I have heard of this. I was diagnosed the same then after radiation on the rib it was changed to MGUS. They now say the don't know what it was that was radiated. I have now went for a 3rd opinion as the back pain is intolerable. Either way the diagnosis has been made and coded to the insurance company. This will create problems with getting future insurances. Seek another opinion and get a correct diagnosis.

I think I can help..they are very similar:

1) Plasmacytomas are tumors composed of plasma cells of which are histologically identical to those seen in multiple myeloma
2) IF THEY OCCUR IN BONE AND THERE IS ONLY ONE SOLITARY LESION they are designated as: solitary plasmacytoma of bone.
3) IF THEY ARISE OUTSIDE OF THE BONE in soft tissues they are called extramedullary plasmacytoma 4) Why some patients develop MM and others plasmacytoma is not understood
5) The diagnosis of solitary plasmacytoma of bone requires the following (and can be distinguished from MM by the following:)

evidence of a plasma cell tumor involving bone.
Complete skeletal xray must show no other lytic lesions (unlike with MM), Bone marrow aspirate and biopsy must contain no evidence for MM. There is NO anemia, hypercalcemia, or renal insufficiency
Hope that helps
Dr O.