Plasma Cell Granuloma Mimicking Plasmacytoma Illustrated by 18F-Fluorodeoxyglucose Positron Emission Tomography

<b>Background:</b> Plasma cell granuloma is generally considered a pseudotumor formed by reactive, polyclonal plasma cells. Although most cases can show polyclonal gammaglobulin production, quite a minority may exhibit monoclonal gammopathy, which mimics plasma cell neoplasms such as multiple myeloma or plasmacytoma. Because of this overlap, distinguishing reactive monoclonal proliferation from true malignancy is clinically essential. <b>Case report:</b> A 79-year-old man was presented with an anterior chest wall mass that had grown during investigation for fever of unknown origin. ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET) revealed a sternal bone mass (SUVmax 9.04), aortic uptake of bifurcation (SUVmax 7.08), and Th7/8 soft tissue mass (SUVmax 5.32). Results from the FDG-PET revealed infectious reactions. A chest wall biopsy revealed high degree proliferation of plasma cells. Hematologists suspected plasmacytoma. The pathologist did not diagnose plasmacytoma; thus, there remains a possibility of reactive granuloma lesion. Lastly, the patient's vertebral soft tissue mass culture yielded Staphylococcus aureus. The patient was treated with antimicrobials and responded well. <b>Discussion:</b> In the presented case, FDG-PET revealed an aortic mass with an aortic aneurysm, a sternal mass, and a vertebral mass, as multiple lesions. The abscess lesions that initially resembled multiple plasmacytomas were identified as plasma cell granuloma. The final diagnosis required demonstrating biopsy and definitive monoclonality. Light-chain restriction or monoclonal protein should be considered in the clinical context. Ultimately, this case highlights the diagnostic value of FDG-PET and the importance of differentiating reactive plasma cell granuloma from true plasma cell neoplasm to guide appropriate management. <b>In conclusion</b>, a reactive plasma cell granuloma associated with infectious aortitis can exhibit monoclonal gammopathy, mimicking plasma cell neoplasm. Careful pathological and clinical evaluation is essential to avoid misdiagnosis and ensure proper treatment.