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Multiple myeloma (MM) is a heterogeneous and ultimately fatal disease. Risk stratification using prognostic biomarkers is crucial to individualize treatments. We sought to investigate the role of CD99, a transmembrane protein highly expressed in many hematopoietic cells including subpopulations of normal and neoplastic plasma cells, for MM risk stratification.
CD99 expression was measured in paraffin samples of bone marrow and extramedullary biopsies of 170 patients with MM. Patients were divided into those with high score (moderately and strongly positive) and low score (negative and weakly positive), with all staining being cytoplasmic and/or membranous.
High anti-CD99 immunostaining was observed in 72 of 136 (52.9%) bone marrow biopsies and 24 of 87 (27.6%) extramedullary biopsies in MM. High CD99 expression of extramedullary specimens was associated with significantly longer overall survival (OS; p=.016). High CD99 expression of extramedullary specimens was also associated with better prognosis in the nonautologous stem cell transplantation group of MM patients (p=.044). In multivariate analysis, International Staging System stage was an independent prognostic factor, whereas CD99 expression was no longer statistically significant.
Expression of CD99 in extramedullary specimens was correlated with longer OS, suggesting that CD99 may be a helpful immunohistochemical marker for risk stratification.
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Myeloid sarcoma is a rare extramedullary myeloid tumor, which is frequently misdiagnosed when no evidence of leukemia is initially observed. Here, we report on a peculiar case of a 49-year-old man afflicted with multiple masses in the jejunum, the superior mesentery, and the serosa of the transverse colon, without leukemic manifestation. The tumor was composed of undifferentiated small round cells containing eosinophilic cytoplasm, which were negative for myeloperoxidase, nonspecific esterase, lysozyme, terminal deoxynucleotidyl transferase, leukocyte common antigen, CD3, CD4, CD15, CD20, CD30, CD43, CD56, CD68/PG-M1, CD79a, human melanoma black-45, c-kit, and CD34 with positivity only for CD68/KP1, CD99, and vimentin. Under electron microscopy, those cells had abundant membrane-bound cytoplasmic granules that measured 200 to 300 nm in diameter, which were consistent with granulocytic azurophilic granules. The tumor was finally diagnosed as a myeloid sarcoma. The presence of non-leukemic myeloid sarcomas showing immunonegativity for conventional myeloid-leukemic markers necessitated a diagnosis by ultrastructural observation.
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