Plasmablastic plasmacytoma followed by a plasmacytic plasma cell myeloma: insights into discordant extramedullary transformation-a case report and literature review.

Plasma cell neoplasms encompass a spectrum of disorders characterized by the clonal proliferation of plasma cells. Plasmablastic transformation in these neoplasms poses diagnostic and clinical challenges due to its aggressive nature and morphological overlap with other malignancies, including plasmablastic lymphoma (PBL). We report a case of discordant extramedullary plasmablastic transformation in a 55-year-old HIV-negative female presenting with an oral lesion diagnosed as plasmablastic plasmacytoma. Initial imaging indicated localized disease without systemic involvement. Despite undergoing chemotherapy and achieving a partial response, the patient developed osteolytic lesions 2 years later. Subsequent pathology evaluation confirmed mature plasma cell myeloma (PCM) morphology. Whole exome sequencing (WES) and whole RNA expression analysis revealed shared mutations (PTPN13, KRAS, LTK) and a MYC::BMP6 translocation in both lesions, supporting a clonal relationship. Additionally, the oral plasmablastic lesion revealed a KLHL6 mutation and an extra MYC::IGL translocation, which were absent in the tibial lesion. The KLHL6 mutation has not been previously reported in studies of discordant extramedullary plasmacytoma with plasmablastic transformation. This case highlights the diagnostic complexity of plasmablastic plasmacytoma presenting as the initial manifestation of plasma cell myeloma. It underscores the necessity of a thorough evaluation, including bone marrow biopsy, to accurately differentiate plasmablastic transformation from PBL and ensure accurate diagnosis and appropriate management.