In adults the primary tumors most likely to metastasize to the brain are located, in decreasing order, in the lung (minimum 50%), breast (15–25%), skin (melanoma) (5–20%), colon–rectum and kidney, but in general any malignant tumor is able to metastasize to the brain. The primary site is unknown in up to 15% of patients. Brain metastases are more often diagnosed in patients with known malignancy, when they present with neurological findings or complaints. In some cancers with a high risk of brain mets, such as small cell lung cancer, CT of head with contrast or brain MRI are recommended at the initial staging. A non-contrast CAT scan is not considered adequate to completely rule out brain metastasis. Therefore, an MRI or contrast CT is medically necessary at initial staging for small cell lung cancer but not in other cancers, unless there are signs or symptoms suggesting brain metastases.
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