Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
If Tommy has been vaccinated, this may be a Vaccine associated Fibrosarcoma. These are very invasive and, at Tommy's age, it is probably not worth removing this because tehse can be so invasive even when we attempt to remove them. Have your vet examine this to be sure that this is not an abscess that can be drained and cured.
This LINK may help.
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