What is the difference between a plasmacytoma and multiple myeloma?

The difference is the number of tumors. In a plasmacytoma, the National Cancer Institute says, the abnormal plasma cells "are in one place and form one tumor." In multiple myeloma they "build up in the bone marrow and form tumors in many bones of the body." Both start in the same cell, so the count is what separates them.

Plasma cells are the immune system's antibody factories. They grow out of B lymphocytes, a type of white blood cell, and NCI notes they "make antibodies to fight bacteria and viruses, to stop infection and disease." In a plasma cell neoplasm, one plasma cell copies itself over and over. Every copy pumps out the same useless antibody protein, called M protein. Doctors measure that protein in blood and urine, and it becomes the tracking number for the whole disease.

What each diagnosis actually requires

A solitary plasmacytoma is one lesion and nothing more. NCI's summary for clinicians describes it as "a solitary lytic lesion of plasma cells on skeletal survey in an otherwise asymptomatic patient," with a marrow sample "from an uninvolved site" containing less than 10% plasma cells. Lytic means the tumor has eaten a hole in the bone. There are two kinds. One sits in bone. The other, called extramedullary, sits in soft tissue such as the airway or sinuses.

Multiple myeloma is the opposite picture. NCI calls it "a systemic malignancy of plasma cells that typically involves multiple sites within the bone marrow." Systemic means body-wide, not local.

The line that decides treatment

Doctors do not call it myeloma just because there is more than one spot. They look for organ damage. The features are known by the letters CRAB, and NCI lists numbers for each:

• Calcium. Hypercalcemia, meaning high blood calcium, "greater than 1 mg/dL higher than reference range."
• Renal. Kidney damage, with "creatinine greater than 2 mg/dL or creatinine clearance less than 40 mL/min."
• Anemia. Too few red blood cells, with "hemoglobin less than 10.0 g/dL."
• Bone. "Bone lesions (one or more)" seen on imaging.

Two newer lab findings also count on their own. One is a clonal plasma cell share in the marrow of 60% or more. The other is a ratio of involved to uninvolved serum free light chains of 100 or more.

These conditions sit on a spectrum. At the quiet end is MGUS, monoclonal gammopathy of undetermined significance. NCI describes M protein in the blood, fewer than 10% plasma cells in the marrow, and no myeloma findings. Its "annual risk of progression of MGUS to a lymphoid or plasma cell malignancy ranges from 0.5% to 1.0%." Smoldering myeloma sits further along, with more disease but still no organ damage.

Why the label changes what happens next

For a single bone plasmacytoma, NCI says treatment is "usually radiation therapy to the bone lesion." One target, one local treatment, no chemotherapy. Multiple myeloma is body-wide, so it needs drugs that travel everywhere, and for some patients a stem cell transplant.

Here is the part worth knowing. NCI states that "plasmacytoma of the bone often becomes multiple myeloma." A plasmacytoma diagnosis is not a smaller version of myeloma that ends after radiation. It comes with years of follow-up, checking M protein levels and blood counts on a schedule. Rising numbers are the early warning, often long before any symptom appears.

The whole diagnosis rests on proving there really is only one lesion. So it is fair to ask what imaging was used to search for others, and how often your M protein will be rechecked from here.