Cancer Lumps on Dogs: How to Tell If a Lump Is Serious and What to Do

Your hand finds something under the fur that was not there last week, and your stomach drops. It is a common reason owners get in touch, and if that is where you are tonight, the first thing worth saying is that most lumps and bumps turn out to be harmless. The difficulty is that you cannot tell which is which by looking, so every new lump is worth mentioning to your vet rather than watching and worrying about it.

Key takeaways

• Telling a benign lump from a cancerous one is not always possible by eye or by feel alone.
• Finding cancer early gives the best chance of successful treatment, and usually makes surgery simpler.
• Fine needle aspirates and biopsies are the two usual ways of finding out what a lump actually is.

Is the Lump on My Dog Cancer? The Honest Answer

The honest answer is that nobody can tell you from across the room, and we know that is not what you want to hear. A vet may have strong suspicions, but there is no way to know for certain what a lump is without sampling it. An experienced vet can often reassure you about typical fatty lumps and warts, particularly where they are small and have not changed for some time, but reassurance is not the same as a diagnosis.

Signs a Lump on a Dog Could Be Cancerous

There are certain ‘red flag’ signs worth knowing about. None of them means cancer on its own, and harmless lumps can tick one of these boxes, but together they help your vet judge how quickly your dog needs to be seen.

It Is Growing or Changing

A lump that is growing quickly or changing in shape, colour or texture is worth having examined sooner rather than later. Rapid growth can point to cancer, and the smaller a lump is at the point of surgery, the simpler and less invasive the removal tends to be. Photographing the lump next to a ruler every couple of weeks gives your vet something concrete to compare against.

It Is Hard, Irregular or Fixed in Place

Cancerous lumps are more often firm or hard, though this is far from a rule. They also tend to feel fixed, as though anchored to the tissue underneath, rather than sliding freely. Fatty lumps, by contrast, are usually soft and move easily under the skin.

It Is Ulcerated, Bleeding or Weeping

Broken skin over a lump is a concern, particularly when it has been that way for a while and is not healing. As cancerous lumps grow, they stretch the overlying skin until it breaks down, which leads to bleeding and secondary infection.

It Is Painful or Your Dog Is Unwell

Cancer is not always painful and many dogs with a cancerous lump seem entirely well in themselves. Where a cancer is very large or has spread, your dog may start to seem off colour, and you might notice signs such as:

• Lethargy
• Pale gums (due to anaemia)
• Abdominal swelling
• A cough that does not settle
• Vomiting and diarrhoea
• Lameness
• Food refusal
• Sudden weakness or a collapsing episode
• Nose bleeds or unexplained bruising

There Is More Than One

Several lumps appearing at once is worth paying attention to. Multiple swollen lymph glands, for instance, can be the first sign of lymphoma.

Common Cancerous Lumps in Dogs

There are many different types of cancerous lumps, but there are certain ones that vets diagnose more regularly than others.

Mast Cell Tumours

Mast cell tumours are known as ‘the great pretenders’, because they can mimic almost any other lump and are not easy to identify by eye. They can also change in size from day to day, as the histamine they release causes swelling around them. This is one of the main reasons vets sample lumps rather than judging them on appearance.

Soft Tissue Sarcomas

Soft tissue sarcomas account for roughly 15 per cent of skin and subcutaneous tumours in dogs. They tend to grow slowly over months or years, and are graded one to three. Lower grade tumours rarely spread but have a habit of regrowing locally if they are not removed with a wide enough margin.

Melanoma

Melanoma arises from the melanin-producing cells, so many of these tumours are darkly pigmented or black. Unlike in people, they are not linked to sun exposure. The mouth is a typical site, and oral melanoma in particular tends to be aggressive and to spread early.

Squamous Cell Carcinoma

These tumours of the skin cells often look like crusted plaques or patches of irritated skin that will not heal, which is why they are easily mistaken for a sore. They usually stay local rather than spreading, and most often affect lightly pigmented, thinly furred areas such as the nose, belly and toes.

Mammary Tumours

Unspayed females are most at risk of mammary tumours, and the risk climbs with age. They develop in the mammary chain beside the nipples and are typically firm and knobbly. Spaying early in life dramatically reduces the risk, which is one of the reasons it comes up in routine preventative healthcare discussions.

Bone Tumours (Osteosarcoma)

Bone tumours can occur in any dog but are seen mainly in large and giant breeds, including the Scottish Deerhound, Rottweiler and Great Dane. When a leg is affected, dogs become suddenly and severely lame, with pain and a firm swelling over the site. On an X-ray, the bone shows a characteristic ‘sunburst’ pattern. Osteosarcoma is painful, so pain relief is a priority from the moment it is suspected.

Lymphoma and Swollen Lymph Nodes

Lymphoma is a cancer of the lymphatic system and the white blood cells within it. The lymph nodes swell in predictable places: under the jaw, in front of the shoulders, in the armpits and behind the knees. They feel firm and rubbery, and are usually painless, which is why owners often find them by chance.

Common Benign Lumps in Dogs

This is the group most lumps fall into. Benign means the lump is not cancerous and cannot spread elsewhere in the body, and many need nothing more than an occasional check.

Lipomas

Lipomas are benign fatty lumps and are very common, particularly in older and overweight dogs. They grow slowly and feel soft and doughy under the skin, rather like firm jelly, and they usually slide about freely when you move them.

Sebaceous Cysts

These are small fluid-filled sacs under the skin, often around the size of a pea. They tend not to grow much and should not be uncomfortable. Most need no treatment and can simply be monitored, though they occasionally burst or become infected, at which point they need looking at.

Warts and Skin Tags

Dogs develop warty lumps and skin tags as they age, and some breeds, including the Poodle and the Boxer, are particularly prone. Skin tags are harmless, stay small and often hang off the skin on a narrow stalk. Warts are usually described as looking like tiny brains or cauliflowers.

Histiocytomas

These red, button-like lumps arise from Langerhans immune cells and typically appear suddenly. Most resolve on their own within two to three months. The classic patient is a young dog, aged roughly one to three, and Boxers, Boston Terriers and Bulldogs are over-represented.

How Vets Find Out What a Lump Is

A vet will form an opinion based on your dog’s age and breed, where the lump is and how it looks and feels. To know for certain, the lump needs to be sampled and examined under the microscope, and in some cases imaged as well.

Fine Needle Aspirate and Biopsy

A Fine Needle Aspirate (FNA) involves passing a fine needle into the lump and drawing off a small number of cells. It is quick, and can usually be done with your dog awake on the consulting table. The cells are examined under the microscope, though because the sample is small, an FNA does not always give a definitive answer.

A biopsy takes a larger piece of tissue, usually under sedation with local anaesthetic, or under a general anaesthetic. A few stitches may be needed to close the site afterwards. Because the sample includes the tissue architecture rather than loose cells, a biopsy is far more likely to give a specific diagnosis and a grade.

Imaging

Imaging is often recommended alongside sampling. It is most useful for checking whether a cancer has spread, such as taking chest X-rays after a diagnosis of mammary cancer, since the lungs are a common site for secondary tumours.

For large lumps, an ultrasound scan shows whether the swelling is solid or fluid-filled, and any fluid present can be sampled or drained at the same time.

What Happens If the Lump Is Cancer

If cancer is diagnosed, take a breath. It is a great deal to absorb in one conversation, and you do not have to decide anything on the spot. Your vet will work with you on a plan that suits your dog and your circumstances. That usually starts with surgical removal, and may also involve chemotherapy or radiotherapy. Newer options, including therapeutic vaccines and immunotherapy, are becoming more widely available.

Sometimes a lump is too large to remove, or sits somewhere that makes surgery impossible. For these patients, palliative care focuses on keeping them comfortable, and good pain control is central to that.

If a lump is badly infected or ulcerated, it may be that the patient’s quality of life is poor and that euthanasia may be the kindest thing you can do for them. There is no right moment and no decision that will feel easy, and you do not have to reach it alone. ongoing support is available for owners.

When a Lump Is an Emergency

It is rare for a lump itself to be an emergency. A lump that is bleeding heavily, or a dog that is in obvious pain, does need to be seen out of hours. While you arrange this, apply steady pressure over the area with a clean dressing from your pet first aid kit rather than letting it keep bleeding. Contact the emergency vet services straight away.

Get Emergency Veterinary Advice for Your Dog

If you need to talk to someone about your dog’s lump, whatever the time, contact us 24/7. Nothing is too small to ring about, and if all you need is someone to tell you whether this can wait until morning, that is a perfectly good reason to call. We can talk you through the next step and arrange an appointment either at your home or in a clinic. If you are in the capital, an emergency vet in London is available day and night.

Cancer Lumps on Dogs: FAQs

What does a cancerous lump look like on a dog?

There is no single look. Cancerous lumps vary in size, shape, colour and texture, and some are indistinguishable by eye from harmless ones. Appearance alone cannot tell you whether a lump is cancerous, which is why sampling matters.

Can a vet tell if a lump is cancer by feeling it?

Not with certainty. Feeling a lump tells your vet a lot about its texture, how freely it moves and how deep it sits, and an experienced vet will often have a strong suspicion. Confirming that suspicion still needs a sample, which is why they will usually suggest one even when they sound reassuring.

How fast do cancerous lumps grow on dogs?

Aggressive cancers can grow noticeably over a matter of weeks, while a low grade soft tissue sarcoma may take years. Growth rate varies with the type and grade, so a slow-growing lump is not automatically a safe one.

What should I be on the lookout for after a lump removal?

Most dogs recover quickly after a lump removal. Monitor the wound in the days afterwards for warning signs such as a gaping or opening wound, swelling, pus discharge, or a dog that seems dull and off their food.