Tumors are abnormal growths of cells. Tumors affecting the skin or the tissue just under the skin are the most commonly seen tumors in cats. This is partly because they are the most easily seen tumors and partly because the skin is constantly exposed to many tumor-causing factors in the environment. Chemical carcinogens, solar radiation, and viruses all can cause skin tumors. Hormonal and genetic factors may also play a role.
Distinguishing a tumor from an inflammatory disease is difficult. Skin tumors can appear in many forms. They are usually small lumps or bumps, but they also can occur as hairless, discolored patches, rashes, or nonhealing sores. Because skin tumors are so diverse, identifying them should be left to a veterinarian.
Tumors can be benign or malignant (cancerous). Malignant tumors can spread and cause harm to the animal. Distinguishing a benign tumor from a cancerous tumor requires specialized knowledge and laboratory equipment. A veterinarian can perform a fine needle aspiration of cells or a biopsy (which removes a small amount of tissue from a tumor) for evaluation.
Treatment depends largely on the type of tumor, its location and size, and overall physical condition of the animal. For benign tumors that are not ulcerated and do not impair the animal's normal routine, treatment may not be necessary. This could be the most prudent option, especially in aged cats.
For malignant tumors or benign tumors that inhibit normal activities or are cosmetically unpleasant, there are several options. For most, surgical removal is the most effective, the least costly, and the option with the fewest adverse effects. Laser and cryotherapy (freezing) are also options for treating small superficial tumors. If malignancy is suspected, surrounding tissue may also be surgically removed. For tumors that cannot be completely removed, partial removal may prolong the life of the animal. Radiation treatment or chemotherapy may also be used to provide your cat with the best possible outcome.
In addition to skin and hair follicle tumors, there are tumors that affect the ceruminous gland. These are discussed in the section on ear diseases (see Ear Disorders of Cats: Tumors of the Ear Canal in Cats).
Basal Cell Tumors and Carcinomas
Basal cells lie at the base of the top layer of the skin (the epidermis). A benign growth of these cells is a basal cell tumor. A malignant growth is a basal cell carcinoma.
Basal cell tumors are common in older cats. Domestic longhair, Himalayan, and Persian are the breeds most at risk. Tumors may develop almost anywhere on the body. These tumors generally appear as firm, solitary, often hairless or ulcerated lumps. The lumps may stick out like stalks from the skin surface. They vary in size from less than 0.4 inches (1 centimeter) to more than 4 inches (10 centimeters) in diameter. In cats, these tumors are often dark in color. Cysts may form. Although basal cell tumors are benign, some grow rapidly and may cause extensive ulceration and secondary inflammation. Surgical removal is an effective treatment.
Basal cell carcinomas are malignant tumors that occur most frequently in aged cats. Persians are more prone to them. They often appear as ulcers on the head, legs, or neck. Unlike benign basal cell tumors, these carcinomas are not usually raised up from the skin. They spread, forming new ulcers. Surgical removal is the treatment of choice. These tumors spread to neighboring skin but seldom spread to other organs.
Benign Fibroblastic Tumors
Benign fibroblastic tumors include the tumors known as collagenous nevi.
Collagenous nevi are benign buildups of collagen that are uncommon in cats. They generally are found in middle-aged or older cats, most frequently on the legs, head, neck, and areas prone to trauma. They are flat to raised lumps that develop in the dermal layer of the skin or in the fat beneath the skin. Surgical removal of both forms is generally effective. Infrequently, some may grow too large to be surgically removed.
Fibromas usually begin in the dermal layer of the skin and have well defined edges. They resemble collagenous nevi. While they are most common in dogs, all domestic animals can develop fibromas. These are usually benign. They appear as raised, often hairless, lumps that may feel firm or rubbery. Unless fibromas are malignant or interfere with the cat's activities or appearance, treatment is optional. In some cases, fibromas may grow to be quite large. In these cases, surgical removal is recommended to make the cat more comfortable.
Benign, Nonviral, Wart-like Tumors
These tumors are not, in the strictest sense, warts, although they may resemble warts. They are often easy to remove and do not threaten the cat's overall health. (See alsoWarts [Papillomas], see Skin Disorders of Cats: Warts (Papillomas).)
Blood Vessel Tumors
Blood vessel tumors of the skin are benign growths that resemble normal blood vessels. These are uncommon tumors of adult cats and most often develop on the head, legs, and abdomen.
Hemangiomas are single to multiple, circular, often compressible, red to black lumps. They can look like “blood blisters.” Hemangiomas are benign, but they tend to develop ulcers and grow quite large. It is important to identify the type of tumor to be sure it is not a malignant growth, so surgical removal is the treatment of choice. However, the tumors may be large and involve the lower legs, making surgical removal difficult. In these cases, freezing (cryotherapy) or radiation may be necessary. These tumors do not usually spread to new sites after complete surgical removal.
Angiosarcomas are the most likely of all soft-tissue tumors to grow quickly and spread to other locations. These tumors can arise at any time, but seem to occur most often in older, neutered male cats. When present, they are usually found on the legs and trunk of the cat. Some forms of angiosarcomas have been associated with prolonged exposure to the sun, especially in cats with white coats.
Angiosarcomas can vary greatly in appearance. Most commonly, they appear as one or more red lumps in the skin or underlying soft tissues. Less frequently, they appear as a poorly defined bruise. These tumors spread, especially to the lungs and liver. In most cases, surgical removal is the treatment of choice. Because these tumors often spread throughout the body, your veterinarian will remove both the tumor and a wide margin of tissue surrounding it. For surface tumors, cryo-surgery (freezing) can be used as needed. Avoidance of further sun exposure may reduce the development of new tumors.
Keratinized Skin Cysts
Some cats develop cysts that are filled with keratin, a skin protein. These are described as keratinized skin cysts. The cysts are benign and have a hard or solid core. Most such cysts are malformations of hair follicles and may be the same color as the hair. They are only occasionally found in cats. The cysts can be solitary or multiple. Surgical removal is the usual treatment.
Dilated pores of Winer are rare, hair-follicle growths recognized only in aged cats. Males may be predisposed. Growths most often develop on the head. They are solitary and dome-shaped. They may stick out from the surface, giving them the appearance of a skin horn. As with other keratinized skin cysts, these often closely resemble a large blackhead.
Lipomas and Liposarcomas
Lipomas are benign tumors of fatty (adipose) tissue. Older, neutered, male Siamese cats are most at risk for these tumors. They are most commonly found on the abdomen. Obesity does not appear to be a factor in the development of lipomas in cats. Lipomas typically appear as soft, occasionally thin, discrete lumpy masses. Most move freely when touched. Many lipomas merge with the healthy fat tissue next to them, making it difficult for a veterinarian to identify the edge of the tumor.
Despite their benign nature, lipomas should not be ignored because they tend to grow, and they may be indistinguishable from infiltrative lipomas or liposarcomas (see below). Surgical removal is the treatment. In most cases, the surgeon will remove not only the visible lipoma but also tissue around it. This provides the cat with the best possibility for a positive outcome.
Infiltrative lipomas are very rare in cats. When they are found, they are usually treated as malignant tumors even though they rarely spread to other organs. These tumors are soft, lumpy swellings in the fat layer under the skin. They can spread to underlying muscle and connective tissue. Invasive lipomas are considered sarcomas of partial malignancy. They rarely spread to other locations. Failure to remove the entire tumor can result in spread to nearby tissue. Surgery to remove the tumor and a wide margin of surrounding tissue is usually the treatment of choice. Amputation may be necessary.
Liposarcomas are rare tumors that are lumpy and soft to firm. Liposarcomas are malignant tumors that have a low potential to spread to other organs. In cats, feline leukemia virus infection has been infrequently associated with these tumors. Wide surgical removal of the tumor plus surrounding tissue is recommended. Recurrence is common, so followup radiation treatment may be required.
Lymphoid Tumors of the Skin
Lymphoma or lymphosarcoma is a cancer of the blood and lymphatic system. A rare form of lymphoma may first occur as lymphoid infiltrations of the skin, causing distinctive tumors. Lymphosarcoma in cats generally begins in other parts of the body, such as the intestinal tract or the lymphatic system. Lymphoma is associated with infection by feline leukemia virus (see Disorders Affecting Multiple Body Systems of Cats: Feline Leukemia Virus and Related Diseases).
Epitheliotropic skin lymphosarcoma is the most frequently recognized form of lymphoid skin tumors in cats. Among cats, the disease tends to develop in older animals. Signs are often extremely subtle. Tumors often appear initially as a crusty plaque that may itch. Accurate diagnosis in the early stages is difficult. Thus, skin scrapings, biopsies, and other diagnostic tools will be used to identify suspicious tissues. The tumors evolve into unmistakable skin lymphosarcoma. Unfortunately, effective treatment in cats is limited.
Nonepitheliotropic skin lymphosarcoma may occur as either single or multiple tumors in cats. The tumors are lumps or plaques that most commonly develop on the trunk. The disease occurs in middle-aged or older cats. The tumors are thick patches or lumps that can have hair loss or skin ulcers. The disease spreads quickly. Even when a solitary lump is completely surgically removed, recurrence is likely. Surgical removal is the most common treatment; the effectiveness of chemotherapy is limited.
Mast Cell Tumors
Mast cell tumors are a kind of tumor named for the type of skin cell from which they grow. Skin mast cell tumors are common. There are also generalized, blood cell, splenic, and digestive tract forms of mast cell sarcoma.
Two distinct variants occur: a mast cell type and a histiocytic type. The mast cell type is most common. They are found primarily in cats younger than 4 years old. They may develop anywhere on the body but are most commonly found on the head and neck. The tumors are benign, single, hairless lumps, generally 0.8 to 1.6 inches (2 to 3 centimeters) in diameter. Occasionally these tumors may extend into the fat beneath the skin. Surgical removal is the treatment of choice; few tumors of this kind return after surgery and, of those that do, considerably fewer spread. Freezing (cryosurgery) is an option used by veterinarians to treat multiple recurrent small tumors in cats that may have problems with anesthesia.
The histiocytic type of skin mast cell tumor in cats is seen primarily in Siamese cats younger than 4 years old. Tumors may develop anywhere on the body and appear as multiple, small (less than 0.4 inches [1 centimeter] in diameter), firm, lumps under the skin surface. Older cats generally have fewer tumors. This variant may be difficult to distinguish from simple inflammation. Because these tumors usually go away on their own, treatment may not be necessary.
A melanoma is a dark-pigmented skin tumor. They are rare in cats and may be either malignant or benign. Unlike in people, sun damage is seldom associated with melanomas in cats.
Melanomas in cats are found most often on the head (especially the ears and eyes), neck, and lower legs in middle-aged or older cats. They can appear as spots, patches, or raised or flat masses. Most have a dark surface. Although generally solitary, these tumors may be multiple. Surgical removal is the treatment of choice and offers a good outcome for benign tumors.
Neurofibromas and Neurofibrosarcomas
Neurofibromas and neurofibrosarcomas are tumors that grow in the connective tissue around a nerve. They are found in older cats. These tumors appear as white, firm, lumps. There are benign and mildly malignant types. Most are locally invasive but do not spread to distant organs. Complete surgical removal is the treatment of choice. Followup radiation treatment may slow regrowth of the tumor.
Sebaceous Gland Tumors
The sebaceous glands secrete oil (sebum) into the hair follicles and onto the skin. Tumors and tumor-like conditions of sebaceous glands are infrequent in cats. Older cats are more likely to develop these tumors than young or middle-aged cats. Persians are the breed most predisposed. When tumors occur, they are typically found in multiple places. They may occur anywhere on the body but are most often found on the head. Sebaceous adenomas may be covered with a crust and inflamed with pus.
Sebaceous gland adenocarcinomas are the rare malignant form of these tumors. They occur in middle-aged or older cats. Female cats may be predisposed. These are hard to distinguish from sebaceous epitheliomas or other skin carcinomas. A biopsy (laboratory study of a tissue sample from the tumor) is usually required to make a definitive diagnosis. These tumors spread within the skin and may spread to regional lymph nodes late in the disease.
Once a diagnosis is established, treatment is optional for benign sebaceous gland tumors unless they are inflamed and infected. For malignant adenocarcinomas, surgical removal (both the tumor and adjacent tissue including any involved lymph nodes) is the treatment of choice. Chemotherapy and radiation treatment may also be prescribed. Even benign se-baceous gland growths recur following surgery if the tumor is not completely removed. In addition, cats that develop one sebaceous gland tumor often develop new tumors at other sites. Your veterinarian may recommend consultation with a specialist in the treatment of cancer (oncologist) or skin disorders (dermatologist) to provide the best treatment for your cat.
Soft Tissue Giant Cell Tumors (Fibrous Histiocytomas)
Malignant fibrous histiocytomas are most frequently found in the skin and soft tissues of aged cats. They are most common on the lower legs or lower neck but may also occur at vaccination sites. They seldom spread to other locations in the body but tend to return after surgical removal. Because these tumors often recur, your veterinarian will remove the tumor and some surrounding tissue. This is necessary to provide your cat with the best possible outcome.
Soft Tissue Sarcomas
Sarcomas on the surface of the skin tend to have a more benign behavior, whereas deep sarcomas are considered malignant. Also, the larger the tumor, the more likely it is to be malignant. A rapidly growing tumor is more likely to be malignant than one that develops slowly. Some sarcomas are found at vaccine sites in cats.
Many sarcomas are shaped like an octopus, with tentacles that extend deeply into surrounding tissue. Diagnosis consists of finding a mass on the cat and confirming that the cells in the tumor are spindle cells. Cells are best obtained through a biopsy.
Surgical removal is the treatment of choice. Because groups of tumor cells that are too small to see grow out as tentacles from the main tumor, your veterinarian will remove not only the easily visible tumor (the head of the octopus) but also a wide margin of tissue surrounding it. In some cases, amputation may be required because the tumor cannot be completely removed from a leg. The best, if not only, opportunity to completely remove a sarcoma is during the initial surgery. Sarcomas that recur have a greater potential for invasion into deeper tissues and spread to other parts of the body. Later surgeries for recurrent tumors may not increase the longterm survival of the cat. Spindle-cell sarcomas generally do not respond well to conventional doses of radiation. Higher doses have been somewhat successful. Surgical removal followed by radiation treatment, or radiation treatment given prior to surgery may provide a longer tumor-free interval. Chemotherapy for sarcomas has become a more common means of treatment. Although chemotherapy may improve the quality and prolong the life of an affected cat, it seldom provides a complete cure.
Fibromatosis is a thickening and invasive growth in tendon sheaths. Fibromatoses are infrequently diagnosed in cats. They are locally invasive but do not spread to distant organs. If feasible, surgical removal is the treatment of choice. Recurrence is common, and radiation treatment may be helpful.
Fibrosarcomas grow rapidly and may spread to other locations. These are the most common soft tissue tumors in cats. Fibrosarcomas vary greatly in appearance and size. Tumors arising under the skin surface may appear lumpy. Those arising in the fat or other tissues under the skin surface may not be noticed unless a thorough, hands-on physical examination is performed. They are firm and fleshy and appear deep in the skin and the fat underneath. They often invade muscles. When tumors are multiple, they usually occur within the same area of the body.
Three forms of fibrosarcoma are recognized in cats: a multiple form generally found in cats younger than 4 years old and caused by the feline sarcoma virus; a solitary form in the young or old cat, not caused by feline sarcoma virus; and a tumor that develops in the soft tissues where cats are commonly vaccinated. Some rabies and feline leukemia virus vaccines may increase the risk of this type of tumor. These tumors appear as lumps or hard patches between the shoulder blades or in the soft tissues of the hind legs or flanks (see Skin Disorders of Cats: Vaccine-associated Tumors).
Surgical removal by a board-certified surgeon is considered the best treatment; however, complete removal is very difficult to achieve. When removing a fibrosarcoma, the veterinary surgeon will remove not only the tumor but also a wide and deep tissue margin around the tumor as well. Because it is difficult to determine the tumor's edges, recurrence is common. More than 70% recur within 1 year of the initial surgery. The rate of recurrence is higher than 90% for vaccine-associated sarcomas. Even when surgical removal is complete, recurrence is still the rule. Radiation treatment is recommended prior to or following surgery to increase the tumor-free interval. Chemotherapy has been recommended for tumors that cannot be removed.
In the 1990s an increase in the frequency of fibrosarcomas under the skin of cats was noticed by veterinarians. The tumors were often located in places where vaccines were commonly given, such as between the shoulder blades. Some vaccines, such as certain rabies and feline leukemia vaccines, have been linked with an increased risk of this tumor. Adjuvants, which are compounds added to a vaccine to boost the animal's immune response, may be a factor in causing the tumors. Because the tumors often spread locally and can be difficult to remove, vaccine recommendations have been changed. Changes may include vaccinating cats less frequently, matching which vaccines are given to the risk of infections that the cat may be exposed to, and giving the vaccine in the leg rather than between the shoulder blades. Vaccine manufacturers are also researching more effective vaccines and new ways of giving them that eliminate the need to add adjuvants.
Cat owners should be aware, however, that the overall risk of this tumor is low. Estimates run from 1 in 1,000 to as low as 1 in 10,000 cats affected. The disease protection that the vaccines provide will far outweigh the risk in most cases. Your veterinarian can recommend the vaccines and vaccination schedule best suited for your cat, taking into account factors such as your cat's age, health, and exposure to other animals.
Squamous Cell Carcinomas
In cats that are all or partially white, prolonged exposure to sunlight is a major cause of skin squamous cell carcinomas. Abnormalities usually develop on the ears, eye ridges, eyelids, nose, or lips of cats that have white skin in these regions. Before a malignant tumor develops, cats develop solar keratosis. The signs of solar keratosis include thickening and discoloration of skin. Surgery to remove the tumor is the usual treatment, but cryotherapy (freezing), topical chemotherapy, or radiation treatment may be used as needed. Limiting exposure to ultraviolet radiation may help prevent skin squamous cell carcinomas. This may be accomplished by using ultraviolet window screens, sunscreen, and keeping your cat indoors during hours of peak sunlight. Before using a sunscreen on your cat, please consult your veterinarian for recommendations. Sunscreen products prepared for human use are not always safe for cats and should never be used around the eyes.
Oral squamous cell carcinomas may appear in the mouth and on the tongue of cats and are not solar induced. The grooming habits of cats expose them to carcinogens such as cigarette smoke and flea collars, which increase their risk of developing these squamous cell carcinomas. Most oral squamous cell carcinomas are single, ulcerated lesions. They often mimic dental disease because they involve the teeth and the bone of the jaw or maxilla. The ulcerative lesions invade other local tissues. Specialized oral surgery to remove the involved bone is the recommended treatment, but it is difficult for cats because they will require feeding tubes. Surgery is followed by radiation treatment. Chemotherapy may be of limited help in some cases. The outcome is guarded to poor in most cases. Most cats have advanced disease at diagnosis.
Feline Bowen's disease (also known as feline multicentric squamous cell carcinoma in situ) is a disease of aged (over 10 years old) cats that may be associated with reduced immunity. Tumors appear as multiple discrete red, black, or brown patches and bumps. Their development is associated with the presence of a papillomavirus. Lesions grow on the skin surface, but eventually these tumors may change into a form that spreads into underlying tissues. No treatment has yet proven to be completely successful; however, cryotherapy (freezing), localized radiation treatment, and topical treatment may control the lesions initially.
Sweat Gland Tumors
There are 2 types of sweat glands: apocrine and eccrine. Apocrine gland cysts are found in middle-aged or older cats. They can occur either in or outside of hair follicles. They appear most commonly on the head and neck. Treatment is by surgical removal.
Cystic apocrine gland dilations have 2 forms. In the cystic form, one or more cysts develop in the middle to upper dermis but have little association with hair follicles. There is also a more dispersed form in which apocrine glands associated with multiple hair follicles are dilated.
Apocrine gland adenomas occur in older cats as firm to variable cysts, seldom larger than 1.5 inches (4 centimeters) in diameter, at the base of the sweat gland. They contain varying amounts of clear to brownish fluid. In cats, the fluid may be darkly pigmented. Apocrine adenomas are more likely to occur in male than in female cats. The vast majority of these adenomas occur on the head, especially the ears.
Apocrine ductular adenomas are less common. They are found in older cats and appear closer to the surface of the skin. They are often smaller, firmer, and less cystic than apocrine adenomas.
Apocrine adenomas and apocrine ductular adenomas are benign, and complete surgical removal cures the condition.
Apocrine gland adenocarcinomas are malignant tumors of sweat glands. Siamese cats may be predisposed. This tumor most commonly occurs where the legs meet the trunk and in the groin. Apocrine gland adenocarcinomas generally are larger than adenomas. Their appearance varies from thick lumps to ulcers. These tumors often spread to lymph nodes or, less commonly, to internal organs or skin. Complete surgical removal is the treatment of choice for apocrine gland adenocarcinomas. Little is known about response of these tumors to chemotherapy either alone or in combination with surgery.
Tumors of the Hair Follicle
Trichoepitheliomas are uncommon tumors involving hair follicles in cats. Both benign and malignant forms of these tumors appear. Tumors of the hair follicle can develop anywhere on the body but are most common on the head, tail, and legs. Benign forms appear as cysts 0.4 to 2 inches (1 to 5 centimeters) in diameter in the middle and lower layers of the skin. Expansion of the cysts or self-trauma may cause skin ulcers. Treatment is by surgical removal. However, cats that develop one such tumor are prone to develop more at other sites.
Malignant trichoepitheliomas are much less common than the benign form. They spread to the skin surface and cause extensive inflammation, tissue death, and fibrosis. Spread to other organs is uncommon. Surgery is required to remove malignant trichoepitheliomas. Because they spread, your veterinarian will likely remove tissue around the tumor as well.
Tumors Originating Outside the Skin (Metastatic Tumors)
Tumors that spread from a place of origin in the body to other locations in the body are described as metastatic tumors. It is unusual for tumors originating in another part of the body to spread (metastasize) to the skin. When it does happen in cats, the metastatic tumors usually appear on the paws or legs. If a metastatic tumor is found on one or more paws of a cat, veterinarians will usually check for the presence of lung cancer. Metastatic tumors that affect the skin are associated with very poor prospects for recovery.
Undifferentiated and Anaplastic Sarcomas
These are cancers of mesenchymal cells and are not common in cats. Mesenchymal cells are the cells that develop into connective tissues, blood, lymph nodes, and other organs. Cancers of mesenchymal cells are difficult to identify because, like the mesenchymal cells themselves, malignant forms of these cells are loosely organized and often undeveloped or undifferentiated. Once identified, surgery is the usual treatment. When removing these sarcomas, the veterinarian will usually remove not only the visible tumor but also a 2 to 3 centimeter margin of tissue surrounding it. This reduces the chance that the malignancy will recur. Radiation treatment is indicated following surgical removal to increase the tumor-free interval.
Papilloma viruses cause warts in most animal species. They are transmitted by direct contact, and possibly by insects. Infection can also be transmitted when a cat comes into contact with an infected object in its environment such as bedding, dishes, collars, leashes, or toys.
In cats, papillomavirus does not cause the hard, rounded warts seen in most other species. Instead, it leads to formation of a squamous cell carcinoma (see Skin Disorders of Cats: Squamous Cell Carcinomas).
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Thomas R. Klei, PhD; David Stiller, MS, PhD; Stephen D. White, DVM, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; Paul Gibbs, BVSc, PhD, FRCVS; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice E. Villalobos, DVM, DPNAP; Patricia D. White, DVM, MS, DACVD