Malignant melanoma in dogs is a cancerous tumor of pigment-producing cells (melanocytes) that can affect a dog’s mouth, eyes, skin, and paws. To help dog parents understand what their dear dog might be facing, integrative veterinarian Dr. Julie Buzby explains the types, symptoms, diagnosis, treatment, and prognosis for malignant melanoma in dogs.
One of my professors in veterinary school taught clinical pathology, somehow managing to make lectures about cells and all things microscopic entertaining and relatable. During one lecture, she emphasized the importance of always checking new or unusual lumps on our patients by saying, “If it swells, stick it!”
As amusing as it was at the time, her message has stayed with me throughout my veterinary career. That simple comment underscores the importance of proactive care for dogs with new lumps or bumps.
If the mass ends up being cancer, such as a malignant melanoma in dogs, time is of the essence.
What is malignant melanoma in dogs?
A malignant melanoma is a cancerous growth that develops on the skin, paws, mouth, or eyes due to the abnormal replication of melanocytes (pigment cells). These cells, found in the deepest layer of the skin, produce melanin, which gives your dog’s skin and fur its pigmentation.
Like other cell types, melanocytes can form both benign and malignant tumors. Technically, benign tumors of melanocytes are called melanocytomas, but people may refer to them as melanomas. Cancerous tumors can also be called melanomas, but referring to them as malignant melanomas better helps indicate their behavior.
Malignant melanomas represent roughly 70% of all melanin-producing tumors in the body. They also account for about 7% of all tumor diagnoses in dogs. Sadly, malignant melanoma can be highly aggressive. And it often metastasizes to local lymph nodes and other organs such as the liver, lungs, and kidneys.
What are the types of melanomas in dogs, and are they benign or malignant?
Three distinct types of melanomas affect dogs:
- Oral melanoma: Those involving the gums, lips, tongue, or hard palate.
- Ocular melanoma: Masses on the surface of the eye or inside the eye.
- Cutaneous melanomas: Ones that affect the skin, paw pads, nails, or digits.
Depending on the location, the melanoma may have a higher chance of being either benign or malignant. However, diagnostic testing is the only way to know for sure.
Oral melanoma—almost always malignant and aggressive
Oral melanoma is the most common type of melanoma in dogs. It accounts for 60% to 80% of melanoma cases and 40% of all oral tumors in dogs. Growths typically develop on the gums, lips, tongue, and hard palate. These tumors are almost always malignant and have a high rate of metastasis to other parts of the body. They can also be very destructive to the jaw and surrounding tissue.
Ocular melanoma—usually benign but can damage the eye
Ocular melanomas are usually benign. But they can still cause significant pain or vision problems for dogs, depending on their size and location. The dog may even lose the eye as a result of an ocular melanoma. Clinicians estimate that approximately 80% of ocular tumors are benign melanocytomas.
Cutaneous (skin) melanomas—usually benign on haired skin, but cancerous on the toes, nails, foot pads, scrotum, and junctions of haired and non-haired skin
Cutaneous melanomas represent between 15% and 27% of melanoma diagnoses in dogs and vary in malignancy based on their location on the body. In most cases, they are benign on haired skin.
However, melanomas affecting the toe (digital melanoma), nail bed (subungual melanoma), foot pad, scrotal tissue, or mucocutaneous junctions (places where haired and non-haired skin meet, such as the lips or eyelids, are more likely to be malignant.
Which dogs tend to develop malignant melanomas?
Malignant melanomas can develop in dogs of any age. However, dogs 10 years of age or older are at a higher risk. Male dogs are also anecdotally more likely to develop melanomas than females.
Aside from age, several breeds have a genetic predisposition to various types of melanomas:
- Oral melanoma: Golden Retrievers, Cocker Spaniels, Poodles, Scottish Terriers, and Dachshunds
- Ocular melanoma: German Shepherds, Labrador Retrievers, and Golden Retrievers
- Cutaneous melanoma: Doberman Pinschers, Rottweilers, Golden Retrievers, Irish Setters, Schnauzers, and Vizslas
Overall, it seems that dogs with darker pigmentation tend to have a higher chance of developing melanomas.
What are the causes of melanoma in dogs?
In addition to genetic factors, another potential cause of melanoma development is skin trauma. If your dog repeatedly licks or irritates a spot on his or her skin, it can cause the pigment-producing cells (melanocytes) to reproduce faster than usual.
This is why a dog’s skin may darken with chronic allergies or around a lick granuloma. The constant irritation can trigger abnormal cell growth. While not proven, this could increase the risk of a melanoma in that spot.
We know that excessive sun exposure can increase the risk of skin cancer in people. However, veterinary researchers generally don’t think UV light exposure from sunlight plays a similar role in causing dog melanomas.
That said, if you want to be cautious, there are dog-safe sunscreens available for dogs with an active, outdoor lifestyle. (Don’t use human sunscreen as it may contain substances that are dangerous for dogs.)
What are the symptoms of malignant melanoma in dogs?
A melanoma can cause a wide range of symptoms depending on its size and location. Plus, whether or not it is malignant factors into the signs you see.
Symptoms of malignant oral melanoma
In the case of an oral melanoma in dogs, you may notice symptoms such as:
- Drooling (ptyalism): Your dog might have a wet chin or strings of drool hanging from the mouth.
- Bleeding from the mouth or bloody saliva: Drops of blood, blood-tinged drool, or blood on dishes or toys are all possible.
- Difficulty eating: Depending on the size and location of the tumor, the dog may have trouble picking up, chewing, and/or swallowing food.
- Weight loss: Due to not eating well or the effects of cancer spread, the dog may lose weight.
- Foul-smelling breath (halitosis): You may notice your dog has bad breath (or that it is worse than normal).
- Pawing at the mouth: The pain and irritation from the tumor may cause your dog to rub or paw at the face.
- Shifting teeth or lost teeth: As the tumor invades the jaw it may push teeth out, shift their position, or cause jaw swelling.
Malignant melanomas in the mouth may or may not contain pigmented cells. Dogs can develop amelanotic melanomas—masses that appear pink or pale but are also malignant. This makes the diagnosis more challenging, especially in dogs with a normally mottled gum or tongue color.
Regardless of color, oral melanomas can appear as raised lumps along the gumline, flat plaques on the hard palate, or ulcerated lesions on or under the tongue. Secondary infections at the tumor site are also frequently present.
Symptoms of melanomas in the eye (benign or malignant)
While dog eye melanomas tend to be benign most of the time, they can still cause problematic inflammation and damage to the structures of the eye. For this reason, symptoms tend to be similar overall between benign and malignant ocular melanomas.
Melanomas that grow inside the eye on the uvea (iris, choroid, or ciliary body) may or may not be visible. However, you can generally see the ones that occur at the junction of the white and colored parts of the eye (the limbus). They look like a smooth, well-defined brownish mass (or occasionally a pink mass if it is amelanotic).
Some of the other common signs include:
- Red eye: The white part of the eye might be redder than normal.
- Squinting (blepharospasm): Pain may cause your dog to keep the eye mostly or totally shut.
- Pawing at the eye: You might also notice your dog rubbing at the eye due to pain.
- Excessive tear production (epiphora): The fur under the eye may be wet with tears.
- Vision loss: Melanoma can damage the eye enough to cause blindness. However, since melanoma usually only affects one eye, you may not notice signs your dog’s vision has changed.
As a result of the tumor, the dog may develop uveitis in dogs (inflammation of the iris, choroid, or ciliary body) or glaucoma in dogs (increased pressure within the eye). Both of these conditions are painful.
Symptoms of benign melanomas of haired skin
Melanomas on haired skin are usually benign and look like a raised, round, or flattened wrinkly area of skin. They may be brown, black, or pink (amelanotic).
However, just by looking at it, it’s impossible to tell if the mass is benign or malignant. And especially in the case of an amelanotic melanoma, you can’t even tell if it is a melanoma or something else, like a mast cell tumor in dogs. Thus, you should not look at a brown or black mass on your dog’s skin and assume it is a benign melanoma.
Symptoms of malignant melanomas of lips, eyelids, or nose
Skin melanomas are more likely to be malignant around the lips, eyelids, nostrils, or genital area. The mass may look a lot like a benign melanoma, but sometimes it is thicker and may become ulcerated or start bleeding. Again, you can’t determine if the mass is benign or malignant just by external characteristics, though.
Symptoms of malignant melanomas of paw pads, toes, and nails
Melanomas on the dog’s paw (the pad, toe, or nail) tend to be malignant and aggressive. As a result, they may cause pain, limping, and licking of the foot. Additionally, the nail may be misshapen or fall off, and the toe may look swollen.
If you see a mass or concerning symptoms, schedule a vet appointment
As shared earlier, time truly is of the essence if you find a new, suspicious-looking lump or bump on your sweet dog or start noticing other symptoms. Scheduling a visit with your veterinarian is the first critical step on the path to an accurate diagnosis and treatment.
How will the vet diagnose malignant melanoma?
It is impossible to determine if a mass is a melanoma, or if it is benign or malignant, based on appearance alone. Thus, the vet will likely recommend additional testing after gathering a history and completing the physical exam.
Fine Needle Aspiration (FNA)
To perform a fine needle aspiration (FNA), the vet will use a small needle and syringe to pull a cell sample from the mass. Then he or she will examine the cells under the microscope. (Or in some cases, the vet may send the slide to the lab to be read by a veterinary pathologist.)
Unfortunately, FNAs may not always give clear answers for dogs with a suspected melanoma. The tumor cells don’t tend to easily release or “shed” into the syringe for collection.
Biopsy
The benefit of a biopsy over an FNA is that a biopsy involves examining a piece of tissue, not just cells, so it can definitively differentiate between benign and malignant tumors. Biopsy samples also help a veterinary pathologist learn how advanced a tumor is and how quickly it might grow or spread.
A critical part of this evaluation is the mitotic index. This is a measure of how many cells are actively dividing under the microscope. In general, the fewer dividing (or “abnormal”) cells seen, the slower the tumor is likely to grow, and the better the outlook with treatment.
For example, tumors in the mouth with fewer than four dividing cells per 10 high-power fields (<4/10 per hpf) typically carry a more favorable prognosis. In contrast, tumors in other locations require fewer than three dividing cells per 10 high-power fields (<3/10 per hpf) to achieve the same prognosis.
Additionally, the pathologist will use special lab tests and stains (called immunohistochemistry) for amelanotic tumors. This confirms the cells are truly melanoma cells, since the usual black pigment isn’t present.
As it is a more invasive procedure than the FNA, the vet will sedate or anesthetize your dog before collecting the biopsy sample. He or she may also biopsy draining lymph nodes (ones that receive lymphatic fluid from the area of the mass) at the same time.
Blood tests and urinalysis
The vet may recommend a urinalysis and blood tests for dogs, such as a chemistry panel and complete blood count. These baseline tests allow the veterinary team to evaluate your dog’s overall health. Since your dog may need anesthesia or other procedures or treatments, this information is important.
Imaging studies
Imaging studies like X-rays, ultrasound, or CT scans may also be part of the diagnostic workup. They help the veterinary team identify bony infection or tumor spread to the lungs or other organs.
What are the stages of malignant melanoma in dogs?
Staging of malignant melanomas depends on the size of the tumor and whether or not it has metastasized to the local lymph nodes or to other distant sites. (There is no need to stage benign tumors, so the staging guidelines only apply to malignant tumors.)
The staging criteria for malignant melanoma, which are based on those created by the World Health Organization for human melanoma, are as follows:
- Stage 1: Mass with a diameter less than 2 cm and no spread to lymph nodes or other organs.
- Stage 2: Mass with a diameter between 2 cm and 4 cm and no spread to lymph nodes or other organs.
- Stage 3: Mass with a diameter greater than 4 cm with or without spread to lymph nodes, but no spread to distant organs, OR mass of any size with spread to lymph nodes.
- Stage 4: Mass of any size with evidence of metastasis (spread) to lymph nodes and distant sites.
In some cases, such as for malignant melanoma of the toe, vets may use a slightly different staging system. Your vet can let you know what system he or she is using.
What is the treatment for malignant melanoma in dogs?
If your dog is diagnosed with a malignant melanoma, treatment options vary depending on the size, stage, and location of the tumor. When possible, surgery plus adjunctive therapy (if needed) is typically the best approach.
Surgical removal of the tumor
Surgical removal with wide margins is usually the treatment of choice for melanomas. Taking several centimeters of seemingly normal tissue around the growth helps decrease the risk that tumor cells will be left behind.
For less invasive tumors, your veterinarian can perform surgery in-clinic. This allows you to drop off your dog and take him or her home to recover the same day. However, more complicated cases of melanoma may need referral to a veterinary oncologist and/or board-certified surgeon. In some cases, the veterinary specialists may also recommend radiation therapy to help shrink the tumor before surgery.
It is worth noting that in some cases, surgical excision of a melanoma can be radical and invasive. And the dog may need a long recovery period. For example, a nailbed melanoma may require the complete surgical removal (amputation) of the toe.
Likewise, dogs with oral melanoma may need part of the lower or upper jaw removed (mandibulectomy or maxillectomy for dogs). Understandably, this can be quite a visual shock for a worried dog owner after surgery. Plus, it may take time for these dogs to figure out how to eat again.
With jaw surgery, your dog might stay in the hospital for a few days. That way, the veterinary team can manage post-operative pain and monitor your dog’s progress as he or she relearns how to eat.
Radiation therapy
Focal radiation therapy helps shrink a malignant tumor before surgery.
It’s also a useful adjunct treatment after surgery to target tumor cells that continue to persist in the healing tissues. Your veterinary oncologist will likely delay the start of radiation therapy for a couple of weeks following surgery so the incision can heal.
Melanoma vaccine (Oncept)
The Oncept™ canine melanoma vaccine can be helpful for dogs who have already undergone surgery to remove an oral melanoma. This melanoma vaccine for dogs is approved for treating dogs with Stage 2 or Stage 3 oral melanoma. It falls into the category of immunotherapy, meaning it helps the dog’s body target the cancer cells. (This is not a preventive vaccine.)
While Oncept injections can help extend a dog’s survival, the cost can be substantial.
Gilvetmab
Another immunotherapy option is Gilvetmab. It currently has conditional approval for the treatment of malignant melanoma (stages 2 and 3) and mast cell tumors. Gilvetmab is an antibody (immune system protein) that works to assist the dog’s immune system in destroying cancer cells.
Chemotherapy
Chemotherapeutic agents, including carboplatin, doxorubicin, and mitoxantrone, have been studied in conjunction with surgical excision and radiation therapy for the treatment of melanoma. Studies have shown that chemotherapy may provide some palliative benefit. But unfortunately, melanoma is generally poorly responsive to most chemotherapy regimens.
What is the treatment for benign melanomas?
The treatment for benign melanomas varies a bit depending on whether they are on the skin or involve the eye.
Benign skin melanomas
For benign skin melanomas, removing them is often still the best choice. Typically, the vet won’t need to take as large a surgical margin as with a malignant tumor. This is the case because a benign tumor doesn’t have the potential to spread like a malignant one does.
Benign melanomas of the eye
For benign melanomas that are on or in the eye, treatment may simply involve careful monitoring. The vet or ophthalmologist will assess your dog every few months to look for tumor growth or complications such as glaucoma, irritation of the eye, or uveitis.
If the mass is growing rapidly, causing problems, or it isn’t yet known whether it is benign or malignant, the vet may recommend removing it. Laser surgery or cryotherapy (freezing) can be an option for treating some ocular melanomas. However, removing the eye entirely (enucleation) may be the best option for tumors deep within the eye or those causing pain and inflammation.
The ophthalmologist can help you determine what is right for your dog.
What is the prognosis for malignant melanomas in dogs?
The prognosis for dogs with a malignant melanoma varies widely depending on the type, location, and stage of the melanoma, and the treatment approach. With stage 4 malignant melanoma, dogs may only survive a matter of weeks. However, dogs with a stage 1 melanoma have much better survival times, potentially up to around two years with treatment.
There are varied enough survival times in the literature for dogs with malignant melanoma that it isn’t really possible or prudent to give more specifics in this article.
If your dog is facing a malignant melanoma diagnosis, the best way to figure out what to expect is to have a conversation with your veterinarian or veterinary oncologist. He or she can review the treatment options and expected outcomes with your dog’s particular situation in mind.
What is the prognosis for benign melanomas?
By virtue of being benign, the outlook is much more favorable for dogs with benign melanomas. In the case of a benign skin melanoma, surgery is curative. (The dog could still develop unrelated melanomas in other locations, though.)
With benign melanomas of the eye, the dog may, unfortunately, end up losing the eye in some situations. However, he or she should be able to compensate just fine with the remaining eye, assuming it is healthy.
Partner with your vet after a malignant melanoma diagnosis
Understandably, finding out that your dog has malignant melanoma is frightening. However, by acting quickly and working closely with your veterinary team, you are giving your dog the best possible chance to enjoy his or her remaining days as comfortably as possible.
Finally, remember, if you feel or see a new lump on your dog, it is important to bring it to the attention of your veterinarian promptly. Then follow through with the recommended testing to identify the mass. The sooner you know what you are dealing with, the better. Even if the news isn’t good (like a malignant melanoma), you can take comfort in knowing that your veterinary team will be with you every step of the way.
Was your dog diagnosed with malignant melanoma?
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