While there's no need to panic if your dog develops bumps and lumps on his skin, you should always have these growths checked out by your veterinarian. While the majority of them end up benign, some are possibly cancerous round cell tumors. Different types of round cell tumors can mimic each other, so your veterinarian must biopsy the tumor in order to make a definite identification.
Mast Cell Tumor
One of the most common round cell tumors, mast cell tumors appear in various guises. They can be flat or raised, small or quite large. The skin on the growth might appear normal with hair intact, or ulcerated and hairless. Mast cell tumors can feel soft to the touch or very hard. Your dog might have just one, or several close together. Most of these tumors are malignant. If surgically removed, it's not uncommon for the tumor to grow back. Mast cell tumors usually metastasize to the liver and spleen.
Lymphoma, which affects a dog's lymph glands, doesn't present itself as a tumor but dogs with the disease experience swollen lymph glands that look like lumps. These usually appear on the primary joints and aren't painful to the touch. While there's no actual cure for this systemic malignancy, some dogs survive for extended periods after undergoing chemotherapy. During this treatment and after, most of these animals have a good quality of life.
Transmissible Venereal Tumor
The venereal tumor is called transmissible because dogs can catch it from each other, and sex isn't necessarily involved. If a transmissible venereal tumor on a dog's body touches an open wound or damaged skin on another canine, the other dog can "catch" it. Although these tumors usually appear on the genitals, they also show up on the mouth, nose and anal areas. If on the latter, another dog can pick up the disease through sniffing. Fortunately, this tumor is rarely malignant. Surgery isn't effective, as the tumor generally grows back. Eradicating the tumor usually requires the dog to undergo chemotherapy and radiation.
Other Round Cell Tumors
Young dogs might develop histiocytomas, small, ulcerated bumps that usually crop up on the head. Most are benign and eventually disappear without treatment. Older dogs are prone to extramedullary plasmacytomas, which are single, small tumors usually appearing on the head, inside the mouth or on the feet. These generally benign tumors require surgical removal if they interfere with the dog's ability to eat. Histiocytic neoplasias usually affect the lungs, so they're a type of round cell tumor that an owner won't notice on his dog. These tumors might or might not be malignant.
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