About 6 to 21%% of metastatic bone disease from all extraosseous primaries will appear initially as a single focus. In Boxer et al's series of breast cancer patients with relapse to bone, 21%% of patients relapsed to bone with a single demonstrable metastasis. Solitary bone scintigraphic abnormality was due to metastasis in 11%% of cases in Jacobson et al' s report on breast cancer patients. This variation in probability of metastatic bone disease may be due to the different location of the solitary bone abnormality as well as the different types of malignant tumor. Lesions in the axial skeleton are the most suspicious for metastatic disease. Boxer et al reported that 88%% of patients with a single bone metastasis were in the spine. Jacobson et al looked at the scintigraphic pattern of metastatic bone disease in breast cancer patients with less than 5 new bone scan abnormalities and reported that 56%% of the bone metastasis are in the spine and only 6%% in the ribs. Interestingly enough, rib lesions were the most common new bone scintigraphic abnormality. In Jacobson et al's series, 85 out of 306 new bone scan abnormalities were rib lesions, but only 7%% of these lesions were malignant.
When solitary lesion is identified on bone scintigraphy, further evaluation is required. Appropriate plain X-rays should be obtained to seek benign etiology. It has been reported that when the correlative X-rays were interpreted as normal, 17 %% of the lesions turned out to be malignant; but when the plain X-rays revealed a benign process, only 1%% of the lesions were malignant.
Bone scintigraphy is a very sensitive test to detect bone abnormality, but it lacks specificity. New solitary bone abnormality on bone scintigraphy requires further evaluation. As a rule, if the plain X-ray reveals a benign process, a malignant cause is unlikely, but if it is unrevealing, further investigation with CT, MRI or follow-up studies are needed to determine the diagnosis.
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