Worry, meaning book a needle test this fortnight, when a lump is growing, appeared suddenly, changes size day to day, is ulcerated or irritated, sits on a lower leg or the face, or when your dog is bothered by it. A soft, stable, slow lump on the body of an older dog can reasonably wait for the next scheduled visit, but it still gets tested then.
The rule that beats all guessing: lumps get named, not watched on vibes. A fine needle aspirate costs a few hundred dollars and turns a mystery into a plan; experienced vets guessing by feel are wrong often enough that guessing is not a plan. Mast cell tumours in particular are shape shifters that can look exactly like harmless fatty lumps.
Keep a lump map: photograph each lump next to a coin, note the date, and you will never lose track of which one is new or bigger. It is the single most useful thing an owner of a lumpy older dog can do.
Send the case to the surgeon. A written independent opinion, from $250.
Ask the surgeon. A written independent opinion, from $250.