Metastases to the brain occur by hematogenous spread, and multiple lesions are found in 70% of cases. The most common primaries are lung, breast, and melanoma, in that order of frequency. Other potential sources include the gastrointestinal tract, kidney, and thyroid. Metastases from other locations are uncommon. Clinical symptoms are nonspecific and no different from primary brain tumors. If a parenchymal lesion breaks through the cortex, tumor can extend and seed along the leptomeninges.

            Metastatic lesions can be found anywhere in the brain but a favorite site is near the brain surface at the corticomedullary junction of both the cerebrum and cerebellum. They are hyperintense on plain T2-weighted images. Areas of necrosis are prevalent in the larger lesions, accounting for their heterogeneous internal texture. Peritumoral edema is a prominent feature, but multiplicity is the most helpful sign to suggest metastatic disease as the likely diagnosis. Correlative studies have shown MR to be more sensitive than CT for detecting metastases, particularly lesions near the base of the brain and in the posterior fossa. One limitation of plain MR is the frequency of periventricular white matter hyperintensities found in the same older age group at risk for metastatic disease.

            Gadolinium enhanced MR has resulted in improved delineation of metastatic disease compared with nonenhanced scans. Moderate to marked enhancement is the rule, nodular for the smaller lesions and ringlike with central nonenhancing areas for the larger ones. Endnote Controlled clinical trials have also shown that contrast-enhanced MR is more sensitive than both plain MR and contrast-enhanced CT for detecting cerebral metastases. In patients with a known primary, T1-weighted enhanced MR is probably sufficient to screen the brain for metastatic disease.

            Hemorrhage is present in 3 to 14% of brain metastases, mainly in melanoma, choriocarcinoma, renal cell carcinoma, bronchogenic carcinoma, and thyroid carcinoma. The presence of nonhemorrhagic tissue and pronounced surrounding vasogenic edema are clues to the underlying neoplasm.

            Metastatic melanoma has been a topic of special interest in the MR literature because of the presence of paramagnetic, stable free radicals within melanin. The MR appearance is variable depending on the histology of the melanoma and the components of hemoglobin. Most are hyperintense to white matter on T1-weighted scans and hypointense on T2-weighted scans. Atlas and coworkers Endnote observed three distinct signal intensity patterns. Nonhemorrhagic melanotic melanoma was markedly hyperintense on T1-weighted images and isointense or mildly hypointense on T2-weighted images. Nonhemorrhagic amelanotic melanoma appeared isointense or slightly hypointense on T1-weighted scans and isointense or slightly hyperintense on T2-weighted scans. The signal pattern for hemorrhagic melanoma was variable depending on the components of hemoglobin. Some uncertainty remains as to whether the predominant effect on signal intensity within melanomas is due to stable free radicals, chelated metal ions, or hemoglobin.  

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