How Do Tumors Metastasize?
The mechanisms by which primary tumors produce brain metastases is thought to be hematogenous spread from primary or secondary sites in the lung. Since the brain has no lymphatic system, all tumors metastasizing to the brain do so by spreading through the bloodstream. Arterial blood passes through the lungs before entering the brain, and collects tumor cells filtered out in capillaries, which subsequently embolize to the brain. This is correlated with sites of localization: the cerebrum is involved in 80 to 85% of all brain metastases, the cerebellum in 10 to 15% and the brainstem in 3 to 5% (6, 10, 29). The overall distribution corresponds roughly to the relative size of blood flow regions in the brain. Different types of primary tumors have different relative frequencies of single versus multiple metastases. Melanoma has the highest tendency to produce multiple lesions, followed by lung and breast cancers (19, 22). Though many studies have indicated that 37 to 50% of patients present
