18F-FDG-PET/CT in Multiple Myeloma: High Intense PET Positive Lesions at Diagnosis Harbor Cytogenetic Adverse Subclones

Aim:In multiple myeloma (MM), high intensity of FDG uptake in focal lesions measured as the standard uptake value (SUV) at diagnosis is associated to aggressive disease and reduced overall survival. However, the reason for high intensity FDG uptake in some focal lesions is unknown, but hypothetically such "hot" lesions could represent evolving myeloma sub-clones with particular characteristics, e.g. higher proliferative activity. We aimed to explore and characterize focal lesions with high FDG uptake and to compare the findings with random diagnostic bone marrow biopsies with lower FDG intensity or a biopsy from another lesion with lower FDG uptake. Thus, we have created a paired biopsy biobank that will be explored for molecular, biological, physiological and myeloma prognostic markers. Here we present our first data including results on morphology, immunohistochemistry, mutational, proliferative and cytogenetic status in the biopsies

Material and methods: CT-guided biopsies from FDG-PET positive CT-visible focal lesions in sternum, sacrum, humerus, femoral, pubic and iliac bones were taken without complications depending on accessibility and safety in 14 newly diagnosed, untreated MM patients, 2 females and 12 males, aged 53-77 years. Patients that had received steroids or bisphosphonates were excluded. Bone marrow biopsies were taken as part of the normal diagnostic work-up, but included an extra biopsy and aspirate for research. FDG uptake in the regions of interest (ROI) at focal and random bone marrow biopsy was quantified by dedicated software (ROVER, ABX, Radeberg, Germany) to obtain the following variables: Lesion volumes, SUVmax, SUVpeak, cSUVmean (SUVmean corrected for partial volume effect). The paired biopsies were analyzed for: Myeloma plasma cells percentage (PC%), myeloma cell proliferation (Ki67 positive fraction of CD138 positive cells (Ki67/CD138%), myeloma cell MYC protein expression (MYC/CD138%), FISH aberrations in % of myeloma cells (del17p, del13q, del1p, amp1q, t(11;14), t(4;14), BRAF mutation and specific p16, p27, and p53 protein expressions by IHC in % of MM cells.

Results: 13 patients were evaluable for analysis with paired datasets. One patient was excluded due to a normal bone marrow examination (multifocal myeloma). ROI SUVmax values ranged from 2.6 to 22.16 and differences in SUVmax between paired biopsies ranged from 0.4 to 17.1. First of all, we found myeloma malignancy in all PET-positive CT-guided biopsies. Comparing the findings in "hot" versus "random" biopsy groups we found significantly higher PC%s in the "hot" biopsy group (p=0.01) but this was not a consistent finding in all patients. PC proliferation rate (Ki67/CD138) was higher in some of the "hot" biopsies but this was neither a uniform observation. No difference in "primary event" chromosome 14q translocations was observed, whereas we identified subclones with typical cytogenetic secondary or late events in several "hot" biopsies that were not present in the random bone marrow biopsy: amp1q in 3 patients, del1p in 1 patient, del13q and del17p in 1 patient. MYC protein expression was higher in "hot" biopsies in 4 patients, and downregulation of p27 was evident in 2 patients. We found no unbalanced expression of p53 or p16, and in only one patient we identified a BRAF mutated subclone that was equally present in the "hot" and "random" biopsy (20% vs 30%). Few patients presented more adverse findings in the "hot" biopsies. Overall, MM adverse findings were present unbalancedly in 8 of 13 patients.

Conclusion: We did not identify a mutual factor that explained the more intense FDG uptake in CT guided biopsies than in random bone marrow, e.g. a higher PC Ki67 expression. However, in 8 of 13 patients we identified 1 or more prognostic adverse, unbalanced findings in the PET positive focal lesions indicating presence of more aggressive subclones. These findings are concordant with the adverse prognostic importance of finding high-intense PET-positive focal lesions on FDG-PET/CT in newly diagnosed MM patients.