Glossary
Mast cell tumor
The most common skin cancer in dogs, which can look like any lump, so every new skin lump should be sampled by a veterinarian.
Also called: MCT, mastocytoma, mast cell cancer, skin tumor in dogs
Mast cells are immune cells that release histamine. A mast cell tumor, or MCT, is a cancer of these cells and the most frequent malignant skin tumor in dogs. It can look like a wart, a soft fatty lump, a red bump or an ulcer, and it may swell and shrink as the cells release histamine.
Boxers, Pugs, Labrador Retrievers and other breeds are overrepresented, and most cases are in older dogs. The behavior varies, from slow, low-grade tumors to aggressive ones.
Nobody can tell a harmless lump from an MCT by looking or feeling, so vets collect cells with a fine needle and may take a biopsy to grade the tumor. Do not squeeze or poke a lump. Have a vet check every new or changing lump.
Last reviewed November 8, 2026