Biopsy of a reasonably well-demarcated mass of the nasopharynx from a 30 year old man, demonstrates a plasma cell proliferation. Serum electrophoresis shows a small monoclonal IgG spike. Bone marrow evaluation fails to demonstrate plasma cell proliferation and no lesions are seen on extensive skeletal x-rays. Which of the following is the most likely diagnosis?

Plasmacytoma (solitary myeloma) involving soft tissue (lungs, nasopharynx, nasal sinuses) is a plasma cell proliferation resembling multiple myeloma but without significant metastatic potential. In contrast, some plasmacytomas involving bone eventually (up to 10 to 20 years) develop into frank multiple myeloma. Heavy chain diseases constitute a group of rare lymphoplasmacytic malignancies in which excessive amounts of a defective heavy immunoglobulin chain are produced. They may take the form of gamma heavy-chain disease (from IgG), alpha heavy-chain disease (from IgA) or mu heavy-chain disease (from IgM); malignant cells are usually present in marrow in all of these conditions. Monoclonal gammopathy of undetermined significance is a disease of elderly patients with a monoclonal spike on serum or urine electrophoresis, but no identifiable mass or bone marrow lesion; 20% of these patients eventually develop one of the other diseases listed in the answer choices. Multiple myeloma is a malignancy derived from a single plasma cell clone with significant metastatic potential. Multiple lytic bone lesions are usually seen.