Multiple myeloma is a multifocal malignant proliferation of plasma cells and is the most common primary malignant tumor of the skeleton.
There is a male predominance, with a male-to-female ratio of 1.5: 1. It is common after age 50 years and is rare before age 30 years.
Because this neoplasm derives from bone marrow elements, it involves bones containing red marrow: the skull, ribs, sternum, pelvis, proximal humeral metaphysis, and proximal femoral metaphysis. At the time of diagnosis, it is often disseminated throughout the red marrow of the axial skeleton.
Infiltration of bone marrow has two forms:
In the difise form, myeloma cells are mixed with hematopoietic cells. This form is difficult to image; the only MRI manifestation is an inhomogeneity of the bone marrow signal intensity that is difficult to quantitate and is subjective CT, plain radiographs, and scintigraphy may result in negative findings.
In the focal form, normal bone marrow is displaced by nodules composed entirely of myeloma cells. Untreated myelomatous lesions reflect decreased signal intensity on TI-weighted images and increased signal intensity on T2-weighted images when compared with the surrounding bone marrow. CT demonstrates purely osteolytic lesions in the trabecular bone, which occasionally extend to involve the cortex. After irradiation, these lesions show low signal intensity on both TI-weighted and T2-weighted images, and a sclerotic border develops, as seen on CT.