Overview of Canine Skin Cancer
Skin cancer encompasses a broad category of tumors that includes any uncontrolled growth of cells of the skin or associated structures such as glands, hair follicles and supportive tissues (fat and connective tissue). Metastasis to the skin from tumors elsewhere in the body can occur but these are not considered skin tumors because they did not start in the skin.
The skin is the most common site of cancer in the dog and the second most common site of cancer in the cat. Dogs have up to six times the number of skin tumors as cats; however, skin tumors in dogs are more likely to be benign than those in cats. Skin cancer frequently occurs in dogs that are middle-aged to older (six to 14 years of age), although there are several types of skin cancers that occur in young animals.
Breeds of dogs that have been found to have a higher incidence of skin cancer include boxers, Scottish terriers, bull mastiffs, Basset hounds, Weimaraners, Kerry blue terriers and Norwegian elkhounds.
The cause of most skin cancers is unknown. Exposure to the sun has been shown to cause a higher incidence of two types of skin cancer: squamous cell carcinoma and hemangioma. Light-colored dogs with thin haircoats that spend time in the sun have a higher risk of developing either squamous cell carcinoma or hemangioma. There may be a genetic basis for the development of certain cancers.
Skin cancers can be divided into four categories, depending upon the cell type that is involved. These categories are epithelial tumors, mesenchymal tumors, round cell tumors and melanomas.
- Epithelial tumors. These tumors involve the skin itself, glands in the skin or the hair follicles.
- Mesenchymal tumors. These tumors are derived from cells that surround or support the skin such as fat, connective tissue, blood vessels and nerves.
- Round cell tumors. These tumors are named because of their appearance under the microscope and include lymphosarcoma, mast cell tumors, histiocytomas, plasma cell tumors and transmissable venereal tumors.
- melanomas. These tumors are derived from melanocytes – cells that provide pigment in the skin.
In dogs, the most common tumors are lipomas, mast cell tumors, sebaceous gland adenomas/hyperplasia and papillomas. Most skin cancers appear as a lump in or underneath the skin or as a sore that does not heal. Animals with skin cancer may experience discomfort or itchiness because of the cancer. This may cause them to chew or scratch at the affected area. With certain types of cancer, there may also be redness to the skin or it may have a flaky appearance.
Diagnosis of Skin Cancer in Dogs
Diagnostic tests are needed to recognize skin cancer and exclude other diseases in dogs. The ability to treat a skin cancer successfully depends upon the type of cancer and how advanced it is at the time of diagnosis. Tests that your veterinarian may wish to perform include:
- Cytology, which is microscopic examination of cells obtained from an aspirate of the tumor or a biopsy
- Biopsy, which is removal of portion of the tumor so that it can be evaluated cytologically or with histopathology, in which the tissue is fixed and then sectioned prior to examination under a microscope
- Complete blood count (CBC)
- Serum chemistry panel
- Chest radiographs (X-rays)to determine if the tumor has spread to the lungs
Treatment of Skin Cancer in Dogs
Treatment for skin cancer depends on the type of tumor that is present and how advanced the disease is at the time of diagnosis.
- Surgery may be performed if the tumor has not spread and the entire tumor can be removed without compromising function of the associated tissues. Occasionally it is used to reduce the size of a tumor so that other treatment can be more successful.
- Radiation therapy may be considered for some tumors, particularly when the entire tumor cannot be removed surgically.
- Chemotherapy may be considered for some types of tumors, particularly for those that have spread to other tissues.
- Cryosurgery is a procedure where the tumor and adjacent skin are frozen. It is generally considered for use only with small tumors.
- Photodynamic therapy is a new treatment modality that uses a dye injected into the blood stream that localizes in cancer cells. A laser of a particular wavelength is then used to excite the cells and cause cell death.
Home Care and Prevention
Examine your dog’s skin on a regular basis. If you note a new lump, a sore that does not appear to heal, or other changes in the skin seek veterinary attention as soon as possible.
After surgery monitor the incision for redness, swelling or discharge. Alert your veterinarian if any of these signs are noted.
Avoid prolonged exposure of your dog to sunlight. This is particularly important if your dog has light colored skin.
In-depth Information on Skin Cancer in Dogs
Other diseases may cause symptoms similar to skin cancer. These must be dismissed before a proper diagnosis for skin cancer can be obtained. They may include the following:
- Infections of the skin (bacterial or parasitic) can cause hair loss, itchiness of the skin, redness of the skin and flakiness. These signs are very similar to those found with certain types of skin cancers (lymphosarcoma). If a dog is not responding to treatment for either a bacterial or parasitic skin disease, a skin biopsy may be recommended.
- Infections of the nail can cause lameness, swelling in the toe and a discharge. Both melanoma and squamous cell carcinoma can occur in the toe, usually at the nail bed and can look like an infection.
- An abscess, which is caused by a bacterial infection, can cause a swelling in the skin. This can look like any number of different types of skin tumors.
Immune Mediated Diseases
Several immune mediated diseases like systemic lupus and pemphigus can cause ulcers and sores in the skin that may look cancerous. The most common areas affected by immune mediated diseases include the tips of the ears, lips, nose and footpads.
Several different types of cysts can be found in the skin and occur as a swelling that opens and drains intermittently. The fluid that comes from a cyst can be anywhere from a clear fluid to a thick “toothpaste” type material.
An injury to the skin can look like certain skin cancers. You should seek veterinary attention if a suspected injury does not heal within a reasonable amount of time.
In-depth Information on the Diagnosis of Canine Skin Cancer
Veterinary care should include diagnostic tests and subsequent treatment recommendations. Diagnostic tests are needed to recognize skin cancer and exclude other diseases. Tests may include:
If there is a discrete mass, your veterinarian may recommend an aspirate of the mass. A needle is placed into the mass and suction is applied to collect some of the cells into the needle. These cells are then spread onto a slide an examined under the microscope.
Cytology is a quick, relatively non-invasive way to evaluate many skin masses. However, it should be noted that certain cancers are difficult to diagnose by cytology and a biopsy may be required. An aspirate can only sample a few cells so it may not be representative of the entire mass.
A biopsy is the most accurate way to diagnose a skin tumor in dogs. There are multiple ways to biopsy a suspected skin cancer. The type of biopsy will depend upon the location and size of the tumor as well as the overall health of your dog. It is essential that any skin mass that is removed be submitted for biopsy. A diagnosis of a benign tumor cannot be made without a biopsy. It is also important that all tissue that is removed be submitted, even if the tumor is large.
- A punch biopsy may be used when the lesions are near the surface of the skin. This type uses the same principle as a cookie cutter, and a small circle of skin is removed with an instrument called a punch. Multiple samples can be taken from different areas in order to increase the chance of making a diagnosis. This can be done with local anesthesia and/or sedation.
- A needle biopsy can be used to take a core of tissue from a discrete mass – one that is in separate parts. This can be done with local anesthesia and/or sedation.
- An incisional biopsy is one where only a part of the tumor is removed for evaluation. An incisional biopsy is performed when the tumor cannot be easily removed, when a malignant tumor is suspected or when there is already evidence of metastasis. It requires general anesthesia.
- An excisional biopsy means that the entire tumor is excised or removed along with some margin of normal tissue. It requires general anesthesia.
Complete Blood Count (CBC)
A complete blood count is drawn to evaluate your dog’s red blood cells, white blood cells and platelets.
Serum Chemistry Profile
A serum chemistry profile is drawn to evaluate your dog’s overall health. If your animal is to undergo anesthesia or receive radiation therapy or chemotherapy, it is important to know if there are underlying health problems that may complicate treatment.
A urinalysis is another useful test to help evaluate your dog’s overall health.
Chest radiographs (X-rays)
A common location for metastasis or spread of a malignant cancer is the lungs. If a malignant skin cancer is diagnosed, your veterinarian will recommend chest X-rays to look for spread of the tumor. The presence of metastasis to the lungs will change the recommended treatment plan as well as make successful treatment less likely.
In some cases, it is difficult for the pathologist to determine the tumor type using only the stains that are routinely applied to the biopsy sample. Special stains may be required to identify the type of tumor. Your veterinarian and the pathologist will decide what special stains are needed based on the tumor types they are suspecting. These tests take extra time to complete and there is generally an added fee. However, they are important because the results may change both type of treatment and the prognosis.
A buffy coat is a blood test that looks for mast cells out in the bloodstream. Normal animals rarely have mast cells out in circulation. This test is requested only when a mast cell tumor is diagnosed.
Lymph Node Evaluation
Lymph nodes are another common site for metastasis. When a malignant skin tumor is diagnosed, the lymph node that is closest to that tumor is examined for spread of the tumor. This can be done either by an aspirate or biopsy of the lymph node.
In-depth Information on the Treatment of Canine Skin Cancer
Treatment for skin cancer may include one or more of the following:
Surgery is the most commonly employed treatment of skin tumors. For benign skin tumors, surgery can be curative. For some malignant skin tumors, surgery can also be curative, although the chance of recurrence at the same site is higher because malignant tumors tend to be more invasive. Malignant tumors can also spread and surgery will have no effect on the potential for metastasis.
When removing a potentially malignant tumor, a wide margin of normal tissue needs to be taken as well as the tumor. Ideally, this margin should be 1 1/2 to 2 inches of normal tissue. In some locations, such as the leg or face, there may not be enough normal tissue that can be removed to ensure a wide excision. It should be expected that after a surgery for a malignant tumor that the incision is usually much larger than the original tumor.
Your veterinarian may biopsy the tumor before attempting to remove it. This is desirable in certain situations because it allows your veterinarian to determine how aggressive the surgery should be or whether surgery is the best option.
Radiation therapy is used in the treatment of several different types of skin cancer. The most common tumor types that are treated with radiation therapy include: mast cell tumors, malignant mesenchymal tumors (a family of tumors called soft tissue sarcomas) and squamous cell carcinoma. Radiation therapy is most effective when surgery can be performed to reduce the amount of tumors to microscopic levels. This means that the tumor is no longer visible.
Radiation therapy usually involves multiple treatments given over several weeks. Each treatment requires a brief period of anesthesia since animals cannot move during their treatment. The most common side effects associated with radiation therapy are changes in the skin, such as redness of the skin, hair loss, oozing of the skin, which heal after the completion of radiation therapy.
If your veterinarian feels that radiation therapy may be helpful to your pet, he will refer you to a facility that has radiation therapy available.
The use of chemotherapy in the treatment of skin tumors is limited to certain situations. Chemotherapy may be recommended if the tumor has already metastasized, if there is a high potential for metastasis or if other treatments are not possible. The most common tumor types for which chemotherapy is recommended are lymphosarcoma and mast cell tumors.
The type of drugs used, the frequency at which they are administered and the length of treatment will depend upon several different factors, including: the type of tumor, whether the tumor was removed and if there are any metastases already present.
Chemotherapy is well tolerated in both cats and dogs and the risk of serious side effects is low. The most common side effects include gastrointestinal symptoms, such as vomiting, diarrhea, and loss of appetite, or a drop in the white blood cell count.
Cryosurgery involves the use of liquid nitrogen to freeze the tumor. Tumor cells that are frozen will die so cryosurgery can be used to treat tumors. This type of therapy is not frequently used. It should be noted that recurrence of many tumors is likely because of the inability to kill the cells at the edges without having serious effects of healthy tissue.
This new therapy can be used to treat certain types of skin cancer. It is used most frequently for the treatment of squamous cell carcinoma on the ears and nose of cats. It involves giving the patient an intravenous drug and then exposing the tumor to laser light. The drug sensitizes the tumor to the laser light and thus the tumor is killed. This type of therapy can be effective, but it requires special equipment and can leave animals sensitive to light for a period of time after treatment.
Follow-up Care for Dogs with Skin Cancer
Examine your dog’s skin on a regular basis. If you note a new lump, a sore that does heal or other changes in the skin, seek veterinary attention.
If surgery was part of the treatment of his skin cancer, your dog may need some additional care. Animals that have had surgery should be kept quiet for the first two weeks to prevent tension on the incision. The incision should be monitored for redness, swelling or discharge. It is important to keep your dog from licking or chewing at the incision because this can cause the incision to come apart or cause an incisional infection. If there are sutures or staples, these will need to be removed approximately two weeks after surgery.
The surgery site should be evaluated on a frequent basis to detect any cancer recurrence. If recurrence is suspected, it should be brought to your veterinarian’s attention immediately. Retreatment is more likely to be successful if the tumor is still small.
There is no known cause for the majority of skin cancers so prevention is difficult. Exposure to sunlight should be limited especially for white or light colored dogs.