A Rectal Mass That Deceived: Solitary Plasmacytoma Masquerading as Mantle Cell Lymphoma With Subsequent Progression to Multiple Myeloma

Solitary extramedullary plasmacytomas (SEPs) are rare plasma cell tumors, particularly when located in the gastrointestinal tract. Misdiagnosis may occur due to overlapping histological and immunophenotypic features with other hematologic malignancies, such as mantle cell lymphoma (MCL). We present the case of a 63-year-old female patient initially diagnosed with MCL based on rectal mass biopsy, who underwent lymphoma-directed therapy. Surgical excision and further histopathological evaluation revised the diagnosis to SEP. Over time, the patient developed serum monoclonal gammopathy and progressed to multiple myeloma (MM). This case highlights the diagnostic pitfalls of rectal lymphoid lesions, the value of surgical pathology, and the potential for SEPs to evolve into systemic plasma cell neoplasia.