What are lytic and focal bone lesions?

What are lytic and focal bone lesions and how common are they? Myeloma cells live in the bone marrow, which is at the center of the bones. And in addition to that, one of the primary complications that we see from multiple myeloma is that the myeloma cells can actually stimulate other cells in the bones to eat away at the bones and form holes in the bones, weakening them, predisposing to fracture, causing bone pain and releasing calcium out of the bones into the blood, which can raise up the blood calcium level and make people feel worse.

When we do see evidence that the myeloma cells have caused the bone to be eaten away, those holes in the bones which appear as dark spots on x-rays we call lytic bone lesions. Lytic comes from the Greek word for eating away or lysis or breaking down, and those lytic bone lesions are very important to identify because we know for sure that in patients with lytic bone lesions, giving bone strengthening medications like zoledronic or pamidronate or denosumab can decrease the appearance of new lytic bone lesions, can improve bone pain, decrease the risk of fracture, and perhaps also help patients with myeloma to live longer.

In addition to seeing lytic bone lesions, we sometimes talk about focal bone lesions, and that really refers to the detection of collections of plasma cells within the bone marrow, whether or not they're actually injuring the bones, which usually involves imaging techniques. Other than the regular x-rays that we use to find lytic lesions. So lytic lesions we would usually find on plain X-rays or CT scans, whereas focal bone lesions we would usually find on MRI scans or PET scans, which have the ability to actually see into the bone marrow and find collections of abnormal cells that are presumably plasma cells.

While lytic bone lesions are often painful and predispose to fracture and might require specific therapy to try to make them better, focal bone lesions are more an indication of whether the myeloma has the ability to clump together in cells, and whether that might result in focal bone lesions. While lytic bone lesions might require therapies in and of themselves in order to reduce pain or the risk of fracture, focal bone lesions are generally more useful as a prognostic marker, so that patients with smoldering myeloma who have a lot of focal bone lesions might be at a particularly high risk of progression to active myeloma, therefore benefit from earlier institution of chemotherapy and the appearance of focal bone lesions on a PET scan after successful treatment of myeloma might indicate that although the patient's in a good response, that it wouldn't qualify as a complete or stringent complete response.

Do lytic lesions ever go away? In general, while focal bone lesions can go away with therapy because we're killing the plasma cells there, lytic bone lesions which involve destruction of the bone often don't change in appearance over the course of myeloma therapy. We do think that as we treat patients with myeloma and give them bone strengthening treatments, their bones do get stronger and the risk of fracture decreases. But often the appearance of the bones on x-ray stays the same. Occasionally, some patients will be able to see that some of those holes seem to fill in, but most people, the bones still look the same, even when they're in a complete response and feel much better. And the risk of fracture is much better.

Can lytic and focal lesions happen in the same location? Sometimes focal bone lesions happen at the same location as lytic bone lesions, so that a collection of plasma cells will directly stimulate destruction of the bone in that spot. But often they're at different places completely, and that's most often easily seen on PET CT scans, where one can see focal bone lesions on the PET part of the scan, and lytic bone lesions on the CT scan. And often those will be in the same place. But sometimes you'll be able to see some lytic lesions that don't have any uptake of the radioactive sugar used in the PET scan, and at the same time, see areas where there's increased uptake of the radioactive sugar without any evidence of bone destruction at that spot.