Lytic bone metastases
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Lytic bone (osteolytic) metastases are distant tumor deposits of a primary tumor within bone characterized by a loss of bone with the destruction of the bone matrix.
Lytic bone metastases are more common than sclerotic bone metastases.
The diagnosis is usually established by a combination of imaging and the known presence of a primary tumor. It can be also proven histologically.
Bone metastases represent a major cause of morbidity, with symptoms that include severe pain and impaired mobility 1.
Complications include the following 1-3:
Osteolytic bone metastases are characterized by destruction and loss of normal bone or bone matrix 1,2 in which parathyroid hormone-related peptide (PTHrP) features a significant part in the evolution of osteolytic lesions by stimulating the differentiation and activating osteoclasts via the RANKL pathway, which primarily mediate the degradation of bone 1,4,5.
Lytic bone metastases can arise from a long list of different primary malignancies including 1-4,9:
In patients with malignancies, osteolytic lesions should be considered bone metastases unless there are atypical imaging characteristics 2.
Lytic bone metastases typically present as lucent bone lesions with thinned or absent trabeculae and ill-defined margins 3. It is important to point out that radiographs depict the bone destruction caused by the metastatic lesion rather than the tumor deposit itself 2. However, plain radiographs are not very sensitive for the detection of osteolytic bone metastases because more than 50% of trabecular bone must be destroyed in a medullary lesion to become visible on radiographs 2,3. Thus smaller lesions can be easily missed especially in patients with osteoporosis 2.
On CT osteolytic metastases are characterized as lucent bone lesions 6 with a soft tissue density, meaning it is higher than fatty tissue starting from -30 HU or more. In the presence of malignancy with a propensity to bone metastases and they should be considered as such and might require additional imaging or follow-up 6.
CT can detect osteolytic metastases better than plain radiographs 3. It is good to characterize bone lesions in a cortical and periosteal location and can demonstrate cortical destruction and/or periosteal reaction 2. Due to the high spatial resolution, it can nicely detect metastases in the small bones and the skull.
It can demonstrate and characterize aggressive features such as pathological fractures 2.
MRI displays high sensitivity and high specificity for the detection of bone metastases in general and the evaluation of the bone marrow including the detection and assessment of complications such as pathological fractures or cord compression 2,3. Due to the increased water content, osteolytic bone metastases are nicely depicted on T1, STIR and diffusion-weighted images 3,7. MRI can also reliably differentiate predominantly osteolytic from osteoblastic bone metastases 7.
Lytic bone metastases typically have the following signal characteristics 2:
- T1: low signal intensity
- T2: high signal intensity
- DWI: high signal intensity on high b-value, low signal intensity on ADC
- SWI: low signal intensity on the inverted magnitude and phase images 7
- T1 C+ (Gd): enhancement
Purely lytic bone metastases are less conspicuous compared to sclerotic bone metastases unless there is secondary osteoblastic activity and/or pathological fracture 8. Lytic lesions can appear as photopenic ("cold") lesions 9.
On F18 FDG PET-CT, osteolytic metastases are typically photopenic with increased activity peripherally 10.
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
- aggressive features
- cortical destruction/breach
- pathologic fracture
- soft tissue extension
- aggressive periosteal reaction
- pain attributable to the lesion (if known)
Treatment and prognosis
As for all kinds of bone metastases treatment is usually planned by a multidisciplinary team 9.
See article: bone metastases
Osteolytic metastases have a higher fracture risk than mixed or sclerotic bone metastases 10-12.
Sclerosis of lytic bone metastases without evidence of new metastases is accepted to represent treatment response. Sclerosis may start 3-6 months after treatment starts and can take up to a year to fully develop 1.