Lipomas, Fatty Tumours & Lumps Under Dogs Skin

Updated October 18th, 2020

It’s no accident that one of our most visited pages is a guide to the skin lumps of dogs. Tumours and masses cause a lot of concern to dog owners. However, with some basic knowledge, and the right action, there’s nothing to fear.

Here I will help you identify a lump found under your dog’s skin. First though I need to clear up a common misconception.

Have something to add? Comments are welcome below and will appear within 24 hours.
By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. These help topics are from a series regularly posted on email and Twitter. Subscribe via email here to never miss a story! The information provided here is not intended to be used as a substitute for going to the vet. If your pet is unwell, please seek veterinary attention.

Do Lumps Appear Overnight?

Many lumps seem to appear suddenly. The good news for dog owners is that this is almost never really true.

But, you say, it definitely wasn’t there yesterday! What’s going on here?

A lump will creep up gradually, ‘under the radar’. Because it changes slowly, it doesn’t trigger your awareness. You’re not looking for problems, so it just hides in the contours.

Then, one of three things happens:

  • it reaches a sufficient size to be noticed
  • the surrounding coat gets a bath or shave
  • your dog loses weight

Now, you can’t stop seeing it. Jeepers, you can even see it across the room.

Trust me on this: definitely see a vet, but the lump could have been there six months. And you want it that way; a slow growing lump is unlikely to be cancerous. It’s the same way you don’t notice a dent or scratch on your car, and then once you see it, it drives you crazy.

Types Of Lumps Under The Skin

Most of the time, a lump under the skin of an old dog is a lipoma, or fatty tumour. However, the biggest mistake is assuming this without testing. It’s no good being right 99% of the time if the other 1% ends up with amputation or worse.

Warning: the idea that you can identify a lump by whether it’s hard or soft, or whether it moves under the skin is false. And at this stage, even the nastiest tumour won’t have any effect on health or behaviour. After this list of common lumps, I’ll show you the only way to tell them apart.


Lipomas are benign tumours of fat tissue. They are most commonly found:

  • in the armpit
  • on the chest
  • on the flank just in front of the hind leg

The picture above shows a large lipoma in the armpit (axilla) and my hand on a smaller one on the chest just behind. It goes to show how hard these lumps are to see.

A lipoma can appear at any age, but most often over 10 years. They grow so slowly that you certainly can’t tell they’ve grown in a month, and usually not even in 3-6 months.

Lipomas are easy to take out and almost never recur after surgery. However, not all need removal as they are painless and slow-growing. Whether we operate depends on five factors:

  1. the position: lipomas in the armpit or on the chest are especially troublesome
  2. the dog’s age: if a dog is 14 there’s not much chance that a small lipoma will trouble them in their expected lifespan
  3. the owner: tolerance of skin lumps and ability to afford surgery vary a lot from person to person
  4. the dog’s health: other illnesses might increase the anaesthetic risk
  5. the speed of growth: rapid enlargement will cause problems just through size and weight alone

Occasionally, a fatty lump isn’t a lipoma, but a liposarcoma or infiltrative lipoma. These are nastier versions of fatty tumours that definitely need removal before it’s too late. You can usually get a clue by how fast they grow.


dog sarcoma tumour

This is the lump that’s most often mistaken for a lipoma because it often feels exactly the same: soft, rubbery and round.

Fibrosarcomas are aggressive, invasive cancers that need urgent attention. Just look at the surgical margins necessary for removal and you can see why.

Speed of growth and position are the best clues. If you can tell that a soft lump is growing, think sarcoma. If it isn’t in the typical places for a lipoma, think sarcoma. However, they can occur anywhere.


dog sebaceous cyst
Sebaceous cyst

A build up of liquid or even solid material under the skin is called a cyst. The most common is a sebaceous cyst, caused by a blocked sebaceous gland. These can grow quickly or slowly, and often burst, releasing a thick brown paste.

You’ll find sebaceous cysts most commonly along the top of the dog from neck to tail.

A soft, painless, cool lump that appears under a surgery site is most often a seroma. This is caused by fluid collecting in the space created by the operation. Seromas can also occur over bony points like the elbow or head after injury.

Cysts rarely need treatment. Sebaceous cysts will grow slowly but normally cause few or no problems. Seromas come up quickly but should go down by themselves over a few weeks.


A special form of cyst filled with pus is called an abscess. Abscesses are well-known in cats, but when we see them in dogs there’s usually an underlying cause. Common examples are stick injury, grass seeds or other foreign bodies.

An abscess comes up quickly and is usually soft, warm, and painful. Its appearance prompts us to perform an anaesthetic and carefully explore the cavity.

Lymph Node

dog lymph nodes

Lumps in the places indicated are often caused by lymph node enlargement. If you can feel a lymph node, this is a bad sign, as that means it’s quite large.

Lymph nodes are very firm and immobile. The only common cause is lymphoma or other cancers, but sometimes a single enlarged node might be caused by infection.

Mammary Tumour

dog breast lumps
Firm masses under the nipples of an older entire female dog. The abdomen is enlarged due to pyometra, a uterine infection.

If your dog is female, and has been desexed later in life, consider mammary tumours. These are firm lumps growing anywhere along the mammary chain from armpit to groin. They often feel quite irregular in shape.

Unlike people, mammary carcinoma usually spreads slowly and small lumps can be removed without radical surgery. Desexing at the time will reduce further tumour development but not stop it completely.

Other Lumps

Two common lumps often get mistaken for tumours, but are in fact normal.

There’s a salivary gland in the neck behind the jaw that feels a lot like the lymph node just in front. It’s also firm and round, and roughly 2cm in a medium dog. However, it shouldn’t stick out, and the opposite one should feel exactly the same.

A hand-shaped firm mass is often found on the rump of overweight dogs. This is usually a fat pad which feels firmer than usual due to the pelvis underneath. For this to be true, there should be an identical one on each side, though one will stick out more if the dog is not standing straight.

perineal hernia dog
Perineal hernia

There are many other rarer possibilities too numerous to mention, that probably together only make up an extra 1%. Pictured above is a perineal hernia, which occurs in older undesexed male dogs. Inguinal hernia, by contrast, is a bulge in the groin where the leg meets the body.

What To Do If Your Dog Has A Lump

The only way to identify a lump with certainty is with a visit to the vet.

Most vets will start with a fine needle aspiration as shown in the link. This will be able to immediately identify a lipoma or cyst.

Other tumours require further testing. Here we have three choices:

  1. Send the slide of aspirated material to a pathologist. This can be diagnostic but is unreliable as the tissue architecture has been lost.
  2. Schedule an ‘incisional biopsy’, where we only take a small piece and get it analysed. This is the best way as it allows us to plan for a single curative surgery.
  3. Schedule an ‘excisional biopsy’, where we remove the entire mass and get it analysed. Without a wide margin, this will only be curative for very benign lumps and require a second surgery for most others.

The decision we take will depend on many factors. Here are some thoughts we may have:

  • It’s always good to send the slide, but the extra cost might not be justified by its unreliability
  • An excisional biopsy is never a good idea without analysis unless the vet is certain the lump is benign, or there is plenty of loose tissue for a second surgery
  • An incisional biopsy followed by definitive surgery will usually give the highest total cost

The single greatest influence on a successful outcome will be how quickly you choose to get a lump checked. This is especially true in places with poor skin coverage, like the head, legs and tail. No vet likes to see a lump for the first time and think, “I hope this is benign, because if it’s not…”

Related: Common Mouth Lumps | Lumps Under The Eye

By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. These blogs are from a series regularly posted on email and Twitter. Subscribe via email here to never miss a story!
Have something to add? Comments are welcome below and will appear within 24 hours.


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