Gastric involvement by multiple myeloma: report of a rare clinical case with an atypical initial extramedullary manifestation.

<title>Abstract</title> Plasma cell neoplasms include solitary plasmacytoma and multiple myeloma (MM), the latter being characterized by clonal proliferation of plasma cells within the bone marrow, often accompanied by systemic organ damage. Although extramedullary involvement is relatively uncommon, its presence is associated with increased disease aggressiveness and a poorer prognosis. We report the case of a 54-year-old woman with no significant medical history who presented with cervical neurological symptoms. Imaging studies revealed a gastric mass initially suggestive of a gastrointestinal stromal tumor (GIST). Histopathological examination demonstrated infiltration by clonal plasma cells with a double monoclonal component—IgA lambda and free lambda light chains—confirming the diagnosis of multiple myeloma with gastric involvement. This case underscores the importance of considering multiple myeloma in the differential diagnosis of atypical gastric masses and highlights the need for a comprehensive diagnostic workup when extramedullary disease is suspected.