Detection
There are two main reasons to detect skeletal metastases: diagnosis of painful lesions staging of cancer before start treatment. Diagnosis of Symptomatic Lesion.
Clinical Features
•pain-the most common symptom ,but not always present
•swelling-rare presentation
•tenderness
Biochemical parameters
•alkaline phosphatase
•urinary hydroxiproline
•urinary hydroxiproline/creatinine ratio
The biochemical parameters currently avaiable lack specificity and are of little value in diagnosis metastases.
Radiographs
Skeletal metastases are usually multiple,but isolated lesions do occur. The lesions may be lytic, sclerotic, or mixed.
•sclerotic-prostatic carcinoma, mammary, gastrointestinal, and blader cancer.
•lytic-occur in every type of primary tumor.
•mixed-most occur from mammary carcinomaThere are 3 types of lytic lesions:
•geografic-large, solitary, well defined (>1cm).
•Moth eaten- multiple smaller lytic areas (range, 2-5mm), that may coalesce to form larger areas.
•Permeative- multiple tiny lytic areas (usually < 1mm) seem principally in cortical bone
Patologic facture is a major complication of skeletal metastases.Usually occur with lytic lesions,and the risk of this complication occuring is related to the amount of cortical bone destruction.
Most metastases develop in the medula and only involve the cortex late; therefore, radiologic examination is insensitive.At least 50% of the medullary bone must be destroyed in the bean axis of the xray before the lesion could be seen radiographically.
Although radiographic examinationis insensitive,radigraphs of painfull lesions must be taken to determine the amount of bone destruction. if 30% of the cortex has been destroyed, the risk of fracture is high and the lesion must be treated.
Skeletal Scintigraphy
Skeletal scintigraphy is much more sensitive than radiography. The lesions usually are seen as localized areas of increased uptake.Occasionally they may appear as areas of diminished uptake caused by infaction,or in large very rapidly growing aggressive metastases where there is much more bone destruction with minimal bone formation.(Virtually all matastases evoke an osteoblastic response)
Doughnut lesions also may be seen.this usually occurs where there is a rim of new bone formation around a large destructive metastasis. Not all metastases can be detected by scintigraphy. These include the following:
•Tumors that do not evoke an osteoblastic response(many myelomata,some lynphomata, and rapidly growing lesions)
•Small deposits (< 2 mm)
•widespread metastatic disease when the uptake is symetrical and there are no areas of incresead concentration. In these circunstances there may be a redution in urinary excretion of the isotopes
If multiple areas of incrased uptake are found, plains radiographs of these areas should be taken to confirm the presence of skeletal metastases or to exclude a benign lesion.Even the radiographs are positive, CT scanning or MRI imaging me be indicate to demonstrate the extent of the soft tissue component and the presence or absence of neural compression.
The disadvantages are low speficty and the lentgh of the examination.See a a patological fracture in a lumbar vertebra ( hole body bone scan using 18F )
Computed Tomography
Is extremely useful in the confirmation of isolated lesions and can demontrate the extent of the soft tissue involvement.
Magnetic Ressonance Imaging
It is the method of choice for examining the spine,and is more sensitive in detection of early metastases within medulla.It also demonstrates the soft tissue and the bone,but does not demonstrate the cortical architeture as CT scans.
If the facilities are avaiable,MRI should be used as the first procedure for patients with suspected neural compression.
Although it is probably the modality of choice for examing the spine,MRI is not as useful as skeletal scintigraphy inthe assessment of metastatic cancer,because it cannot be used to examine the axial skeleton or long bones.
Arteriography
Used for lesions that is thougth to be very vascular(for example,renal metastsis).And, if surgery is likely to be associated with major hemorrage, preoperative embolization may be required.
Biopsy
If there is still any doubt about diagnosis, a bone biopsy may be required.
Conclusion
There is no ideal method for detecting skeletal metastases.Currently, skeletal scintigraphyis the most sensitive technique available, but the uptake of bone-seeking radionuclides is not specific.Radiography, CT scans, or MR images may be required of areas where there is a focus of increased uptake, particulary if such a focus is solitary.Ocasionally, a biopsy may even be necessary.
Bibliography
1.C.S.B. Galasko, MSc, ChM,: Diagnosys of Skeletal Metastases and Assessment of Response to Treatment.Clinical Ortopaedics and related research 312:64-75,1995
2.Adams JE,Isherwood I:Conventional and New Techniques in Radiological Diagnosis.In Stoll BA, Parbhoo S (eds).Bone Metastasis, Monitoing and Treatment.New York, Raven Press 107-148, 1983.
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