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

Background: 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. Case report: 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. Discussion: 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. In conclusion, 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.