I am not an expert on veterinary matters, so please put that in perspective.
Our 6-year-old boy developed a growth that looked a lot like that. We took him to the vet the next day. The vet aspirated the growth, and found that it was a mast cell tumor. I don't mean to scare you, as statistics show that 75% of mast cell tumors are either grade 1 or 2. Our boy's tumor, unfortunately, was grade 3, and we are working to treat him.
As a precaution, we urge you not to press on the growth or palpate it any way. If it is a mast cell tumor, touching it can release histamines into the dog's system. Until we sort out our treatment program with an oncologist, we have been urged to give our dog low doses of benadryl 2/x day and Pepcid to offset the release of histamine, which is responsible for allergic reaction.
As you know, most of these things are benign, but it is prudent to be cautious.
Please keep us posted.