Skin Tumours in Dogs and Cats

Skin tumours are some of the most common tumours seen in the dog and cat, especially in Australia and New Zealand where there is a high incidence of solar-induced skin cancer (squamous cell carcinoma).

There are many different types of skin tumours but the tricky part is that different skin tumours can look alike and the same tumour may not always look the same. It is also not possible to tell if a tumour is benign or malignant by look or feel alone. Therefore, whenever you notice a lump or bump, or even an itchy or bleeding sore on your pet that isn't healing, it is best to have it looked at by your vet.

Cause

There is no consistent cause for skin tumours. There are solar radiation induced tumours, such as squamous cell carcinoma and haemangiosarcoma. Some have genetic links, such as sebaceous adenomata in poodles, while others have infectious causes, for example with viral papillomatosis.

Diagnosis

The majority of skin tumours in dogs are benign whereas in cats they are mostly malignant. Benign skin tumours are usually slow growing, well defined and mobile over underlying structures. Malignant tumours tend to grow faster, infiltrate into adjacent tissue and may become red, inflamed and ulcerated.

However, the physical appearance of the tumour is not always a guide to how it will behave. Leaving it and waiting to see what happens isn't usually a good option - they aren't called growths for nothing and may become too large to remove or spread and become life-threatneing - and simply removing the lump may not be warranted, for example if it is a secondary tumour. The extent of the surgery required is also dependant on the type of tumour. A malignant tumour needs a much wider margin of excision than a benign tumour to prevent recurrence. Surgery may need to be repeated to remove more tissue if the pathology report reveals it is a malignant tumour.

A diagnosis is essential before a tumour can be treated successfully. After palpating the tumour and surrounding lymph nodes, along with a thorough physical examination to check for other lumps or unusual signs, your vet may wish to take a sample of the tumour. The simplest method is by fine needle aspirate but for more information a core needle biposy or dermal punch may be recommended. The latter methods may require heavy sedation plus a local or general anaesthetic, making them more expensive but offer the best sample for a definitve diagnosis.

Treatment

Complete surgical excision is the primary treatment of choice for most tumours. The pathology report gives the information required to determine the extent of surgery required plus the choice of follow-up therapies. For malignant tumours a wide surgical excision is required and chemotherapy may also be recommended. Radiation may be useful to treat local disease that is non-resectable or where there was incomplete resection of the tumour.

Mast Cell Tumours

The most important skin tumour in dogs is the mast cell tumour - it is the most common malignant skin tumour in the dog. They originate from mast cells, which contain a large array of bioactive molecules that can have many different effects on the body. For example, ulcers in the stomach and intestines, enlarged spleen and liver, and changes in the blood cells.

Mast cell tumours may develop anywhere on the body surface as well as in internal organs, but the limbs, ventral abdomen, and thorax are the most common sites. They can be single or multiple. The difficulty with mast cell tumours is that they can look and feel like anything. This can include soft, subcutaneous masses that can feel exactly like lipomas.

A fine needle aspirate is sufficent for diagnosis of mast cell tumours. While the majority of mast cell tumours do not metastasize, they are very aggressive locally and wide surgical excision is indicated. Additional treatment, such as radiation and chemotherapy, depends on the grade of tumour identified by pathology. Regular re-checks for recurrence, metastasis, or new cutaneous masses are recommended.

Cutaneous Squamous Cell Carcinoma

In the dog the most common form of this disease is solar-induced, usually on the nose or ventral abdomen. It occurs most commonly in dogs with less pigmentation and sparser hair in these areas, especially those with a history of sunbathing. These tumours are malignant but metastasis is uncommon. Surgical resection is often difficult due to the wide extent of local disease so prevention is the best treatment - keep susceptible dogs indoors during the middle of the day or provide a sunsuit. Tattooing has been found to be ineffective as it does not offer protection to the deeper layers of the skin where the UV rays penetrate.

Another form of the disease in dogs is seen under the toenails (subungual). These tumours may be mistaken for nail-bed infections in the early stages. Subungual disease has a higher risk of malignancy and chemotherapy may be useful in conjunction with surgical resection in these cases.

In cats, solar-induced squamous cell carcinoma is the most common form of the disease, accounting for around 50% of skin tumours seen in cats in Australia and New Zealand. It is seen most often on the face involving the nose, ears and eyelids. Again, the metastatic potential for these tumours is low, but surgical resection is often difficult. The highest success rates are seen with surgery and radiation. Cryotherapy has been used with variable success.

Soft Tissue Sarcoma

There are a variety of forms of soft tissue sarcomas. While many have a low metastatic potential, they can be highly invasive locally. In the dog these tumours can arise anywhere on or in the body. The location can determine the amount of problem that they cause due to the difficulty in achieving complete surgical resection. Palpation can be misleading and in some cases the tumour can extend up to 5cm from the edge of where the mass can be felt. These tumours can also feel similar to other skin tumours, including the lipoma.

Surgery is the best form of treatment. Referral to a specialist surgeon may be required. Radiation can be useful as an adjunct therapy for local control, while chemotherapy may be useful for high grade tumours with a high metastatic potential.

Dermal Haemangiosarcoma

This is seen most often in dogs but can also be seen in cats. The solar-induced form of the disease has been seen in both species. Multiple lesions may be present but they are usually small, confined to the dermis and are mostly benign. Surgery is the treatment of choice.

Non-solar induced disease in dogs may be dermal (relatively benign), subcutaneous (moderately malignant) or involving the muscle layers (highly malignant). Treatment varies accordingly and is also dependent on whether the tumour is a primary skin tumour or a metastatic lesion from an internal primary tumour.

Melanoma

Melanomas are relatively common in dogs, however, they are not associated with solar damage as with humans. The can be benign or malignant and can occur anywhere on the body. Tumours that arise on hairless areas, such as the lips or nail beds are more likely to be malignant. Breed is also important with most tumours in Doberman Pinschers and miniature Schnauzers being benign, while the majority in miniature poodles are malignant.

The treatment of choice is surgical resection. With malignant melanomas, chemotherapy is often used as an adjunct therapy.

Histiocytic Tumours

The most common tumour in the dog is the histiocytoma or button tumour. This is a benign tumour that can spontaneously resolve. Occasionally multiple lesions can be seen. The main concern is histiocytic sarcoma, which can be an internal disease with lesions in the skin and subcutaneous tissues in many cases. The Bernese Mountain Dog is most at risk with the disease also seen in Labradors, retrievers and Rottweilers.

Secondary Skin Tumours

There are some tumours that have some predilection to metastasis to the skin, inclduing renal tumours, salivary tumours and lung tumours.

- Last updated 16 November 2012

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