If you’ve found a lump under your dog’s skin, it can be easy to fear the worst. The good news is that not all lumps are tumors, and even if a lump is a tumor, not all tumors are cancer. Integrative veterinarian Dr. Julie Buzby discusses the different reasons for a lump under a dog’s skin and what to do about it.
Did you know that your dog’s skin is the largest organ in his or her body? Being able to see and feel the skin and associated tissue makes it easy to quickly notice new problems. Sometimes, the changes are more superficial, like signs of an infection or a skin tag. Other times, a new lump under a dog’s skin might be what catches your attention.
Why do lumps form under a dog’s skin?
There are many reasons for a lump under a dog’s skin. But in general, they occur when something—be that more cells, fluid, pus, foreign objects, etc.—gets added to a discrete area in the dog’s subcutaneous (below the skin) space. The resulting mass may be:
- Large or small
- Hard, firm, soft, or fluctuant (feels like fluid)
- Painful or non-painful
- Round or irregularly-shaped
- Moveable or firmly attached
- Involving the skin and subcutaneous tissue or only the area under the skin
- Located most places on the body—shoulders, neck, head, back, belly, legs, etc.
Subcutaneous types of lumps and bumps can be divided into three categories: non-tumors, benign tumors, and malignant tumors.
8 lumps under a dog’s skin that are not tumors
A dog can develop a lump under the skin that is not a true tumor. Sometimes the bump comes up suddenly, as is the case with those caused by infection, inflammation, or fluid accumulation. Other times, the bump grows more gradually.
Some examples of non-tumor subcutaneous lumps include:
1. Follicular cysts
Follicular cysts in dogs form when debris or infection gets trapped inside hair follicles. They can become inflamed and may sometimes rupture.
2. Sebaceous cysts
Sebaceous cysts in dogs are a type of follicular cyst that forms when sebum from oil glands becomes trapped inside a hair follicle. They may occur on top of the skin or under the skin and contain a white to grey thick material.
3. Dog bites, insect bites, or other trauma
The body will release inflammatory mediators in response to an insect or animal bite or other type of trauma. These chemical signals increase blood flow, draw white blood cells to the area, and cause fluid to leak into the tissue from the vessels. This can create a swelling or lump which may be painful or hot. Plus, the skin in the region might be red or damaged.
4. Abscess
Bacterial infections can lead to a pocket of pus (abscess) forming beneath the skin. Dog tooth root abscesses are one of the most common culprits. They can occur on the face or underneath the eye, and some can rupture outward. However, dogs can also develop a subcutaneous abscess from an infected bite or the presence of foreign material in the skin.
5. Foreign object
If a sharp item or plant material (such as a foxtail in dogs) gets lodged under the skin, it can cause an area of swelling and pain. There may also be a draining tract.
6. Seroma or hematoma
Dogs may develop a hematoma, or a collection of blood under the skin, after trauma. Or sometimes, especially after surgery or an injury that pulls the skin away from the underlying tissue, the dog can develop a pocket of serum, the fluid portion of blood. This is called a seroma.
7. Enlarged lymph node(s)
Dog lymph nodes are small, bean-shaped structures that are part of the immune system. They are located in many places throughout a dog’s body, including under the jaw, in front of the shoulders, and behind the knees. When enlarged due to infection, inflammation, or cancer, lymph nodes can become more prominent. It is easy to mistake them for a new lump.
8. Lump from a vaccination
Vaccinations contain substances meant to stimulate the immune system, which is a good thing. However, sometimes the resulting inflammatory reaction may cause the dog to temporarily develop a lump under the skin at the site of the vaccination. Generally, the lump will get smaller over a few weeks and doesn’t bother the dog. However, if it persists for more than three months or so, it is best to check in with your veterinarian.
4 benign tumors under a dog’s skin
Dogs can also develop non-cancerous or cancerous tumors under their skin. There is a higher chance that a lump is a tumor (a mass that develops when there is uncontrolled cellular growth) in middle-aged or older dogs. However, young dogs can develop tumors, too.
Benign tumors remain localized to their site of origin and seldom cause discomfort. They can vary in overall size, though many are 2 to 3 centimeters wide on average.
Examples of benign subcutaneous tumors in dogs include:
1. Lipomas
Lipomas in dogs are by far the most common type of benign subcutaneous tumor in dogs. They are comprised of fat cells (adipocytes) and typically feel like soft, movable lumps under a dog’s skin.
2. Fibromas
As the name indicates, fibromas are made up of fibrous tissue. They can form from chronic inflammation or past trauma and are usually firm to the touch.
3. Hair follicle tumors (trichoepitheliomas and pilomatricomas)
Trichoepitheliomas are tumors that form within hair follicles and are more common in cats but can occur in dogs. The most common locations include the face and chest.
Pilomatricomas also form from hair follicles. They can act like cysts but usually contain gritty material instead of fluid or sebum.
4. Hamartomas
Hamartomas are dome-shaped, firm swellings that may appear on the lower legs, on pressure points, and between a dog’s toes.
5 cancerous lumps under a dog’s skin
Unfortunately, some lumps under a dog’s skin end up being malignant tumors (cancer), which have the ability to grow quickly. They may be round or have a poorly defined or oddly shaped appearance. Sometimes the cancer can be locally invasive. Or it may have the potential to spread to other internal organs like the liver or the lungs.
Examples of malignant tumors include:
1. Mast cell tumors
Mast cell tumors are the most common type of malignant skin tumor in dogs. Known as “great pretenders,” mast cell tumors in dogs can look like anything, from a pea-sized lump under a dog’s skin to a large bump with a red ulcerated surface.
2. Fibrosarcomas
Fibrosarcomas are malignant tumors that come from connective tissue. They can be hard to the touch and typically form on the trunk, on the nose, or in the mouth.
3. Squamous cell carcinomas
Most squamous cell carcinomas occur around the lips, mouth, and nail beds. But they can sometimes grow underneath the skin’s surface instead.
4. Liposarcomas
Like lipomas, liposarcomas originate from fat cells. However, they are firmer to the touch than lipomas, can be locally invasive, and, while it is uncommon, can metastasize.
5. Leiomyosarcomas
Leiomyosarcomas originate from smooth muscle and are more likely to develop from the digestive tract. However, cutaneous leiomyosarcoma in dogs can appear as firm nodules under the skin.
You found a lump under your dog’s skin. What happens next?
Whenever you find a new lump or bump on your dog, the best course of action is to have your veterinarian evaluate it. Based on your dog’s history, any additional symptoms, and the appearance of the lump, the vet will create a diagnostic and treatment plan.
Often, the vet’s first step may be to perform a fine needle aspirate (FNA) on the lump. This test involves using a needle to pierce the lump, collecting cells from inside the lump, and then evaluating them under a microscope. Veterinarians and veterinary pathologists can usually determine what the lump might be based on cellular appearance.
However, in some cases, fine needle aspirates can be inconclusive. This may be because a bump was too firm to yield adequate cells, or the cells were not representative of the mass as a whole. In these cases, your veterinarian may recommend a biopsy prior to or after removal. Since a biopsy looks at a chunk of tissue rather than just cells, it provides more information than the FNA. However, it is also more invasive.
Additionally, especially if the vet suspects the mass is cancerous and no biopsy has been performed yet, he or she may recommend one after removing the mass. This way, the vet knows exactly what kind of mass it ended up being and if the margins were clean (free from cancer cells).
What is the treatment for a lump under a dog’s skin?
The treatment plan depends in large part on the type of lump the dog has.
Treatment for lumps that aren’t tumors
The vet may use a combination of topical and systemic medications to treat bites, infections, and inflammatory cysts. If the lump is draining and the fur is matted over it, he or she may recommend clipping and scrubbing the site before starting therapy, too.
Sometimes topical sprays and ointments are the right choice for treating the infection and inflammation. However, other swellings, especially ones that haven’t erupted to the surface, may require oral antibiotics and anti-inflammatories.
Treatment for tumors (cancerous and non-cancerous)
If a tumor is present, the results of the fine needle aspirate and/or biopsy will dictate the plan. For masses that are potentially cancerous or causing problems, surgical removal is the treatment of choice. On the other hand, benign masses usually don’t need to be removed. But it is important to keep an eye on them and report any changes in size or appearance to the vet.
For tumors that the vet is able to remove with complete margins (no cells are left behind), the surgery may be considered fully curative. However, sometimes the cancer cells may have already spread to another organ, or the margins weren’t wide enough. In those cases, the vet or veterinary oncologist may recommend additional therapies such as chemotherapy, radiation, and immunotherapy.
Be proactive if you find a lump under your dog’s skin
Especially if you have an older dog who keeps having lumps and bumps pop up, or the last few lumps the vet aspirated were benign, it can be easy to become lax about testing new bumps on your dog. However, this is a risky approach.
It is impossible for a dog parent or veterinarian to be 100% certain that a lump under a dog’s skin is harmless without testing. Unfortunately, there are plenty of times when a soft, movable lump under a dog’s skin “felt” like a lipoma but ended up being a malignant tumor.
As they say in veterinary oncology, “Why wait? Aspirate!” It is best to test each new lump or bump on or under your dog’s skin, so you know what you are dealing with and how best to proceed. That way, you don’t run the risk of accidentally ignoring a mass that later turns out to be problematic.
If your dog had a lump under the skin, what did it end up being?
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