A melanoma is a tumor that grows from melanocytes, the cells that produce pigment in the skin [1]. In dogs and cats these tumors can be benign, in which case they are called melanocytomas, or malignant, in which case they are called melanomas or malignant melanomas [1].
The look of a lump does not tell you which kind it is. A vet can collect cells with a fine needle or take a biopsy (a small piece of tissue) and send it to a laboratory; poorly pigmented tumors in particular often need a biopsy to confirm what they are [1]. So any new lump, dark spot that changes, or swollen toe should be checked by your vet.
For other kinds of lumps, see our posts on lumps and bumps on dogs and lumps and bumps on cats. Nail bed problems in general are covered in nail and nail bed problems in dogs and cats.
Melanocytoma or malignant melanoma?
A 2024 consensus guideline written by veterinary cancer specialists asks vets to use the words this way: “melanoma” and “malignant melanoma” both mean a malignant tumor of melanocytes, “melanocytoma” means a benign one, and “melanocytic tumor” covers both [1]. In people, by contrast, “melanoma” always refers to a cancer.
The same guideline points out that canine melanoma is not caused by ultraviolet light, unlike many melanomas in people [1]. For cats, it says a different cause in sun-exposed and covered areas is plausible, but there is no evidence that the disease behaves differently [1].
Melanocytic tumors are common in dogs but rare in cats [1]. In one Italian pathology service (University of Perugia), 845 of the canine tumors examined from 2005 to 2024 were melanocytic, compared with 60 feline ones [2].
Melanoma in dogs
Benign melanocytomas
- The guideline says melanocytic tumors of the haired skin in dogs are commonly benign [1]. The Perugia authors also describe canine skin melanocytic tumors as mostly benign, although in their records skin melanomas (228) were only slightly fewer than skin melanocytomas (253) [2].
- The Perugia paper lists breeds that some studies report at higher risk of melanocytic tumors, including Scottish, Airedale and Boston Terriers, Miniature and Standard Schnauzers, Doberman Pinschers, Vizslas, Golden and Chesapeake Bay Retrievers, Irish Setters, Chow Chows, Boxers, and Cocker and Springer Spaniels [2].
- On haired skin these tumors usually show up as a raised, pigmented lump, and signs of inflammation are usually absent [1].
A tumor that looks harmless can still be malignant, so only your vet’s tests can show which kind a lump is [1].
Malignant melanomas
- The guideline names the mouth (oral) and the nail bed (subungual) as the sites of the most common malignant types in dogs [1]. Lumps on the toe, footpad and scrotum are more likely to be malignant [1].
- Schnauzers, Rottweilers, Scottish Terriers, Golden Retrievers and Irish Setters are at increased risk of nail bed melanoma, according to the guideline [1].
- A nail bed melanoma usually shows up as a swollen, painful end of the toe or a sore near the claw that does not heal [1].
- Some melanomas have little or no pigment, which makes them harder to identify on a needle sample [1].
- Melanomas on the lip or tongue tend to have a better outlook than those inside the mouth or on a toe [1].
In the Perugia records, older dogs were more likely to have melanomas than melanocytomas, and the average age of affected dogs was 10 years [2].
Melanoma in the mouth is a separate subject and is not covered here; if you see a lump in your dog’s mouth, ask your vet.
Melanoma in cats
- In cats, melanocytic tumors are found most often in the eye, on haired skin (especially the ear flap) and in the mouth [1].
- Affected cats are usually 11 to 13 years old, but melanoma of the ear flap has been reported in younger cats, with a median age of 7 years [1].
- No sex or breed predisposition has been reported in cats [1].
- In the Perugia records, 22 of the 28 feline skin melanocytic tumors were melanomas rather than melanocytomas [2].
A French study of 30 cats with melanomas outside the eye, using cases from the animal histopathology laboratory of the National Veterinary School of Nantes, found that cats with tumors on the ear flap were significantly younger than cats with tumors elsewhere. The authors concluded that most of these feline melanomas are malignant [5].
In a study of 10 cats with melanocytic tumors on the nasal planum (the hairless tip of the nose), the authors thought that a pigmented nose might make these tumors more likely. In four of the cats, a dark mass that had been unchanged for a long time suddenly began to grow quickly, which the authors took as a sign that it had turned malignant [6]. The guideline adds that benign nasal planum melanomas in cats may progress to malignancy [1]. This is one reason a long-standing dark spot that starts to change should be checked.
How it is diagnosed
- Your vet can examine cells from a fine needle sample. Tumors with little pigment are harder to identify this way, and a biopsy examined under the microscope is then needed [1].
- The guideline recommends a large or deep biopsy that avoids ulcerated or dead areas so the diagnosis is accurate [1].
- Chest x-rays and checks of the nearby lymph nodes help the vet find out whether the tumor has spread (staging) [1].
- Certain features seen under the microscope help predict outcome and guide treatment decisions [1]. A study of 324 cats proposed a grading scheme based on the tumor’s site, how fast its cells were dividing (mitotic count) and dead tissue inside the tumor (intratumoral necrosis) [4].
Treatment
- The guideline describes surgical removal with wide margins as currently the mainstay of local treatment, wherever the melanoma is [1].
- For a nail bed melanoma, removing the affected toe at the joint where it meets the foot is the treatment of choice [1].
- For a nasal planum melanoma in a cat, the guideline recommends wide removal of the nose tip, whether the tumor is benign or malignant [1].
- Added treatments such as immunotherapy or chemotherapy may be offered when the risk of spread to distant organs is high [1].
- In the French study of 30 cats, the authors recommended surgery as the first option to consider [5].
Outlook
The guideline says location is the major factor in predicting outcome, although it does not fully predict how invasive a tumor will be or whether it will spread [1]. Melanomas of the haired skin are often cured when they are removed with clean margins, while a high rate of cell division is linked to spread and shorter survival [1].
Outcomes vary between studies and between body sites:
- In 87 dogs treated with surgery for malignant melanoma of the haired skin, the median overall survival time after surgery was 1363 days, the post-surgery metastatic rate was 21.8% and the local recurrence rate was 8%. Older age and a higher mitotic index were linked to shorter survival. The authors concluded that long survival can be expected in many of these dogs after surgery alone [3].
- Dogs with a nail bed melanoma and no spread, treated by toe amputation, have reported median survival times of about 12 months [1].
- In the French study of 30 cats, tumors without pigment (achromic) were associated with a poorer prognosis, and 20 cats treated with surgery survived significantly longer than cats that received only medical treatment or no treatment [5].
- In the 324-cat study, of 79 tumors with outcome data, median survival for cats with low-grade tumors was not reached, and median survival was 90 days for cats with a high-grade tumor [4].
- In the 10 cats with nasal planum tumors, most (7) had to be euthanased due to tumor progression, with a median survival time of 265 days [6].
These are figures and statements about groups of animals in particular studies. They do not prove that a treatment caused an outcome, and they cannot predict how one pet will do. Your vet can explain what the biopsy results mean for your dog or cat.
When to see your vet
See your vet if your dog or cat has:
- any new lump or spot on the skin, dark or not [1]
- a dark spot or lump that changes, grows or ulcerates [1][6]
- a swollen or painful toe, a lost nail or a sore near the claw that does not heal [1]
- a lump on the lip [1]
- a spot or lump on the ear flap or elsewhere on the skin of a cat [1]
- a pigmented spot on the nose that changes [6]
FAQ
Is every black lump on a dog a melanoma?
No. Many lumps can be dark, and a melanoma can also have little or no pigment [1]. Only testing tells what a lump is.
My vet says it is a melanocytoma. Is that cancer?
The guideline uses “melanocytoma” for a benign melanocytic tumor [1]. Ask your vet what the lab report says and what follow-up they recommend.
Does sun cause melanoma in pets?
The guideline says canine melanoma is not caused by ultraviolet light. In cats, a link with sun exposure has been considered but not shown to change how the disease behaves [1].
Why would my vet x-ray a swollen toe?
Nail bed melanomas are among the most common malignant melanomas in dogs [1], and x-rays help the vet see whether the bone of the toe is involved and plan treatment.
Are melanomas in cats common?
No. The 2024 guideline calls melanocytic tumors rare in cats [1].
The bottom line
Melanomas in dogs and cats may be benign (melanocytomas) or malignant, and only your vet’s tests can tell which [1]. In dogs, the mouth and nail beds are the sites of the most common malignant types, while tumors of the haired skin are often benign [1]. In cats, melanocytic tumors are rare and are found most often in the eye, on the skin (especially the ear flap) and in the mouth [1]. Surgical removal with wide margins is currently the mainstay of local treatment [1]. Outcomes vary by body site and between studies, and your vet can explain what the results mean for your pet. Have any new or changing lump, spot or swollen toe checked.
This article is for general information and does not replace a visit to your veterinarian. If you find a lump or spot on your pet, contact your vet.
Sources
- Polton G, Borrego JF, Clemente-Vicario F, et al. Melanoma of the dog and cat: consensus and guidelines. Front Vet Sci. 2024;11:1359426. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC11026649/
- Lo Giudice A, Porcellato I, Giglia G, et al. Exploring the Epidemiology of Melanocytic Tumors in Canine and Feline Populations: A Comprehensive Analysis of Diagnostic Records from a Single Pathology Institution in Italy. Vet Sci. 2024;11(9):435. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC11436034/
- Laver T, Feldhaeusser BR, Robat CS, et al. Post-surgical outcome and prognostic factors in canine malignant melanomas of the haired skin: 87 cases (2003-2015). Can Vet J. 2018;59(9):981-987. https://pubmed.ncbi.nlm.nih.gov/30197441/
- Pittaway R, Dobromylskyj MJ, Erles K, et al. Nonocular Melanocytic Neoplasia in Cats: Characterization and Proposal of a Histologic Classification Scheme to More Accurately Predict Clinical Outcome. Vet Pathol. 2019;56(6):868-877. https://pubmed.ncbi.nlm.nih.gov/31221042/
- Chamel G, Abadie J, Albaric O, et al. Non-ocular melanomas in cats: a retrospective study of 30 cases. J Feline Med Surg. 2017;19(4):351-357. https://pubmed.ncbi.nlm.nih.gov/26767979/
- Reck A, Kessler M. Melanocytic tumours of the nasal planum in cats: 10 cases (2004-2019). J Small Anim Pract. 2021;62(2):131-136. https://pubmed.ncbi.nlm.nih.gov/33244779/