Different tests

There is a cancer of bones called multiple myeloma that has an association with CIDP. Some of those posting with monoclonal gammopathy might have multiple myeloma or Waldenstrom’s macroglobulemia or MGUS (which is different but can be an early sign of myeloma). Myeloma is an abnormality in the bone marrow–the soft part in the middle of the bones where blood is made, but it can affect the hard part of bones and be seen as holes in the bones. I was tested for this with a skeletal survey in which they take pictures of all your bones looking for areas in which the bone is eaten away (lytic areas) by the presence of tumor cells. I had this done because my MRI showed a strange looking marrow signal and I had a blood test for a monoclonal antibody (serum electrophoresis) that was abnormal. A skeletal survey is a better test for myeloma than a bone scan.
the bone density test (also called a DEXA scan) looks at the density of the bones. It does not look for tumor. It is used to see if prednisone has made the bones weak or to look for osteoporosis.
The bone scan is used to look for solid tumors that have gone to the bones and sometimes to see if there is an infection of the bone. Bone scans are typically not used as screening tests for cancer, because they use radiation–more than even the skeletal survey, unless one is having a lot of unexplained bone pain. Most tumors that go to bones are not in your age range or are not common. So breast cancer (not common in men), prostate cancer (typically in older men than you), osteosarcoma (typically in adolescents and the 20-year-old group), thyroid cancer (rare), etc. Some kinds of lymphomas and lung cancer can go to bones often late and usually when there are other signs or symptoms. There are some adult sarcomas that start in bone but these are not common and not commonly associated with a tendency to immune mediated nerve damage.
You might want to ask your friend which tumor he was worried about. If it is multiple myeloma, even though you are on the young age for this, it might be worth considering a serum electrophoresis and 24-hour urine collection for a monoclonal protein. Bone scans look at rebuilding up of the bone after damage and the damage in multiple myeloma is holes in the bones, so a bone scan is not nearly as good a test for it. It is probably reasonable for any of us adults to get a chest xray which will show lung cancer and some lymphomas as well as showing the bones of the spine and ribs. This does not take much radiation to do.
WithHope for a cure of these diseases