Objective: To study morphology, CD56 and cyclin D1 expression and EBER status in PCNs presenting with masses.

Material and Method: Thirty-nine mass-forming PCNs with available materials between 2006 and 2010 were identified from Siriraj Hospital pathology database. H&E slides were reviewed for morphologic grade according to Bartl grading system. Immunohistochemistry for CD56 and cyclin D1 and EBER in situ hybridization were analyzed on tissue microarray sections of the included cases.

Results: Of 39 cases, it comprised 31 (79.5%) plasma cell myelomas (PCMs), five (12.8%) osseous plasmacytomas (OPs), and three (7.7%) extramedullary plasmacytomas (EMPs). Intermediate-grade morphology was common to all types of PCNs. CD56 and cyclin D1 positivity were more often in PCMs comparing with OPs and EMPs; however, differences in expression of these markers among different types of PCNs were insignificant (p>0.05). An EBER-positive EMP was identified.

Conclusion: The majority of mass-forming plasma cell tumors in the studied population are PCM-related. Intermediategrade morphology is common in all types of PCNs. A value of CD56 and cyclin D1 immunostains in discrimination between types of PCNs cannot be confirmed in the current study. Identification of the EBER-positive EMP suggests that EBV association in plasma cell tumor can be encountered in Thais.

Keywords: Plasma cell neoplasms, Plasmacytomas, CD56, Cyclin D1, EBER