What Causes Dent On The Top Of Head?

This might sound a bit odd, but I have a dent on the top of my head about 2" wide and 1.5"long. This has never really bothered me too much in the past but now I have the joy of my hair going thin to the point that I have to shave it off. My problem is that I am so self concious of this dent as it is very visable im wondering if it could be 'filled' in any way.