Lipoma in dogs: diagnosing and treating fatty lumps

Overview

A lipoma consists of fat cells, or adipocytes. The usual site is the subcutaneous fat layer, where fat is normally present anyway. Lipomas develop particularly on the trunk and near the upper parts of the limbs. A middle-aged or older dog may have several separate lipomas.

In British primary veterinary care, about 1.94% of dogs were diagnosed with a lipoma during one year. The figure comes from 2013 records at 215 practices. It describes diagnoses recorded that year, rather than each dog's lifetime risk.

An ordinary lipoma differs from an infiltrative lipoma that grows into surrounding muscle or connective tissue. A malignant fatty tissue tumour is called a liposarcoma. Other tumour types can also resemble a soft fatty lump. Benignity is therefore a diagnosis that a lump's familiar appearance alone cannot confirm.

Fatty tumours can also form inside the abdominal cavity, although a subcutaneous location is most common. A visible swelling at the skin surface therefore does not represent every tumour arising from fat tissue.

Causes

A lipoma is a local tumour of fat cells, rather than the same thing as general weight gain. One known factor does not usually explain tumour development in an individual animal. Tumours can involve both inherited and environmental risk factors. Differences in lipoma prevalence between breeds support unequal susceptibility among dogs.

An association with high bodyweight has been observed, but weight does not directly show a dog's body fat. A high weight can reflect muscle mass, frame size or excess fat. A causal link between obesity and lipoma could therefore not be confirmed without body condition scores. An existing lipoma does not usually shrink on its own.

Risk factors

Lipomas are common in middle-aged and older dogs. In Britain, dogs with lipoma had a median age of 10.02 years, compared with 4.18 years in other dogs. The median is the middle value in a distribution, rather than a minimum age for developing disease. Prevalence increased with age.

In the British review, lipomas were diagnosed in 7.84% of Weimaraners. The proportion in Dobermanns was 6.96%. The corresponding figure for Labrador Retrievers was 5.15%. Miniature Schnauzers and Cocker Spaniels also had a higher diagnosis risk than crossbreeds. These figures concern dogs attending veterinary practices in one country and cannot be directly applied to breed populations elsewhere.

Spayed females and castrated males had a higher diagnosis risk than entire dogs of the same sex. This association may partly relate to weight and fat distribution. The finding does not prove that neutering definitely caused an individual dog's lipoma.

Typical symptoms

A lipoma may be an entirely symptom-free finding while stroking or brushing your dog. Growth may be slow, but different lumps behave differently. Some remain the same size for years, while others become very large. Common features include:

• A soft, rounded or lobulated mass felt beneath the skin.
• A lump that often moves under the fingers relative to surrounding tissue.
• Several lumps at different sites in the same dog.
• An awkwardly located lump altering gait or causing lameness.
• Avoiding stairs, getting up or usual walks.

Sites that can interfere with movement include the armpit, between the legs and the neck. A growth extending into muscle may feel firmer and less mobile than usual.

Complications and consequences

An ordinary lipoma does not spread to internal organs. However, even a benign lump can grow large enough to affect movement or cause discomfort. Recurrence after removal of an ordinary lipoma is fairly unlikely. A new lipoma can still appear elsewhere later, as multiple lumps are common in older dogs.

An infiltrative lipoma can be difficult to remove completely from between muscles. It can recur locally and require further operations. Of lipoma samples at a Canadian diagnostic laboratory, 8–9% were classified as infiltrative. That figure concerned dog and cat samples submitted to the laboratory, rather than every fatty lump in dogs.

After lipoma removal, blood leaking beneath the skin may appear as bruising around the wound. Marked swelling, ongoing bleeding and foul-smelling discharge, however, are reasons to contact the vet promptly. Surgical wound changes are therefore monitored differently from a fatty lump investigated before surgery.

Diagnosis at the vet

The vet assesses the lump's size, shape, location, texture and any signs of inflammation. Appearance is insufficient to distinguish a fatty tumour from other skin changes. A mast cell tumour, for example, can mimic a lipoma. An abscess or cyst can also cause a skin lump. Every new lump is investigated as a separate finding, even if an earlier one was benign.

Fine-needle sampling collects cells from the lump onto a slide using a thin needle. A pathologist assesses their appearance under a microscope. This cell examination is called cytology. Fine-needle sampling is usually fairly quick and straightforward. An inconclusive result may require a tissue sample, or biopsy.

A biopsy examines a small piece of tissue rather than just separate cells. Microscopic tissue examination, called histopathology, helps reach the final diagnosis. In an infiltrative lipoma, cells may look quite bland despite the tumour invading its surroundings. Information about growth pattern and previous recurrence is therefore also needed for interpretation.

CT may be used particularly to investigate an infiltrative lipoma or suspected liposarcoma. Images can show fatty tissue extending between muscle layers. Finding an ordinary fat cell in a sample therefore does not describe every feature of a deep-growing lesion.

Prolonged observation without any initial sample can mean that possible surgery is more extensive later. Particularly on a limb or face, there may be little healthy skin available for wide removal. A diagnosed lipoma is therefore distinguished from an unidentified subcutaneous mass when planning treatment.

Biopsy collection can be performed under sedation or general anaesthesia.

A cell sample is stained to make cells easier to distinguish under the microscope.

Treatment

The vet prescribes treatment and any medicines. A confirmed, symptom-free lipoma can often be monitored without removal. Surgery is considered if the lump affects movement, is awkwardly located or causes other problems. Surgical removal is the most effective treatment for a lipoma. Surgery can become harder as the tumour grows larger.

Removing a small lump is usually less extensive than removing a large one. The wound may also be smaller and less painful. Not every lipoma is clearly distinguishable from surrounding normal fat. Planned weight loss before surgery in an overweight dog may make tumour boundaries easier to identify.

Removal of an infiltrative lipoma may also require removal of surrounding tissue. A difficult tumour may need a vet with surgical expertise. Radiotherapy after surgery can be considered for a recurring infiltrative lipoma. Treatment of malignant liposarcoma is more extensive than that of an ordinary lipoma.

Protect wound healing by avoiding running, jumping and other vigorous activity. Exercise is restricted for about 7–10 days after many operations, but your dog's instructions depend on the procedure. Removable skin sutures are often taken out after 7–14 days. Some sutures beneath the skin dissolve and do not need removal.

Early in recovery, sedation or general anaesthesia may make your dog sleepier than usual. Behaviour should move towards normal over the next 24–48 hours. Nausea after surgery may ease if food is divided into smaller portions.

There are no known home remedies that remove a lipoma.

Prevention

Lipomas are difficult to prevent because their exact cause is unknown. Monitoring weight and body condition is sensible, but weight loss has not been proven to prevent lipomas. Regularly feeling the skin helps detect changes early. Check the whole dog, as a new lump may differ in type from previous lipomas.

Looking for lipomas fits into routine veterinary examination, particularly in susceptible breeds. Monitoring the size of an identified lump helps detect a change in behaviour. The purpose is to identify changing treatment needs, because even a benign lipoma can grow enough to affect movement.

Areas to check include the ears, around the eyes, mouth, legs and tail, and skin around the bottom. Gently feel the whole body in the direction of hair growth. Male dogs also normally have nipples on the abdomen, which should not be mistaken for new skin tumours.

Important for dog owners

Draw your dog's outline on paper and mark each lump's position. Record the examination date and which lump was sampled. Measure a monitored growth consistently and take a comparison photograph. Note changes in shape, firmness, pain and discharge. Show a new lump to the vet rather than naming it yourself after a previous fatty tumour.

After surgery, prevent wound licking with the supplied collar or protective clothing. Keep the wound dry: swimming and bathing can expose it to bacteria. Wound edges should stay together and the surface remain clean. Ongoing discharge or a bad smell requires contact with the veterinary team. Keep the pathology report too, so a lump growing at the same site later can be compared with the previous diagnosis.

Frequently asked questions

Does every lipoma need surgery?

A confirmed, symptom-free lipoma can often be monitored. Removal becomes relevant, for example, when growth starts affecting walking or the location causes discomfort. The decision depends on effects on your dog's daily life, the location and the confirmed tumour type.

Can a soft lump be something other than a lipoma?

Yes. A mast cell tumour, for example, can resemble a lipoma, so softness and mobility do not confirm the diagnosis. The vet can take a fine-needle sample and a biopsy if needed. A new lump also needs investigation even if your dog already has fatty tumours.

Is a lipoma cancer?

An ordinary lipoma is a benign fatty tissue tumour and does not spread to internal organs. Liposarcoma is a separate malignant fatty tissue tumour requiring more extensive treatment. A lump's diagnosis therefore needs veterinary investigation before moving to monitoring at home alone.

Why does a removed fatty tumour grow back?

Recurrence after removal of an ordinary lipoma is fairly uncommon. An infiltrative lipoma growing between muscles can be difficult to remove completely and may require further surgery. A growth at the same site is reassessed alongside previous surgical and sampling records.

What do several fatty lumps in the same dog mean?

Middle-aged and older dogs can have several separate lipomas. However, the number of lumps alone does not identify their type. Every new change must be assessed because it may differ from previous ones. A location map helps identify the agreed lump during monitoring.