Thank you for your question!
Its understandable to be alarmed when you find a lump on your dog but the good news is that most are benign (non-cancerous) growths that often appear as dogs get older.
Nevertheless it is important to have lumps checked by your veterinarian as, if the worst happens, and it is cancerous then the earlier it's diagnosed the better the outcome for treatment. Your vet will examine the area and may sample the lump to evaluate the cells under a microscope to determine what they are. This is the best way to determine whether a lump is benign or malignant.
-Dr Alex Hynes