Donut shaped lump on dogs chest

I noticed a tiny lump on my dogs chest about a month ago. It has grown quite considerably since then so we have him booked it to get it surgically removed in a couple of weeks. I was just wondering if anyone has any idea what it could be as I can’t find any other pictures of similar ones online and our vet isn’t sure either. Many thanks.

Hello!  I can't remember seeing one exactly like this either; I wonder if that's necrosis (cells dying off) or infection / foreign body reaction / hair follicle cyst centrally?  Your vet is right in that there is no way to know the full story from looking and biopsy or removal sound like good ways to find out.  Please do let us know what they find!

What did this end up being? My dog has the same thing and I'm waiting to have it removed in a few weeks. My vet hasn't seen this before either.  

I had the mass removed and biopsied. It was a cutaneous inverted papaillomas.