A careful history regarding recent medication is very important for proper interpretation of the results of a sensitive functional imaging study such as FDG PET-CT, as well as an understanding of its physical limitations.