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Malignant Melanomas in Cats


Melanomas or tumors arising from pigment producing cells can be benign or malignant. In cats, melanomas are found most often on the head (especially ears and eyes), neck and lower legs. Luckily, malignant melanomas are relatively rare in cats1. For instance, melanomas, in general, comprise less than 3% of skin tumors with approximately 42% to 68% of those being malignant2. And malignant melanomas account for less than 1% of oral tumors in cats, according to Tufts University.

Why worry about malignant melanomas then?
Unfortunately, in their malignant form, melanomas tend to be very destructive locally, to re-grow even after surgical removal and to metastasize or spread to other locations in the body. In cats melanomas involving the eyes are more common than those in the mouth or in other locations on the skin; and both ocular and oral melanomas tend to be more malignant than skin melanomas3.

This is where things can be especially tricky in cats because many cats develop benign pigmented changes in these same locations. These changes look a lot like freckles. You might know cats (especially orange, calico or sometimes silver cats) that develop dark spots on their lips, gums, eyelids and/or nose. This condition, called lentigo, can start showing up on cats as young as a year of age, but is completely benign and does not turn into cancer4.

This is not necessarily the case when pigment changes occur on a cat’s iris. In that location pigment changes can progress from the occasional freckle to a more generalized and enlarging, but still benign, melanosis that may eventually undergo a malignant transformation5.

So you should always be concerned with any new growths (pigmented or not since melanomas are not always brown) or color changes that occur on your cat. You may also discover tumors because of clinical symptoms associated with their location. For instance, tumors in the mouth may result in bleeding, drooling, bad breath, or difficultly eating/swallowing, while intraocular tumors can cause abnormal pupil size, pain, and ultimately vision problems.

How do you know for sure your cat has malignant melanoma?
As always, anything that concerns you should be checked by your veterinarian. In the case of melanomas of the iris, careful examination and/or ultrasound can aid in distinguishing between benign pigment changes and actual tumors. But to know for sure, your veterinarian will have to send a tissue sample to the laboratory in order to get a definitive diagnosis either by a needle aspirate or surgical biopsy. If the diagnosis is malignant melanoma your veterinarian will want to run other tests (blood works, XRays, ultrasounds, aspirates) to establish your cat’s general overall health (to make sure she doesn’t have any other illnesses) and to stage the cancer (establish as best as possible if it exists in any other locations). This information is important in order to give you a clearer picture of your cat’s individual condition and prognosis.

What can you do if your cat has malignant melanoma?
Sadly, malignant melanomas are very aggressive cancers. The prognosis is guarded, and the odds of long term survival are not good. Complete surgical removal of the original tumor is always the best first step, provided that the size and location of the tumor allow for it. Unfortunately that is not always the case. Radiation therapy may be indicated in some instances for non-surgical tumor control or even after surgery where excision of the cancer was not complete. Some form of chemotherapy may also be an option.

Once your veterinarian has gathered all available information, he can then counsel you on the best approach for treating your cat. Just remember that treating cancer is not something you do expecting to cure the condition, but rather to control it and to slow down the progression. That is why it is particularly important that you make sure that you fully understand what your veterinarian is telling you regarding your cat’s prognosis, the treatment plan being proposed, and what reasonable expectations you should have for achieving happy, quality time with and for your cat before moving forward.

If you have any questions or concerns, you should always visit or call your veterinarian -- they are your best resource to ensure the health and well-being of your pets.

Resources:

  1. "Tumors of the Skin in Cats." The Merck Manuals. Web.
  2. Gross, Thelma Lee, Peter J. Ihrke, Emily J. Walder, and Verena K. Affolter. Skin Diseases of the Dog and Cat: Clinical and Histopathologic Diagnosis. Oxford: Blackwell Science, 2005. Print.
  3. "Feline Melanoma: A Comparative Study of Ocular, Oral and Dermal Neoplasms." U.S. National Library of Medicine. Web.
  4. "Congenital and Inherited Skin Disorders of Cats." The Merck Manuals. Web.
  5. Ionascu, Iuliana, Georgeta Dinescu, and Cucos C. Anca. "Iris Melanoma in Cats." Veterinary Medicine Journal. N.p., n.d. Web.

Reviewed on:

Tuesday, February 3, 2015


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Oral tumors in dogs and cats are among the most commonly occurring cancers encountered in our patients. The most common location for oral melanoma is the gum (gingiva) or the buccal mucosa (inside of the cheek), but other locations have been reported such as the inner lining of the lips, palate and tongue. Oral melanomas are locally aggressive and also have a high likelihood (80%) of metastasis (spread) to other organs such as the regional lymph nodes and lungs. Although most oral melanomas are very aggressive, there may be a small subset of these tumors, possibly including lingual (tongue) melanomas that may be less metastatic.

Symptoms of Oral Melanoma

Most cats and dogs with oral cancer have a mass in the mouth noticed by the owner. Pets with oral tumors will typically have symptoms of increased salivation (drooling), facial swelling, bleeding from the mouth, weight loss, foul breath, oral discharge, difficulty swallowing, or pain when opening the mouth. Loose teeth could be indicative of bone destruction due to the tumor.

A thorough diagnostic evaluation of oral tumors is critical due to the variety of different tumors that could be present. Sedation or anesthesia is often required in order to examine the pet’s mouth, especially if the suspected tumors are located in the back of the mouth or involve the tongue. If the tumor is suspected to be malignant, chest X-rays can be done prior to biopsy to check for metastasis (spread) to the lungs. Bone destruction is not typically seen on X-rays of the jaw until greater than 40% of the bone is destroyed so what appears to be a normal X-ray cannot exclude the tumor’s bone invasion. Advanced imaging such as CT (computed tomography) or MRI (magnetic resonance imaging) can be valuable tools in staging the disease (determining how advanced it is), especially for evaluating bone invasion and the potential extension of the tumor into the nasal cavity, pharynx or the eye. The use of CT may eliminate the need for regular X-rays but CT/MRI imaging is more expensive.

Regional lymph nodes should be carefully assessed for any abnormalities, although lymph node size is not an accurate predictor of metastasis. In a study of 100 dogs, 40% showed normal sized lymph nodes despite being positive for cancer cells, and 49% of dogs who showed lymph node enlargement did not actually have lymph node metastasis. Lymph node aspirates (isolation of cells for microscopic analysis to check for the presence of any cancer cells) are recommended for pets with oral cancers.

The final diagnostic step, which is done under anesthesia, is incisional biopsy. Biopsy is preferred over cytology to definitively differentiate between benign (noncancerous) and malignant (cancerous) tumors and determine the exact type of the tumor present.

Treatment Options

Surgical removal of oral melanoma is always the primary method of treatment whenever possible. Since many oral melanomas invade the bone, the surgery will aim to remove not only the tumor itself but also any associated boney structures. For oral melanomas, an aggressive surgery is usually recommended given the aggressive nature of these tumors. Cosmetic appearance is generally good after most upper or lower jaw surgeries but can be challenging when both sides have to be surgically treated. While surgery can reduce the risk of local recurrence (tumor coming back), unfortunately, most dogs will eventually succumb to the tumor’s spread to other organs without further treatment. In a published study of 37 dogs whose oral melanomas in the lower jaw were removed by partial mandibulectomies (surgery where part of the lower jaw was removed), median survival time was 9.9 months (range 1-36 months), and 21% of dogs were alive after one year. Twenty of the 37 dogs were euthanized due to recurrent or metastatic disease.

Types of Surgical Intervention

Mandibulectomy (lower jaw) Maxillectomy (upper jaw)

Classified according to the portion(s) excised

A. Unilateral rostral A/B. Unilateral premaxillectomy

B. Bilateral rostral C. Bilateral premaxillectomy


Melanoma In Dogs And Cats

Melanoma is a moderately common, locally invasive and frequently malignant cancer in domestic dogs and cats. We see it less commonly in cats than dogs, but it still has a significant prevalence in the feline species. There are several forms of melanoma, which typically are classified by their location. These include tumors that tend to localize in the skin (cutaneous melanoma), eyes (ocular melanoma), nail bed (subungual melanoma) and oral cavity (oral melanoma).

The causes of melanoma in dogs and cats are not fully known, but the skin form seems to be over-represented in white colored dogs and cats, so like people, less protective pigment of the skin and hair seems to increase cellular mutations from UV light, and lead to melanoma more commonly. Where melanoma differs significantly from melanoma in people from dogs and cats, however, is that black dogs and cats also seem over-represented, which is contrary to one one would expect. The fact that some domestic dog and cat breeds are more commonly affected by melanoma also suggests that there is a genetic component to the disease.

Canine and feline melanoma is difficult and in some cases impossible to prevent, because veterinary medicine has yet to fully identify its underlying cause. Given the over-representation of white colored dogs and cats, especially those with very short hair coats, it would be wise to limit prolonged sun exsposure n these animals. Recognition of melanoma and early detection remain our best method to combat this disease. Given the suspected genetic contribution to this disease, affected animals probably should not be bred. However, with early detection, the adverse effects of melanoma may be manageable. Dog and cat owners, especially those with predisposed breeds, should familiarize themselves with the symptoms of melanoma and should perform routine, hands-on examinations of their pets to identify any suspicious lumps or bumps at the very earliest opportunity. These should be examined by a veterinarian as soon as they are identified.

Currently, there is no vaccine proven to prevent the occurrence of canine and feline melanoma, although there is a vaccine that is licensed for dogs only, to help treat the condition by prolonging survival rates and minimizing clinical signs of the disease. The license is conditional at this time and not readily available to all practitioners. Its benefit is still questionable and also remains very expensive – too much so for many dog owners.

While surgical removal is the most common treatment for melanoma, the tumors tend to recur in the same or in other areas post-operatively. Metastasis (spread to other tissues) is suspected to be present in 85% of cases at time of diagnosis. Chemotherapy has been shown to effectively increase survivial rates following surgery, but not significantly so, only a matter of months at this time. Early detection remains our best weapon against this disease.


Melanoma In Pets

Melanoma is a tumor that can develop in the skin, toenail bed, lips, eye, intestine and other unusual places.

These tumors originate from the pigment-producing cells called melanocytes, which are responsible for giving humans a tan with sun exposure. Melanomas located on the haired skin (except the toenail bed) generally do not spread to other organs (metastasis).

Of all digital tumors, 17.3% have been reported to be melanomas. These tumors have a very high metastasis rate. At the time of diagnosis of a digital melanoma, roughly one-third of dogs will have detectable spread of the tumor to the lungs.

Melanoma is the most common oral tumor. In general, these tumors are locally invasive and spread rapidly to the regional lymph nodes and lungs. There are certain types of melanomas on the lips and oral cavity that have less tendency to spread and a good chance to be cured with surgery.

Clinical Signs

A brown or black mass (lump) is the most common sign of a melanoma however, some are not pigmented and may appear as a pink mass. A melanoma that involves the toenail bed may cause the toe to swell and cause lameness. Initially, this tumor of the digit may mimic the appearance of an infected toe however, treatment with antibiotics does not resolve the problem.

A melanoma in the oral cavity may produce signs of drooling, bad breath, reluctance to chew hard food and blood coming from the mouth. Enlargement of lymph nodes in the area of the tumor may be a sign of spread of the tumor to these nodes. If the tumor has spread to the lungs, potential clinical signs may include breathing difficulty, coughing, weight loss, poor appetite and malaise.

Diagnosis

The diagnosis of a melanoma is based upon a fine needle biopsy or surgically collecting a piece of tissue from the mass. If the fine needle biopsy does not provide a definitive diagnosis, a core of tissue may be required. A complete blood count, chemistry profile and urine testing are done to evaluate the health of your companion’s internal organs prior to anesthesia and surgery.

Chest x-rays and abdominal ultrasound are used to identify visible metastasis. (Please note that microscopic spread of tumor to other organs cannot be detected with x-rays or ultrasound.) Lymph nodes in the area of the tumor are evaluated with a fine needle biopsy to rule out spread of the cancer. If this evaluation of the lymph node is not definitive, removal and microscopic analysis of a regional lymph node is recommended.

Treatments

Surgery is the treatment for a melanoma. Melanomas located in the haired skin (except for the nail bed) are typically benign and only require conservative simple surgical removal. If the tumor is located on the toenail bed, amputation of the entire toe is essential.

Tumors located in the oral cavity require an aggressive surgery to remove all local disease. Commonly, these tumors invade the bone therefore, a portion of the jawbone must be removed with the tumor to achieve local control of the disease.

In certain cases, an oncologist may recommend chemotherapy for melanomas. Typically, one treatment is performed every 3 weeks for a total of four to six treatments. Most patients tolerate the chemotherapy medication with transient mild side effects.

Radiation therapy has been shown to prevent or delay the onset of tumor regrowth. Six to seven radiation treatments are administered to the tumor site and regional lymph nodes, starting 2 weeks after the tumor has been removed. Radiation treatments are administered twice weekly. A short-duration anesthesia is required to administer each radiation treatment.

A melanoma vaccine, which requires administration of a series of injections, can help the body kill residual tumor cells. Best results are achieved when all visible tumor has been removed.

Results

Skin melanomas: Melanomas that are located in the skin (except for the nail bed) carry an excellent prognosis with simple surgical removal or digit amputation, as they have a very low spread rate.

Digital melanomas: Digital melanomas treated with surgical amputation of the digit resulted in a median survival time of 365 days. Digital melanomas that are not located on the nail bed and have benign characteristics (low mitotic index) can be potentially cured with surgery alone.

Oral melanomas: The size of a melanoma and stage of the disease (presence of metastasis) directly affects the survival time.

Median survival times
  • Patients with stage 1 tumors (less than a 2 cm diameter mass and no metastatic disease) that are surgically removed have medial survival times of 17 to 18 months.
  • Patients with stage II tumors (2 cm but less than 4 cm in diameter mass with no metastasis) that are surgically removed have median survival times of 5 to 6 months.
  • Patients with stage III tumors (4 cm and larger mass with spread to regional lymph nodes) have median survival times of 3 months.
  • Stage IV tumors are those that have already spread to distant sites such as the lungs, and survival times are weeks to a few months.

Negative prognostic factors include advanced stage of the disease, evidence of metastasis, and the mitotic index on the biopsy report. Death of the patient is usually due to metastatic disease (spread of tumor) typically to the lungs.

Radiation therapy has been reported to have excellent local control of oral melanomas, yet does not prevent or treat the spread of the tumor to the lungs and other distant sites. Therefore, this modality should be used in conjunction with the melanoma vaccine for improved survival times.

Vaccination with the melanoma vaccine reportedly gives excellent long-term control of the disease and survival times with minimal to no side effects. The best result was seen with the intradermal vaccination administered with a special injector system.


A malignant melanoma is a tumor involving the pigmented cells of the skin. Melanoma is a classical skin cancer of humans, and in humans it is associated with sun exposure. While a good fur coat generally protects our pets from sun-induced malignant melanoma, a melanoma diagnosis is still just as serious and potentially deadly in our pets. Most patients are middle-aged or older dogs.

The melanoma is a tumor of pigmented cells called melanocytes.

Benign versus Malignant Melanomas

  • The mouth, lips, oral cavity
  • The toe or foot
  • The eye
  • Haired skin

Each of these areas produces a somewhat different syndrome and we will review each separately.

Melanoma of the Mouth

The oral melanoma is a highly malignant tumor. It is locally destructive in the mouth plus it readily and quickly spreads to other areas of the body. This means that there are three aspects of the cancer requiring attention: the local destruction in the oral cavity undetectable tumor cells in the process of spreading and the cancer that is detectable in distant organs.

Once a melanoma is confirmed by biopsy, a staging process must begin to determine the extent of tumor spread. Chest radiographs are scrutinized for evidence of spread, local lymph nodes are sampled, and the abdomen is screened with an ultrasound exam.

In the mouth, the size of the tumor is extremely important when considering the prognosis. Veterinary medicine has adopted the World Health Organization staging system, where Stage I disease is a tumor less than 2 cm (just less than 1 inch) in diameter Stage II disease is a tumor 2 – 4 cm in diameter and Stage III disease consists of tumors 4 cm or larger or any tumor with local lymph node involvement. Stage IV disease includes any tumor with evidence of distant spread. Median survival times for oral melanoma have been reported as:

  • Stage I: approximately 17-18 months (with surgery alone)
  • Stage II: approximately 6 months (with surgery alone)
  • Stage III: approximately 3 months (with surgery alone)
  • Stage IV: approximately 1 month (surgery not applicable)

The goal of additional adjunctive therapy (radiation, chemotherapy, immunotherapy) is to extend this time.

Local Disease Control

Local disease control refers to control of the disease in the mouth. Ideally most of the tumor can be surgically removed however, even an extensive resection is not likely to remove the entire tumor with certainty and some kind of adjunctive treatment is prudent. Radiation in the area has extended the time until the tumor regrows in areas where complete margins cannot be obtained (i.e. biopsy shows there is still more tumor left). If margins are clean, immunotherapy (see later) may be all that is recommended.

Distant Disease Control

If there is documentation that the tumor has escaped and colonized another site in the body (i.e. the patient is in Stage IV), more extensive treatment is needed and this usually means chemotherapy. The melanoma is notoriously not responsive to chemotherapy in people, but research and new drug development continues. In one study carboplatin combined with piroxicam increased median survival time from 30 days to 119 days in Stage IV patients.

Microscopic Disease

After no disease is detectable to the eye, we cannot be sure about undetectable disease, which is malignant cells in transit looking for a place to set up shop. Chemotherapy helps reach these cells as does immunotherapy, a newer mode of cancer treatment. Immunotherapy involves generating an immune-response against the tumor cells and attacking them with the body’s own natural system. Periodic screening tests/staging are needed to see if the tumor has succeeded in settling in a distant organ and therapy can be ramped up to address this event.

Melanoma of the Digit (Toe)

Developing melanoma in the toe or toenail bed seems to be a particular problem for black dogs. The tumor is particularly destructive to the bone and quite painful, starting usually as a swelling that seems associated with an infected toenail. The toe infection may improve with treatment but the swelling does not resolve and ultimately gets worse. If the tumor has not spread beyond the toe, amputation should theoretically be curative but reality is that median survival time after toe amputation in cases where further tumor is not detectable is approximately one year. It is believed that 30-40 percent of these tumors have already spread at the time of diagnosis so if a longer survival is the goal, further treatment should be explored.

Because this is such an aggressive tumor, staging is important after melanoma is confirmed. This means chest radiographs, lymph node sampling, and ultrasound of the abdominal organs to identify distant spread. As before, local disease, distant disease and microscopic disease must be addressed.

Haired Skin Melanoma

The behavior of a melanoma is highly dependent on where it develops. Most areas of skin grow benign versions of the melanoma that are called melanocytomas. These tumors typically do not spread and do not behave in a malignant manner. Since some haired skin melanomas are definitely malignant in behavior, it is important to have all removed tumors (pigmented or not) analyzed by a pathologist. There is some suspicion that haired skin melanomas developing within 1 cm of a mucosal margin (like the mouth) behave more malignantly than one would expect based on what is seen under the microscope.

Many melanomas removed from the body look malignant under the microscope but do not behave in a malignant manner. To get a better sense of how a given tumor will behave, the number of cells in the process of division are counted in the sample. This is reflected by a number called the mitotic index of the tumor and is expressed as the number of mitotic figures per high power field (microscopic view). Less than three mitotic figures per high power field indicates the tumor will behave in a benign manner. There is evidence that staining the sample for certain tissue markers (such as Ki-67) can help predict behavior as well.

As with the other melanoma forms, malignant tumors should be staged and treated for local, distant and microscopic disease.

Melanoma of the Feline Eye

Uveal/Iris Melanoma – Dogs The pigments of the iris and ciliary body within the eye are especially vulnerable to melanoma development. These tumors are particularly difficult to see until they are fairly large. The clinical situation is different depending on whether the patient is a cat or a dog.

The good news is that this is benign condition in at least 80 percent of patients. The bad news is that an expanding growth inside the eye, even a benign growth, can cause pain and blindness. If glaucoma (increased eye pressure) has resulted or if there is deep inflammation in the eye, rapid tumor growth, or vision loss, it is probably worth simply removing the eye (enucleation) though smaller growths might be amenable to laser therapy.

Uveal/Iris Melanoma – Cats

The feline situation is different in that 60-70 percent of these tumors are malignant and will spread. Tumors can be large and bulgy as in the dog’s eye or they can be more subtle, starting as flat areas of pigment on the iris (colored part of the eye) and later gaining some 3-dimensional growth. The eye will likely need removal to manage the pain from the expanding growth but it is important to do proper staging first (radiographs of the chest, local lymph node sampling, ultrasound of the abdominal organs) to determine the extent of the existing tumor spread.

Iris melanosis, which is a benign condition, can be impossible to distinguish from an early melanoma so often periodic evaluation by a specialist is in order. Iris melanosis refers to the “freckles” and flat dark spots that older cats develop on their irises. These spots are common but should not have any bulging or changing roughness when the iris is viewed from the side.

Epibulbar Melanoma

This form of melanoma is usually benign in either dogs or cats and arises on the outer eye portion where the sclera (white part) meets the cornea (clear part over the iris). Smaller growths may not need treatment. Larger ones can be surgically removed, treated with laser or with cryosurgery. Tumor spread is not expected with this form of melanoma.

As mentioned, treatment is divided into local disease control, distant disease control, and microscopic disease control. Surgery and radiotherapy are the fundamental treatments for local disease whereas chemotherapy is the foundation of distant disease control. Different medications and protocols are being developed all the time. Immunotherapy is a little harder to classify.

The Melanoma Vaccine

A melanoma vaccine has been in use for nearly a decade now, not to prevent melanoma development but to generate an active immune response against an existing tumor. The vaccine was tested on dogs with Stage II and Stage III oral melanoma after all detectable tumor was surgically removed with improved survival times achieved over what was expected from surgery alone. The vaccine has also been found helpful for toe/digit melanomas as well.

The vaccine is for dogs without grossly detectable tumor it will probably not be helpful for dogs where there is already distant tumor spread.

How it Works

An enzyme called tyrosinase is crucial to the melanocyte’s ability to produce melanin (pigment). By using human tyrosinase as a stimulator, the patient’s immune system can be tricked into attacking the melanoma cells that contain the patient’s natural tyrosinase. The vaccine is available only through veterinary oncology specialists. It is given in four single doses at 2-week intervals. Booster shots are given every 6 months and is best used for dogs with oral melanoma without node involvement. Life expectancy has been extended to over one year in many cases.


Watch the video: Malignant Melanoma State of the Art Global Overview (July 2021).