What is the difference between metastasized brain cancer and cancer that began in the brain?

Prognosis/tx differs: Cancers that begin in the brain and cancers that metastasize to the brain from another primary site (e.g., lung, breast, colon, melanoma) have different prognoses and different treatments.

Curiously different: In pediatric tumors, brain metastasis is unusal except for rhabdoid kidney tumors. In adults, 100k brain mets per year, led by lung, breast, melanoma, kidney, but not prostate. Brain primary tumors are very resistant, and their edges and tentacles hard to find. Metastasis usually are quite responsive, but almost always recur. The biology is fascinating and a puzzle!