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My first assumption is that your dog is 5 years of age or older. If not, the possibilities are a bit different, so let me know if my assumption is not correct. Although most soft lumps are not cancerous, there are a few that could be. If the lump is movable under the skin (i.e., not attached to the skin above or the deeper tissues below) and is of the exact consistency of fat, then there is a very good chance that it is a benign (NON-cancerous) tumor called a lipoma. However, it would still be wise to have your vet confirm that. If the skin above the lump moves with the lump (i.e., is attached to the lump), then there are several possibilities. One is a simple skin cyst (often called a sebaceous cyst or an epidermal cyst). These are usually spherical, have a portion that is raised a little above the skin, and the center is usually softer than the outer portion. If closely examined, you might see a small central pore in the center. There are a few soft lumps that can fool you, however. One is an enlarged lymph node. The lymph nodes on a dog's chest are just in front of the shoulder, between the neck and the "shoulder blade" (the prescapular lymph nodes) or against the chest in the "armpit" region. A lymph node can be enlarged because of infection and, yes, cancer, so it should always be evaluated by a vet. Another soft lump that can fool you is a "mast cell tumor." These may seem at first like a lipoma, but are attached to the skin. One hallmark of a mast cell tumor is that its size may go both up and down, and it may periodically turn red and cause the dog to scratch at it. This is because a mast cell tumor's cells contain histamine, which, when released, cause a reaction similar to an allergic reaction. These tumors can be quite serious, depending upon the grade, which can only be determined with a biopsy. However, they can be diagnosed by a needle-aspirate, which is examined in the office, under a microscope.
I hope his helps. Let me know if you have any questions.