Carcinoma in Dogs vs Sarcoma: Key Differences Explained

A carcinoma is a cancer that starts in epithelial cells, the cells that cover the skin, line organs and passages like the bladder, and make up glands. Carcinomas are malignant by definition, and many of them can spread, often reaching the nearby lymph nodes first.

A sarcoma is a cancer that starts in connective tissue instead: fat, bone, muscle, blood vessels, nerve sheaths and the fibrous tissue that holds everything together. Sarcomas are malignant by definition too.

So the difference is where the cancer starts. That shapes how a tumor tends to behave, how your veterinarian samples it, and where they look for spread. What the label does not tell you is how serious your dog's cancer is. That depends on the specific tumor type, its grade and its stage. For a broader introduction, see our guide to neoplasia in dogs.

What Carcinoma and Sarcoma Mean

Cancer develops when cells that would usually die and be replaced grow out of control instead. These extra cells may form a mass called a tumor, although some cancers, such as leukemias, do not.

Carcinomas and sarcomas share the ending -oma, which means tumor, as do adenoma and lymphoma. Carcino- means cancer; sarco- means flesh. Some common dog tumors, like mast cell tumors, have names of their own.

The name also tells you whether a tumor is benign. "Carcinoma" and "sarcoma" are reserved for malignant tumors. A benign tumor of the same tissue gets a different name: an adenoma or papilloma is the benign epithelial version, while a lipoma (fat) or fibroma (fibrous tissue) is the benign connective tissue version. If a report says "sarcoma," it is describing a cancer, not a harmless lump.

Carcinoma vs sarcoma at a glance

How Carcinomas and Sarcomas Spread

Spread to a distant part of the body is called metastasis. A common rule of thumb is that carcinomas tend to travel through lymph vessels to the nearest lymph nodes first, while sarcomas more often spread through the bloodstream, frequently to the lungs. Treat that as a starting point, not a law. Some carcinomas spread through the blood, and some sarcomas reach lymph nodes.

Behavior also varies a lot within each family. Transitional cell carcinoma of the bladder spreads frequently, while hepatocellular carcinoma is slow to spread. Many soft tissue sarcomas rarely spread unless they are high grade, while osteosarcoma and hemangiosarcoma commonly reach the lungs.

That is why neither word is a verdict. A low-grade sarcoma removed with wide margins and a carcinoma that has already reached the lymph nodes are very different situations, even though one sounds scarier than the other.

Common Carcinomas in Dogs

Carcinomas can develop anywhere epithelial cells are found, including the skin, nasal cavity, chest cavity, bladder, liver and mammary glands.

Squamous cell carcinoma

A cancer of the skin's surface cells. Most are solitary, a single lesion in one location. The skin form is linked to sun exposure on lightly pigmented, thinly haired skin such as the belly, and Dalmatians, Bull Terriers and Beagles are among the breeds at higher risk. A separate form grows in the nail bed of the toe, more often in breeds such as Standard Poodles, Giant and Standard Schnauzers, Gordon Setters and Briards, and it may spread to the lymph nodes and lungs.

The outlook depends on how malignant the tumor is, how big it is, whether it can be removed, and how far it has invaded local structures.

Transitional cell carcinoma

A cancer of the lining of the bladder and urethra, now often called urothelial carcinoma. It most commonly involves the neck of the bladder and the urethra, and it can grow into the prostate in male dogs. Scottish Terriers are at the highest risk by a wide margin; Shetland Sheepdogs, Beagles, West Highland White Terriers and Wire Fox Terriers are also predisposed.

It spreads frequently, most often to the regional lymph nodes and lungs. As it grows, it can block the urethra or ureters and stop urine from flowing, which is an emergency. The Merck Veterinary Manual notes that treatment is not usually curative, but that treated dogs often live a year or more with a good quality of life.

Hepatocellular carcinoma

A cancer of the liver cells themselves, and the most common primary liver tumor in dogs. It may first show up as a mass your veterinarian can feel in the abdomen, or as rising liver enzymes on routine bloodwork. Ultrasound is the more sensitive way to find it, and a tissue sample confirms what it is.

When it forms one large mass, it can often be removed surgically, and wide removal can be curative. The outlook depends heavily on whether the tumor can be removed.

Mammary carcinoma

About half of mammary (breast) tumors in dogs are malignant; the rest are benign. The malignant ones come in several subtypes, and the outlook depends on the tumor's size, grade and stage, including whether it has reached the lymph nodes or invaded blood vessels. Mammary tumors are most common in unspayed female dogs, and the risk is low in dogs spayed before their first heat. Our guide to mammary cancer in dogs covers this in more detail.

Common Sarcomas in Dogs

The group dog owners hear about most is soft tissue sarcoma, but bone and blood vessel cancers are sarcomas too, and they behave quite differently.

Soft tissue sarcoma

An umbrella term for several connective tissue cancers, including fibrosarcoma, liposarcoma and peripheral nerve sheath tumors. They usually show up as a lump under the skin or in the muscle, often on a limb or the chest or abdominal wall.

Pathologists grade them from I to III (low, intermediate or high), based on how the cells look and how fast they are dividing. The grade drives behavior: most are locally invasive with a low chance of spreading, but high-grade tumors are more likely to spread, most often to the lungs.

They are also bigger than they feel. They can seem neatly contained, but tumor cells often extend beyond the edge you can feel, which is why surgeons aim to remove a wide margin of normal-looking tissue around them.

Liposarcoma

Liposarcomas develop from fat cells, usually in older dogs. They are uncommon and malignant, unlike the far more common benign lipoma. They are often firm with an ill-defined border, locally invasive, and have a low potential to spread. Shetland Sheepdogs and Beagles are listed among the breeds most at risk.

Dogs whose liposarcoma is removed with wide margins generally do much better than dogs whose tumor is only partly removed or sampled.

Osteosarcoma

Osteosarcoma is the most common primary bone tumor in dogs, accounting for more than 80% of them. It usually affects the limbs and occasionally the skull, spine or ribs, and it is most common in large and giant breeds such as Great Danes, Rottweilers and Scottish Deerhounds.

The outlook is guarded because it so often spreads to the lungs. According to the American College of Veterinary Surgeons, about 10% of dogs already have spread at the time of diagnosis, but up to 90% develop it during the course of the disease. Read more in our guide to osteosarcoma in dogs and cats.

Hemangiosarcoma

A cancer of the cells that line blood vessels, so it can appear wherever there are blood vessels. The spleen is the most common site; others include the heart (the right atrium), the liver, the skin and the muscle. It is more common in older dogs, and German Shepherds, Golden Retrievers and Labrador Retrievers are among the breeds most often affected. It tends to spread through the bloodstream to the lungs and other organs.

Peripheral nerve sheath tumors

These malignant tumors, sometimes called neurofibrosarcomas, develop from the protective sheath that covers peripheral nerves. In dogs, they most often arise from the brachial plexus, the bundle of nerves that supplies the front leg, and less often from the trigeminal nerve in the head. They are slow-growing and invade locally, but most do not spread to other organs.

How Vets Tell Carcinoma and Sarcoma Apart

The first step is usually a fine needle aspirate, especially for a lump that can be felt through the skin. A small needle is passed into the tumor, negative pressure is applied with an attached syringe, and the needle is redirected a few times to collect cells. The cells are spread onto a glass slide and sent to the lab. This fine needle aspirate technique video shows how it is done.

A pathologist then examines the cells under a microscope (cytology). Cytology can suggest what kind of tumor it is, but it is not always diagnostic. Sarcomas are a particular challenge: their cells sometimes hold together so tightly that an aspirate collects none, and even a good sample cannot show the tumor type or grade.

That is where a biopsy comes in. A piece of the tumor (incisional biopsy) or the whole mass (excisional biopsy) is removed and sent to a pathologist for histopathology, the examination of the tissue structure under a microscope. Histopathology confirms the tumor type and gives its grade, a measure of how abnormal the cells look and how quickly they are dividing. If the mass is internal, your dog will need anesthesia anyway, so a biopsy is usually more useful than an aspirate because it is more likely to give a definite answer.

Next comes staging: working out how big the tumor is and how far it has spread. Depending on the tumor, it may include:

• bloodwork and a urinalysis to check general health;
• a fine needle aspirate of the nearest lymph nodes;
• chest x-rays or a CT scan to look for spread to the lungs;
• an abdominal ultrasound or CT scan when the tumor type calls for it.

Grade and stage, along with the tumor type and whether it can be fully removed, drive your dog's outlook and treatment options.

Questions to Ask Your Veterinarian

If your dog has been diagnosed with a carcinoma or sarcoma, these questions help turn the label into a plan:

• What type of tumor is it exactly? "Carcinoma" or "sarcoma" is the family; the specific type matters more.
• What grade is it? And was that grade based on a biopsy or on cytology alone?
• What stage is it? Has it reached the lymph nodes, lungs or other organs, and which tests checked for that?
• If it was removed, were the margins clean? That is, did the pathologist find tumor cells at the edge of the removed tissue?
• Should we see a veterinary oncologist? A referral can help you weigh options for this specific tumor.

The Bottom Line

Carcinomas and sarcomas are both malignant. They differ in where they start, epithelial cells versus connective tissue, and that shapes how they tend to spread, but the label alone does not tell you how aggressive a particular tumor is. The type, grade and stage do.

If you notice an abnormal growth or a change in your dog's behavior, have your dog examined by a veterinarian. Both types of cancer can be treated with surgery, chemotherapy or radiation, and your veterinarian or a veterinary oncologist will recommend the right approach for your dog.

Originally written by Helen Roberts. Updated by the I Love Veterinary editorial team.

Sources

• Merck Veterinary Manual: Introduction to Cancer and Tumors in Animals
• Merck Veterinary Manual: Connective Tissue Tumors in Animals
• Merck Veterinary Manual: Tumors of the Skin in Dogs
• Merck Veterinary Manual: Neoplasia of the Urinary System in Dogs and Cats
• Merck Veterinary Manual: Mammary Tumors in Dogs
• ACVS: Bone Tumors in Cats and Dogs
• Cornell Riney Canine Health Center: Hemangiosarcoma in Dogs

This article is for general information and is not a substitute for advice from your veterinarian.