Dog Cyst vs. Tumor: How to Tell the Difference

If you’ve ever pet your dog and suddenly felt a lump, you know the feeling of panic that can overcome you. What could it be? Well, there are a few different things, including cysts and benign or malignant tumors. One thing is for sure: You know it wasn’t there before, so now you’re worried.

Before you get too overwhelmed, realize that not every lump is cancer, which is what most people are worried about. There are many lumps on dogs that are entirely benign, meaning non-cancerous. The problem is that cysts and tumors can look and feel very much alike from the outside, and it’s going to require a visit to your vet and maybe a test or two to know for sure what it is. 

This article breaks down what a cyst is, what a tumor is, what “benign” and “malignant” mean, and which of those are we talking about when we mention cancer. Read on to learn when to be concerned and when not to, what these things really are, how they’re different, and what to do.

Understanding dog cysts

Cysts are not the same thing as tumors. Let’s go over precisely what a cyst is, different types of cysts, why they occur, and symptoms.

What is a cyst?

If it’s a cyst that you’re feeling, it will feel similar to a tiny balloon in or under the skin. This occurs if something gets blocked or trapped where it shouldn’t be. Another way of describing it is as a closed sac that can be within or under the skin, and it usually contains fluid, a semi-solid material, or keratin, a thicker, toothpaste-like substance made of skin cells. It’s typically closed and does not leak any fluid unless it ruptures. Many cysts are benign (non-cancerous), often cause no discomfort, can be monitored, and do not need to be removed.

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Types of cysts in dogs

There are several different types of cysts that dogs can get, and while we often just refer to them all as a cyst, they don’t all act the same way.

Sebaceous cysts

These occur when oil glands become blocked and are the most common. They’re often round, slow-growing, and can rupture, leaking thick white material.

Follicular (epidermoid) cysts 

This type of cyst is also filled with keratin, but forms around hair follicles. You can commonly find them on the neck, trunk, and legs.

Apocrine sweat gland cysts

These are not as common and are often associated with a specific breed. They tend to be softer and filled with fluid.

False cysts

They’re really not cysts. They don’t have a genuine cyst lining. They’re usually seromas that form after trauma or surgery, when fluid accumulates under the skin.

Causes of cysts in dogs

Cysts almost always have nothing to do with cancer. They generally form due to blockages or irritation.

Common causes include:

• Blocked hair follicles

• Overactive oil glands

• Minor skin trauma

• Genetic predisposition in some breeds

• Allergies, causing inflammation

• Chronic skin infections

Dogs who have oily skin, recurrent skin infections, or allergies are more likely to develop cysts.

Symptoms and identification

Cysts typically:

• Grow slowly

• Feel smooth and round, and can be moved around under the skin with your fingers

• They are not painful unless they’ve become infected

• May change in size from time to time

• Can rupture, and the material that comes out can be thick, white, or yellow

If your dog has a cyst that’s already been diagnosed and suddenly becomes red, painful, swollen, or starts draining, it may be infected and should be rechecked by your veterinarian.

Understanding dog tumors

Tumors are different from cysts. They can be benign or malignant. 

What is a tumor?

While tumors are abnormal cells caused by uncontrolled cell division, they are not necessarily malignant. In fact, most aren’t. They tend to be benign. Benign tumors grow slowly and do not spread. Malignant tumors (cancer) can invade surrounding tissues and spread to other parts of the body. Unfortunately, there’s no way to tell the difference just by looking or feeling.

Types of tumors in dogs

There are many different types of tumors in dogs, but some are more common than others.

Lipomas

These are benign fatty tumors. If you feel them, they feel soft, squishy, and movable like fat. They’re very common in middle-aged and older dogs.

Mast cell tumors

These tumors are malignant (cancerous) and among the most common malignant skin tumors. Unfortunately, they can look like just about anything, from small to large, firm to soft, ulcerated to normal skin, so there’s no way to identify them just by looking.

Histiocytoma

These are benign tumors that are most commonly seen in young dogs. They can be red, raised, and fast-growing, but fortunately, they often disappear on their own.

Soft tissue sarcomas

Soft tissue sarcomas are malignant and can invade surrounding tissue, but they grow very slowly.

Melanomas

This type of tumor can be either benign or malignant. They are most commonly found in the skin, nail beds, or in the mouth.

Causes of tumors in dogs

Genetic mutations are the cause of most tumors, but different factors can make a dog more susceptible to developing one:

• Age (as dogs get older, they are more likely to develop cancer)

• Chronic inflammation

• Breed predisposition

• Hormonal influences

• Environmental factors

• Immune system changes

Rarely can you tell what the cause is, but it’s seldom anything a pet parent did wrong.

Symptoms and identification

Tumors can:

• Cause pain or limping, depending on the location

• Become ulcerated or bleed

• Change in color, shape, or size

• Grow rapidly

• Feel firm or fixed, unlike cysts, which are soft and movable

Unfortunately, some malignant tumors can feel soft and harmless, while some benign tumors can feel scary. That’s why you can’t rely on appearance and will need your veterinarian's help to determine what you’re dealing with.

Differences between cysts and tumors

To make things even simpler, here’s a quick comparison:

Cysts 

• Often movable

• Frequently benign

• Slow growing

• Usually fluid-filled

• May rupture and drain

Tumors

• Can be benign or malignant

• Can grow quickly

• Solid tissue growth

• May feel fixed or irregular

• Do not tend to drain like cysts

The big takeaway here is that you cannot diagnose a lump by feeling it alone. Even a very experienced veterinarian has to rely on diagnostic tests.

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Lipomas in Dogs: Causes, Symptoms and Treatment of Fatty Tumors

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Treatment options

Naturally, treatment will depend on what the lump is. To find out what you’re dealing with, diagnostic tools will be needed:

Find needle aspirate (FNA)

This is a sample obtained with a quick needle stick and examined under a microscope. This is often the first step to diagnose a lump.

Biopsy

A tissue biopsy removes actual tissue for laboratory analysis. This will provide a definite answer.

Imaging

X-rays or ultrasound are used to look for internal tumors or the spread of the original tumor.

Treatment approaches

Monitoring

Your vet may tell you that a tumor can be watched, such as a benign tumor or cyst. 

Medical management

If the cyst is infected, it will likely require antibiotics or anti-inflammatory medications.

Surgical removal

Masses that are painful, infected, rapidly growing, or malignant may require surgical removal.

Oncology care

If the tumor is malignant, certain cancers will require chemotherapy, radiation, or targeted therapy.

The earlier you detect a lump and the quicker you have it looked at by your veterinarian, the simpler and less invasive the treatment, and the better the outcome.

How to prevent dog cysts and tumors

You can’t prevent every lump, but there are things you can do to reduce risk and catch problems early.

• Every month, run your hands over your dog completely, feeling for any new lumps.

• Keep your dog at a healthy weight.

• Manage allergies.

• Have skin infections looked at immediately.

• Schedule regular vet exams.

• Don’t ignore lumps just because they’re small or unchanged.

Early detection absolutely makes a difference.

Bottom line

If you suddenly feel a lump on your dog, it’s scary, but remember it’s not necessarily bad news. Many times, it’s a benign cyst or tumor, and even if it’s malignant, it's often treatable when caught early.

The best thing to do if you find a new, growing, or changing lump is to see your veterinarian right away. A quick needle stick can often provide peace of mind, or it can catch a serious problem before it becomes too serious to treat. When it comes to lumps, guessing is stressful; knowing is empowering.

References

• Adedeji, Adeyemi O., et al. “Cytologic Features of Cutaneous Follicular Tumors and Cysts in Dogs.” Veterinary Clinical Pathology, vol. 46, no. 1, Mar. 2017, pp. 143–150, Accessed 28 Dec. 2025.

• Dobson, Jane M. “Breed-Predispositions to Cancer in Pedigree Dogs.” ISRN Veterinary Science, vol. 2013, 2013, pp. 1–23.

• Garrett, Laura. “Canine Mast Cell Tumors: Diagnosis, Treatment, and Prognosis.” Veterinary Medicine: Research and Reports, vol. 5, Aug. 2014, p. 49.