Here, we report the outcome of 226 myeloma patients presenting with extramedullary plasmacytoma or paraosseous involvement in a retrospective study conducted in 19 centers from 11 countries. Extramedullary disease was detected at diagnosis or relapse between January 2010 and November 2017. Extramedullary plasmacytoma and paraosseous involvement were observed in 130 patients at diagnosis (92 of 38) and in 96 at relapse (84 of 12). The median time from multiple myeloma diagnosis to the development of extramedullary disease was 25.1 months (range 3.1-106.3 months) in the relapse group (median follow up: 15 months). Imaging approach for extramedullary disease was computed tomography (n=133), positron emission tomography combined with computed tomography (n=50), or magnetic resonance imaging (n=35). The entire group received a median two lines of treatment and autologous stem cell transplantation (44%) following the diagnosis of extramedullary disease. Complete response was higher for paraosseous involvement versus extramedullary plasmacytoma at diagnosis (34.2% vs 19.3%; P=NS.) and relapse (54.5% vs 9%; P=0.001). Also paraosseous involvement patients had a better progression-free survival (PFS) when recognized at initial diagnosis of myeloma than at relapse (51.7 vs 38.9 months). In addition, overall survival was better for paraosseous involvement compared to extramedullary plasmacytoma at diagnosis (not reached vs 46.5 months).Extramedullary plasmacytoma at relapse had the worst prognosis with a PFS of 9.1 months and overall survival of 11.4 months. In the multivariate analysis, paraosseous involvement, extramedullary disease at diagnosis, International Staging System (ISS-I), and undergoing autologous stem cell transplantation improved overall survival independently. This cohort demonstrated that extramedullary disease benefits from front-line autologous stem cell transplantation and extramedullary plasmacytoma differs from paraosseous involvement in terms of rate and duration of response, with even worse outcomes when detected at relapse, constituting an unmet clinical need.

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