What is the cause of brain tumor?

Most are random: All tumors result from accumulated genetic mutations that give a growth advantage to clones of cells. Most strike at random without clear environmental risks. A few familial syndrome (turcot's, neurofibromatosis, von hippel-lindau, lifraumeni, some rarities) feature increased risk for one or more types of brain tumors. If you have a brain tumor, nothing that you did caused it. Wishing you the best.