How much does my cancer risk increase after a ct scan of the head as a teenager?

Almost no risk: Please see: http://www.scientificamerican.com/article/how-much-ct-scans-increase-risk-cancer/ also: 1) CT head exposure is mostly confined to the head, not the whole body. 2) There may be cellular repair at doses as low as CT, limiting cancer rates. (We don't know how much). 3) Whatever the theoretical risk, there must be a good reason to do the CT in the first place.