FDG PET Functional Imaging in Multiple Myeloma - Clinically Important Caveats, Pitfalls, and Pearls.

We have performed 2,000+ Fluorodeoxyglucose PET (PET) scans for multiple myeloma (MM) staging and restaging at our facility since October 2001. While the usefulness of the PET scan for MM is reported by us and others elsewhere, we have reviewed our list of “incidental” but important findings, some of which are unique or occur more commonly with MM patients and some of which are common to many patients, the more common of which we present below: Occult infection occurs very commonly in patients with MM due to direct tumor effect on the immune system and to medication (especially high dose dexamethasone). Of the 2000+ PET scans for MM done at our facility, 300+ infections (about one half occult) have been detected by PET. These most commonly involve central lines (septic thrombophlebitis), diskitis, lung (either bacterial or fungal), and periodontal abscesses (a source of bacteremia in these patients), though non-catheter related spontaneous septic thrombophlebitis also occurs.

While related to MM, extramedullary disease from MM (EMD) is seen more commonly with PET than MRI since PET has a wider field of view. In our series of 172 patients with both baseline PET and MRI, PET detected EMD in 11/11 patients versus MRI detection rate of 7/11. On follow-up, PET detected three times more EMD (29/31 sites) than MRI (9/31sites). Detection of EMD by PET at baseline is a profoundly negative prognostic factor in our series (12 month EFS 20% for EMD+ vs. 47% for EMD−, p = 0.002, and OS 42% for EMD+ vs. 70% for EMD−, p=0.005, n=48). Following tumor response in hypo- or non-secretory disease is difficult by standard prognostic factors (SPFs). FDG PET results were actively used for clinical management in 10 of 11 non-secretory patients at our facility.

A major pitfall of PET scanning for MM comes from the use of steroids. Treatment with chemotherapy in general and steroids (i.e. prednisone or dexamethasone) in particular can result in a false negative PET scan by producing a profound but transient suppression of tumor metabolism. In addition, the hyperglycemic effect of the steroids produces competitive inhibition of FDG (a glucose analog) uptake, also causing suppression of FDG tumor uptake that can lead to an underestimation of disease. Second primary malignancies are frequently seen the MM age group. In our population, we have found unsuspected breast cancer, breast lymphoma, thyroid cancer, melanoma, colon cancer and lung cancer. In addition, we have found several functioning thyroid adenomas and premalignant colon polyps. Being a whole body imaging device for metabolism, FDG PET is a powerful though nonspecific tool for imaging that can not only help stage and restage the malignancy under question but which can yield a plethora of additional clinically useful information if used properly and with its limitations understood.