Bone Metastases

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Malignant bone infiltration occurs due to direct tumour invasion or from haematogenous spread. Direct invasion may be due to erosion by the primary tumour, as in rectal cancer involvement of the sacrum, or due to extra-capsular lymph node infiltration, as in cervical cancer. Bone metastases due to haematogenous spread can occur with any pelvic tumour, most commonly in prostate cancer and rarely in ovarian cancer.

Accuracy and use of MRI in detection of bone metastases

Compared to bone scintigraphy, MRI is known to be more sensitive in detecting metastases with a sensitivity of 82-91% compared with 71-84%. This is because MRI detects metastases by altered signal at sites of early marrow involvement before osteoblastic stimulation (the cause of the radionuclide avidity) occurs. During a routine pelvic staging MRI examination, the pelvis, proximal femora and lumbar spine are imaged. As this coverage includes a large number of common metastatic bone sites, the majority of bone metastases should be detected.

MR imaging technique for assessment of bone metastases

Tl-weighted and fat suppressing STIR sequences are the most useful sequences for identifying lesions. T2*-weighted gradient echo images, though less sensitive than T1 and STIR sequences, may show lesions by their loss of susceptibility artifact, due to disruption of the trabeculae, and may help identify fractures and osteoblastic healing in osteoporosis. T2-weighted spin echo sequences are of limited use. Intravenous contrast may be helpful as the metastatic tumour will enhance, though this is not routinely used.

In the specific case of suspected vertebral metastases, the whole spine should be imaged to detect all metastatic deposits. Parasagittal sections should be performed with axial sections through areas of concern. In addition to Tl-weighted, STIR images and T2*-gradient echo images, diffusion-weighted images may also be helpful. T2-weighted spin echo sequences will help to evaluate concomitant degenerative disc disease.

Imaging features

Normal marrow appearances

Normal bone marrow may be haemopoietically active, containing myeloid elements and known as 'red marrow', or haemopoietically inactive containing mainly fat cells and known as 'yellow marrow'. At birth, virtually the whole skeleton contains red marrow. During normal aging, this is converted to yellow marrow in a uniformly predictable manner, starting from the extremities and moving proximally. In adults the only remaining red marrow is in the axial skeleton and most proximal appendicular skeleton. There is a mix of red and yellow marrow in some portions of the skeleton.

Red and yellow marrow have different imaging characteristics. Because of its fat content, yellow marrow tends to be relatively high signal on Tl- and T2-weighted images and to suppress on STIR sequences resulting in low signal. Red marrow is intermediate signal on Tl -weighted images and higher signal on T2-weighted images and STIR sequences. When there is a mix of red and yellow marrow, the overall result is to increase the signal intensity of the marrow on Tl-weighted images so that the islands of red marrow are often poorly demarcated intermediate areas within the marrow.

Bone metastases

Metastases show as discrete intermediate to low signal areas contrasting well with the predominantly higher signal fatty marrow on Tl-weighted images. On STIR sequences, metastases are usually high signal. In prostate cancer, sclerotic metastases appear as low signal on both sequences.

Pitfalls of MRI

• Normal retraction of red marrow, which partially affects a particular bone, can be a problem, particularly in the femoral head and ilium where patchy signal may be detected. This is often relatively symmetrical within the skeleton, and the red marrow appears band like or ill-defined enabling differentiation from bone metastases.

• Bone marrow reconversion may occur in times of increased demand for haemopoiesis, for example in chronic anaemic states, and as a result of haemopoietic growth factor therapy. Yellow fatty marrow converts back to red haemopoietic marrow starting centrally and spreading more peripherally. Tl-weighted images will show areas of intermediate signal, whereas T2-weighted images and STIR sequences will show variable, high signal, though not as high signal as in metastatic disease. The reconversion is relatively symmetrical with uniform involvement of the marrow spaces. While diffuse malignant infiltration from lymphoma, myeloma or leukaemia could be confused with bone marrow reconversion, the pattern is dissimilar to solid tumour bone metastases.

• Radiotherapy effect tends to cause fatty change within the marrow, with high signal on Tl-weighted and T2-weighted images and suppression of marrow signal on STIR sequences. It can be readily identified as it is usually very uniform with a sharp demarcation, which corresponds to the boundary of the radiotherapy field.

Other effects of radiotherapy include insufficiency fractures, which occur in bone weakened by radiation osteitis, particularly the sacrum. They manifest as bands of low signal on Tl-weighted images, which are high signal on STIR images and within which the fracture line itself may be observed centrally as a fine low signal intensity line. When the sacrum is affected, bilateral vertical sacral fractures may occur with a bridging horizontal fracture producing the classical H pattern or 'Honda' sign. MR scans are the most sensitive imaging modality for detection of insufficiency fractures but CT scans may define the fracture line more readily, therefore increasing diagnostic confidence.

Abnormalities in bone marrow adjacent to the sacroiliac joints have also been demonstrated on MRI in patients treated by radiotherapy with no demonstrable fracture seen on other radiological modalities. These lesions are ill-defined low signal on Tl-weighted images and high signal on T2-weighted images. Biopsy in such cases has shown evidence of peritrabecular fibrosis and inflammatory infiltration.

• Osteoporotic vertebral collapse, when chronic, can usually be distinguished from metastases by the relatively normal signal of the collapsed vertebra, the lack of an associated perivertebral soft tissue mass and lack of enhancement after intravenous contrast medium injection. Acute osteoporotic fracture and collapse may be difficult to differentiate from metastatic disease because of abnormal vertebral signal due to haemorrhage and oedema, enhancement after intravenous contrast medium, and perivertebral haemorrhage into the soft tissues.

On diffusion-weighted images pathological compression fractures are hyperintense, whereas benign vertebral compression fractures are hypo- or isointense compared to adjacent normal vertebral bodies. T2*-weighted images may help by demonstrating the osteoporotic fracture site and reactive trabecular sclerosis producing increased susceptibility low signal intensity.

• Haemangiomas tend to be focal areas with signal varying from intermediate to high on T1-weighted, T2-weighted and STIR

images, depending on the relative proportions of their fat and soft tissue vascular components, as well as any interstitial oedema. Accentuated vertical trabeculation may be seen within larger lesions. They are most commonly seen in the vertebrae, particularly the lower thoracic and upper lumbar spine and are rarely seen in the flat and long bones. They typically occur in females in the 4th and 5th decades.

• Benign bone islands are composed of compact bone within the medullary canal and exhibit low signal on all image sequences. Their small size and lack of cortical involvement or periosteal reaction may help differentiate them from sclerotic metastases.

• Subchondral cysts in degenerative joint disease are of low signal on T1 -weighted images and high signal on T2-weighted and

STIR images. Their typical subchondral location and associated features of loss of joint space, osteophyte formation and low signal subchondral sclerosis should help in their diagnosis.

• Nutrient foramina may be seen in all bones extending from the cortical margin into the medulla. They are small, low signal, well-defined lesions which appear linear when scrolling through the images. They are bilateral, virtually symmetrical and found in typical anatomical locations (e.g. the medial aspect of the ilium).

• Paget's disease occurs most commonly in middle aged males. It may be solitary or multifocal, thereby simulating metastases.

MRI features are nonspecific. The affected bone may appear enlarged, and of heterogenous signal with low signal cortical thickening. The cortex may occasionally be of higher signal due to remodelling of the cortical bone. Correlation should be made with plain radiographs, which have much more characteristic and specific appearances.

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