I've Developed A Dent In My Head Which I Noticed

I've developed a dent in my head which I noticed in just over a month ago.

It's about 2 inches wide, and it's at the crown of my head.
I went to my Dr today and he ordered an X-Ray. He said something about 'diffusion' because if it were protruding he'd worry about a tumor.
Mine is the opposite.
I am just wondering what's going on.

Thank you,

Karly