@Arella Mae,
These types of growths may occur almost anywhere on the human body. They frequently occur on the thumb or elsewhere on the hand and usually are the result of physical damage taking place.
I knew a woman who had a car door slam on her thumb. With time, she developed a growth ( hrony tissue ) on her thumb. This type of growth ( on the thumb ) can usually be removed by a dermatologist .
As far as the growth on the man's skull is concerned, he's lucky the growth was outwards, as opposed to inwards.