Can Susceptibility-weighted MRI Help Differentiate Osteolytic from Osteoblastic Spine Metastases?

Abstract

N early everything has changed in radiology since I was a resident 30 years ago. As a musculoskeletal radiologist, I believe MRI has had much to do with this revolution. The article by Böker and colleagues in this issue of Radiology (1) led me to remember that there have been at least two relative constants. First, metastatic disease is overwhelmingly the most common malignant tumor of bone. Second, osseous metastases are still generally classified as osteoblastic versus osteolytic. The fact that an MRI article brought these to consciousness is somewhat ironic.There are 350 000 cases of osseous metastatic disease in the United States each year (2). Notably, this number is growing due to improved premortem diagnosis and the therapeutic model of cancer becoming more often the management of a chronic disease (3). Because osseous metastatic disease presents less of a systemic burden than brain, hepatic, or pulmonary metastases, there is a greater focus on longitudinal assessment of osseous metastases.Thirty years after I trained, we continue to use the osteolytic versus osteoblastic classification schema. Osteolytic metastases are much more common and often occur with lung, gastrointestinal (usually colon), renal, and thyroid primary tumors. Osteoblastic metastases tend to occur with prostate and breast primary tumors (4). Mixed patterns are less common, and, in my opinion, are often the result of a patient's immune response to initial lytic tumors or lytic tumors that were treated. The latter is increasingly important as more patients with metastatic disease are being treated. Once upon a time, patients with metastatic disease were only treated palliatively; now, aggressive treatments are often pursued. Nearly all of these treatments become ineffective at some point in time and then the regimen must be changed. Continued follow-up with consideration for healing versus growth becomes necessary. By and large, this follow-up is gradually transitioning to the wholebody MRI detection of bone marrow. Because the bulk of osseous metastatic disease is in the axial skeleton, focused advanced imaging of the spine is now common.Even today, the differentiation of osteolytic from osteoblastic metastases remains important. First, it helps ascertain the risk of pathologic fracture: Osteoblastic metastases tend to fracture less frequently but more transversely compared with osteolytic metastases. Second, in these often chronically burdened patients, the differentiation of osteolytic from osteoblastic metastases helps determine if their widespread disease is getting better or worse.Although we often look for the development of fat on MR images as a sign of healing, whether visually or with in-and out-of-phase imaging and/or the Dixon technique, sclerosis is another common way that osseous metastases heal. Hence, the precise assessment of an osteoblastic response remains vital.Metastases appear as a replacement of the normally predominantly fatty marrow, as was confirmed in the study by Böker et al. Edema is common but may be subtle ...