Can you please tell me how metastasized brain cancer and cancer that started in the brain differ?

Different processes: Any type of cancer can spread or metastasize to the brain. Primary brain cancer starts in the cells of the brain itself. There are different types of primary brain malignancies, each with its own prognosis.

Where it originated: There are cancers that develop from the cells already in the brain. Metastasis to the brain means there is a cancer in the body and a few of these cells have traveled in the blood to the brain and are lodged and growing there. The treatment is going to be different depending on the circumstances and where the cancer originates.

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!