Molecular Features and Clinical Outcomes of Extramedullary Plasmacytomas

The treatment of multiple myeloma (MM) continues to evolve with new drugs, resulting in dramatically improved outcomes. Despite these advances, MM remains an incurable disease, with Extraosseous/Extramedullary disease (EMD) playing a major role in the development of relapsed refractory disease. EMD is defined by the development of plasma cell neoplasms that arise in tissues other than bones. EMD can present at diagnosis or develop during the disease course of MM. Sometimes EMD can be solitary without bone marrow (BM) involvement. Determining the molecular underpinnings of EMD are critical to advance the care of such patients. A few reports suggest a possible role of Ras mutations in the intramedullary to extramedullary transition in a limited number of patients. Therefore, the molecular biology of developing EMD has not yet been clearly defined.

We sought to identify molecular features of EMD and associated these with clinical outcomes.

We analyzed samples from 443 MM patients who presented to Massachusetts General Hospital between 2013 and 2021. All patients voluntarily signed informed consent approved by the institutional review board for SNaPshot (molecular) testing. BM aspirate, biopsy and EMD tumor specimens underwent pathological analysis as well as FISH testing. The diagnosis of EMD was determined by either CT, PET-CT or MRI done as part of their clinical course. In some cases, EMD was confirmed by biopsy. Although there were some cases of EMD adjacent to bones, we strictly defined EMD as non-adjacent to bones based on imaging.

Nucleic acids were extracted from BM and EMD samples obtained from MM patients. Multiplexed mutational analysis was done with primers designed to cover 111 genes including genes known to be oncogenic. SNaPshot was done and the threshold for allele frequencies was determined as 8% based on our laboratory cut-offs.

The median follow-up of the entire MM population was 63.7 months (range 1 to 408). Overall, 96 of 443 patients (21.6%) developed EMD as previously defined. Sixty-five out of 96 patients had biopsy confirmed EMD, while 31 patients were diagnosed by imaging only. SNaPshot molecular testing was performed on 30 EMD samples from the 65 patients who had biopsies. Interestingly, all EMD samples except for 1 had either NRAS, KRAS, or BRAF mutations. There were 14 EMD samples with NRAS mutations, 6 with KRAS mutations, 4 with BRAF mutations, and 1 with both KRAS and BRAF mutations. All BRAF mutations coexisted with other mutations, such as TP53, ATM, and ARID1A whereas some of the NRAS and KRAS mutations were observed alone.

Next SNaPshot was analyzed for 8 paired BM and EMD samples. Three BM samples were negative for NRAS mutations while all EMD samples were positive.

Patients with KRAS/NRAS/BRAF mutations who developed EMD had poorer prognosis than non-EMD patients with KRAS/NRAS/BRAF mutations. In patients with KRAS mutations, the median overall survival (OS) was 36.9 months (EMD) vs not reached (non-EMD) (p<0.01); and BRAF mutations, the median OS was 58.5 months (EMD) vs 112.4 months(non-EMD) (p=0.194). In patients with NRAS mutations, the median OS was 94.5 months(EMD) vs 124.5 months (non-EMD) (p=0.11). Average time from diagnosis of MM to developing EMD in 68 patients with longitudinal follow up was 41.6 months and average time from developing EMD to death in 54 patients with longitudinal follow-up was 7.6 months. There were significant differences in BM FISH analysis in KRAS/NRAS/BRAF mutation harboring EMD vs non-EMD; Chromosome 1 abnormalities (79.2% vs 34.2%), TP53 deletion (41.7% vs 22.4%) and hyperdiploidy (37.5% vs 67.1%).

Our data suggest that KRAS/NRAS/BRAF mutations may play an important role in the development of EMD. These and other associated molecular abnormalities portend a poorer prognosis and may provide novel therapeutic insights.