Sclerotic bone metastases
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Sclerotic or osteoblastic bone metastases are distant tumor deposits of a primary tumor within bone characterized by new bone deposition or new bone formation.
Bone metastases are the most common malignancy of bone of which sclerotic bone metastases are less common than lytic bone metastases.
The diagnosis is usually established by a combination of imaging and the known presence of a primary tumor that is associated with sclerotic bone metastases. It can also be proven histologically.
Osteoblastic bone metastases are characterized by increased bone formation 2. However, the exact mechanism that leads to osteoblastic formation is not entirely elucidated. It is assumed that several tumor-derived growth factors increase osteoblast activity while osteoclast activity is restricted 3,4.
Sclerotic bone metastases can arise from several different primary malignancies including 1-3:
prostate carcinoma (most common)
Sclerotic bone metastases typically present as radiodense bone lesions that are round/nodular with relatively well-defined margins 3. Radiographs are specific but suffer from low sensitivity 1.
CT can detect osteoblastic metastases with a higher sensitivity than plain radiographs and shines in the assessment of bones which are characterized by a small bone marrow cavity and a high amount of cortical bone such as the ribs 2,3.
On CT sclerotic bone metastases typically present as hyperdense lesions, but display a lower density than bone islands 5. A mean CT attenuation threshold of 885 HU and a maximum attenuation threshold of 1060 HU has been found supportive in the differentiation of untreated osteoblastic and bone island in one study 7, but the exclusive use of attenuation values for the assessment of sclerotic bone lesions has been discouraged 8.
MRI features high sensitivity and high specificity for the demonstration of bone metastases in general and for assessing the bone marrow 2,3. It can differentiate predominantly osteoblastic from osteolytic bone metastases 9 as well as easily demonstrate and assess complications such as pathological fractures or spinal cord compression 2,3.
A disadvantage of MRI is that the detection is poor in bones with a small marrow cavity such as the ribs and these bones are better investigated with CT 2,3.
T1: low signal intensity
T2: low to isointense 2,9
SWI: low signal intensity on the inverted magnitude and phase images 9
T1 C+ (Gd): variable
Bone scintigraphy (99mTc MDP) is very sensitive for the detection of osteoblastic providing information on osteoblastic activity but suffers from specificity with a false-positivity rate ranging up to 40% 1.
PET features high sensitivity in the detection of bone metastases especially 18 NaF-PET is suitable for the detection of sclerotic metastases since it shows tracer uptake in locations with osteoblastic activity and is more accurate than FDG-PET 3.
The radiological report should include a description of the following 2:
location and size including the whole extent of disease load
tumor margins and transition zone
soft tissue extension
aggressive periosteal reaction
pain attributable to the lesion (if known)
Treatment and prognosis
Treatment of bone metastases, in general, is usually planned by a multidisciplinary team 10. See article: bone metastases
Osteoblastic metastases have a lower fracture risk than lytic or mixed bone metastases 11-13.
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