Hi Mark, My Dobermann has a few large fatty lumps around her chest and sides. I have had them checked...the vet can do a "fine needle aspiration" and look at the cells microscopically while you wait. All that came out of mine were fatty cells, so it's just an age thing (she's almost 12).
However, in saying that, do get it checked, as I also had a Staffy who developed a small lump on his leg, and was found to be cancer. A needle aspiration is practically painless, and instant, so please get it checked to ease your mind.
Good luck with your dog. I'm sure you have nothing to worry about, but better to be safe than sorry.