OldieMast cell tumours are the most common skin cancer of dogs, and they can look like anything: a soft lump, a firm nodule, a rash, a swelling that comes and goes. They affect mainly middle-aged and older dogs, with a median age at diagnosis of seven to nine years, and breeds reported as predisposed include the Boxer, Pug, Staffordshire Bull Terrier, Boston Terrier, Golden Retriever, Shar-Pei and French Bulldog1.
Because they cannot be recognised by eye, every new skin lump on a senior dog should be checked with a needle sample; the lump that has been there for months is also worth a look. Mast cell tumours release histamine, so a lump that swells and shrinks, or that makes the dog itchy or vomit, is a particular flag.
Many are cured by surgery with a wide margin. Higher-grade tumours may need further treatment, and the vet will grade the tumour to decide1.
This is general information, not a diagnosis. Your vet knows your dog.
Sources
- Prevalence and risk factors for mast cell tumours in dogs in England (Shoop SJ et al.). Canine Genetics and Epidemiology 2:1, 2015 (peer reviewed). Accessed 2026-09-22.