CD117 (KIT) is a useful marker in the diagnosis of plasmablastic plasma cell myeloma

Aims: Plasmablastic plasma cell myeloma (PPCM) is a rare morphological presentation of multiple myeloma, and can resemble plasmablastic lymphoma (PBL), a human immunodeficiency virus (HIV)-related lymphoid neoplasm, morphologically and immunophenotypically. The aim of this study was to retrospectively search for factors that could help to differentiate these two entities. Methods and results: We used clinical looking glass, a data mining tool, to identify patients with a diagnosis of either PPCM or PBL with a CD117 test performed. We identified 27 cases from 16 patients in our institution with a diagnosis of either PPCM or PBL. There were 14 cases of PBL from eight patients and 13 cases of PPCM from eight patients. We found that six of eight patients with PPCM were positive for CD117, whereas no patients with PBL (0/8) showed positivity (P = 0.007 for patients). All (8/8) PPCM patients had a paraproteinaemia, whereas only two of eight PBL patients had a paraproteinaemia (P = 0.007). Lytic bone lesions were more indicative of a PPCM, with six of eight patients showing these lesions, and only one of eight PBL patients showing these lesions (P = 0.0406). HIV status was more likely to be positive in PBL patients, with six of seven patients showing positivity, and only one of five PPCM patients showing positivity (P = 0.072). Epstein–Barr virus-encoded RNA in-situ hybridization did not differentiate these two entities, with seven of eight PBL patients showing positivity, and two of five PPCM patients showing positivity (P = 0.2168). Conclusions: This study reveals that CD117 can be a useful marker to help differentiate PPCM from PBL and give the patient an appropriate prognosis and options for treatment.