A brain metastasis is a cancer that has metastasized (spread) to the brain from another location in the body and is therefore considered a secondary brain tumor. The metastasis typically shares a cancer cell type with the original site of the cancer. Metastasis is the most common cause of brain cancer, as primary tumors that originate in the brain are less common. The most common sites of primary cancer which metastasize to the brain are lung, breast, colon, kidney, and skin cancer. Brain metastases can occur months or even years after the original or primary cancer is treated. Brain metastases have a poor prognosis for cure, but modern treatments allow patients to live months and sometimes years after the diagnosis.

Because different parts of the brain are responsible for different functions, symptoms vary depending on the site of metastasis within the brain. However, brain metastases should be considered in any cancer patient who presents with neurological or behavioral changes.

Brain metastases can cause a wide variety of symptoms which can also be present in minor, more common conditions. Neurological symptoms are often caused by increased intracranial pressure, with severe cases resulting in coma. The most common neurological symptoms include:

The most common primary tumors causing brain metastases are, in order of decreasing frequency: Lung, breast, kidney, gastrointestinal, and melanoma. Lung and breast cancers account for over half of brain metastases.

The most common sources of brain metastases in a case series of 2,700 patients undergoing treatment at the Memorial Sloan–Kettering Cancer Center were:

Lung cancer and melanoma are most likely to present with multiple metastasis, whereas breast, colon, and renal cancers are more likely to present with a single metastasis.

Metastatic spread to the brain is usually hematogenous. Metastatic cells often lodge at the interface between gray mater and white mater as arteries narrow in caliber at this site. Spread to the meninges may result in leptomeningeal cancer.

Brain imaging (neuroimaging such as CT or MRI) is needed to determine the presence of brain metastases. In particular, contrast-enhanced MRI is the best method of diagnosing brain metastases, although primary detection may be done using CT. Positron emission tomography (PET) is usually not considered suitable for imaging of brain metastases, because the most frequently used PET-radiotracer, ¹⁸F-FDG, does not only accumulate in the tumor but is naturally taken up by normal brain tissue, usually resulting in insufficient tumor-to-background contrast. PET-imaging of brain metastases is however feasible using alternative radiotracers, such as the Gallium-68 labeled peptide ⁶⁸Ga-Trivehexin, a tracer targeting the tumor cell surface protein αvβ6-integrin. ⁶⁸Ga-Trivehexin demonstrated high uptake in a brain metastasis of tonsil carcinoma but no uptake in the surrounding healthy brain tissue, enabling a delineation of the brain metastasis (see image).