Skin lumps are very common in dogs. There are a whole range of possible scenarios that can present as a lump on the skin, including, insect bites and stings, abscesses, hematomas, cysts, soft tissue swelling following trauma and benign or malignant tumors.

Tumors are probably the most common of these, but two thirds of them are benign and therefore merely cosmetic. Here we look at the most common types of tumor affecting the skin and subdermis in dogs, and what, if any, treatment they require.

When presented with a skin tumor, a veterinarian has 2 issues to deal with:

1. Is it a type of tumor that requires surgical removal, and

2. Is further treatment required once it has been removed. Some skin tumors are so obviously benign that a vet will not even bother to biopsy it, let alone recommend removal. These are the small wart like tumors that are often attached by a narrow strip of skin.

All skin tumors that do not fit this category though should be subjected to a fine needle aspiration (inserting a needle into the lump, sucking out some cells and characterising them under a microscope) or a surgical biopsy. This will determine whether or not complete removal is necessary, and define the prognosis. A thorough clinical examination is important to assess both the tumor and the patient as a whole.

A veterinarian will study the tumor with respect to its size, position and appearance, and examine the local and regional lymph nodes to look for any evidence of metastasis.

Difference between benign and malignant tumors

Benign tumors are generally slow growing and may change little over the course of a year. They usually have a clear margin and cause the dog little trouble, but they can occasionally cause discomfort if they become ulcerated or start to bleed following self trauma.

Malignant tumors often grow rapidly over a few weeks. They tend to become ulcerated early in their course and they can infiltrate surrounding tissues, therefore seeming fixed and less moveable than benign tumors.

Benign Skin Tumors

Nevus (moles) and Skin Tags

These are not tumors, they are harmless, well marginated stable skin growths that are present at birth, but can grow in old age.

Warts and Papillomas

These are benign tumors that may grow spontaneously or may be triggered by a virus.

Sebaceous Cysts

These are common in dogs (around 7% of all skin tumors). They are not tumors, but accumulations of sebum within the gland due to a blocked duct, which can often be resolved by squeezing it.

Sebaceous Hyperplasia

This is proliferation of the sebaceous tissue mentioned above to form cauliflower like warts that may become traumatised, ulcerate and bleed. They are very common in old Cocker Spaniels and Poodles. Surgical excision or cryosurgery with liquid nitrogen is curative.

Basal Cell Tumor

More common in cats than dogs. Usually presents as a domed, well marginated hairless pink mass in the skin. They tend to be moveable over underlying structures and are slow growing. Surgical excision is curative.

Histiocytoma

Usually seen in juvenile dogs around 18months old, though can occur at any age. They grow rapidly over about 6 weeks to a maximum size of around 2cm, and are domed hairless lumps within the skin that can ulcerate. The key step for the veterinarian is to differentiate histiocytomas from mast cell tumors (see later), which can be easily done via a fine needle aspirate. Surgical excision or cryotherapy is curative for histiocytomas, and many will disappear of their own accord without treatment.

Benign Subcutaneous Tumors

Lipoma

The lipoma is the most common subcutaneous tumor in dogs. It is a well defined, well circumscribed mass with a soft rubbery texture to it. They are slow growing and a very common occurrence in older overweight dogs. They can occur both on the trunk or on the limbs, and though they are completely benign, occasionally they can become so large that they are a nuisance to the dog and only then is surgical excision necessary. Before assuming a lump is a lipoma, it is good practice to have a veterinarian perform a fine needle aspirate on the lump as one or two other tumors can mimic a lipoma, such as certain presentations of mast cell tumors (see below) and hemangiopericytomas.

Malignant Skin Tumors

Squamous Cell Carcinoma

This is a common malignant tumor in both cats and dogs. It usually occurs on relatively hairless non pigmented skin and can be triggered by long or repeated exposure to UV light. Surgical excision is usually curative if good margins are achieved, and those cases with incomplete margins benefit from post operative radiotherapy to kill the remaining cancer cells. Some squamous cell carcinomas are very difficult to remove though, such as a nasty infiltrative form which occurs on the nasal planum. Chemotherapy has been used for this tumor type with mixed results.

Melanomas

Relatively common in dogs. The majority are benign and surgical excision is curative. However, those that arise on the nail bed, scrotum or mucocutaneous junction (e.g. on the lips) are usually malignant. Surgical excision is mandatory.

Mast Cell Tumors

These are amongst the most common of skin tumors in dogs. They have a wide range of appearance and behavior, making them sometimes challenging to diagnose and treat. About 10 per cent of dogs suffer them at multiple sites. Mast cells occur naturally in the skin, performing an important function in releasing histamine and heparin in response to various allergic stimuli. Like any cell line, mutations can occur where the cells start to divide uncontrollably, forming a tumor. If a mast cell tumor is squeezed it will therefore release histamine and cause a wheal reaction on the skin. Histamine can also cause vomiting so affected dogs may present with upset stomachs.

If a mast cell tumor is confirmed on a fine needle aspirate, it should be surgically excised as soon as conveniently possible. In cases of well marginated tumors that are caught fairly early, this is usually curative. Margins of 2 to 3cm are advised. However, higher grade tumors tend to be aggressively invasive and complete excision is not always feasible. In these cases, partial excision together with radiotherapy or chemotherapy is the treatment of choice. Dogs that have had mast cell tumors tend to be predisposed to getting them again in the future, even if they are surgically excised promptly. Therefore, owners of such dogs should meticulously examine their dogs skin at periodic intervals to search for any suspicious lumps.