Skin Cancer

Skin cancers are the most common tumor in dogs, accounting for approximately one third of all tumors. There are many types of skin cancer but mast cell tumors (MCTs or mastocytomas) occur most frequently and therefore will receive much focus in this section.  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 (6 to 14 years of age), although there are several types of skin cancers that occur in young dogs.

Skin cancers may be superficial (on or near the surface) or subcutaneous (under the skin) masses and it is important to differentiate the benign lesions from potentially malignant masses. In dogs, approximately 20-30% of skin cancers are malignant.  Some dermal masses with malignant potential are mistaken for benign lesions, therefore it is important to have all skin masses evaluated by a veterinarian quickly after they are detected.

Benign tumors are typically slow growing and will change very little over months and years. They are commonly well circumscribed, freely movable and non-painful. Malignant tumors are more likely to be rapidly growing, have ill-defined margins, infiltrative, and may be ulcerated (a formation of a break on the skin).  Skin cancers are divided into four categories, depending upon the cell type that is involved. These categories are epithelial tumors, mesenchymal tumors, round cell tumors and melanomas. In dogs, the most common tumors are lipomas, mast cell tumors, sebaceous gland adenomas/hyperplasia and papillomas.

Epithelial tumors
These tumors involve the skin itself, glands in the skin or the hair follicles.  Epithelial tumors include papillomas, squamous cell carcinoma, basal cell tumors, sebaceous gland tumors, sweat gland tumors, and perianal tumors.

Papillomas
cutaneous papillomas, warts, squamous cell papilloma. These are cauliflower like growths with small frond like structures. In young dogs they frequently form in multiples, while in older dogs they are usually solitary. They can occur on the head, eyelids, feet, and in and around the mouth. Viral associated papillomas are contagious but treatment is usually not necessary as they will regress within approximately 3 months. Surgery or cryotherapy are sometimes necessary and can be very effective.

Squamous Cell Carcinoma is the most frequently diagnosed carcinomas arising in the skin in dogs and usually develops on nonpigmental and hairless sections of skin. There are two forms—cutaneous and subungual. Cutaneous squamous cell carcinomas are tumors of older dogs, with bloodhounds, basset bounds, and standard poodles at greatest risk. Lesions commonly arise on the head, distal extremities, ventral abdomen, and perineum. Most cutaneous squamous cell carcinomas appear as firm, raised, 
frequently ulcerated plaques and nodules; sometimes they can be extremely exophytic and have a surface that resembles a wart.. These usually develop on ventral abdominal, preputial, scrotal, and inguinal skin in white-skinned, shorthaired breeds such as dalmatians, bull terriers, and beagles.  Subungual squamous cell carcinomas are most commonly found in schnauzers, Gordon setters, briards, Kerry blue terriers, and standard poodles. Generally, all are darkhaired breeds, and a dark coat color has been associated with the development of subungual squamous cell carcinomas arising on multiple digits, often on different extremities. Females have a slightly higher rate of occurrence and both the fore- and hindlimbs are equally predisposed to tumor development.

Surgical removal or amputation of the involved digit is the treatment of choice.  Excision may be combined with radiation or chemotherapy. Limiting exposure to the sun may help prevent solar-induced squamous cell carcinomas.

Basal cell tumors
Basal cells line the deepest layers of the skin and that is what is affected with basal cell tumors.  Basal cell tumors are benign and are usually solitary, well circumscribed, firm, hairless, elevated masses that are freely movable.  They most commonly appear in older dogs and poodles are predisposed. Tumors are most commonly located on the head, neck and shoulders and it does not metastasize. Treatment is surgical removal.  Sun exposure increases the risk for basal cell tumors.

Sebaceous gland tumors
This group of tumors include sebaceous hyperplasia, sebaceous epithelioma, sebaceous adenoma, and sebaceous adenocarcinoma. They are relatively common in older dogs and often appear on the eyelids, other areas of the head, trunk and feet. Sebaceous hyperplasia is the most common and may be solitary or multiple wart-like growths that appear pink and are usually less than 1 cm in diameter. In dogs, miniature schnauzers, beagles, poodles and cocker spaniels are commonly affected. All of the sebaceous gland tumors can be treated effectively with surgery alone.

There is often evidence of lymphatic invasion on Perianal tumors. Perianal (flepatoid) adenomas represent the majority of perianal tumors and are most common in intact male dogs, especially poodles.  It is a tumor of the hepatoid glands which occur in the perianal skin, but also in the skin of the tail, dorsal midline, and ventral abdomen. Castration alone can at times be curative. They tend to occur on the hairless skin around the anus, are raised, sometimes multiple masses that may become ulcerated and secondarily infected. Castrated males are more likely to have the malignant counterpart, perianal adenocarcinoma which may require more aggressive treatment.

Mesenchymal tumors
These tumors are derived from cells that surround or support the skin such as fat, connective tissue, blood vessels and nerves.  Typically, the cells of benign and malignant mesenchymal tumors have poorly-defined cell membranes as compared to epithelial tumors. The cells are not generally round; they may be spindle-shaped, polygonal, or dendritic, but uniform "roundness" is a characteristic of epithelial cells or of cells from a round cell tumor. Types of mesenchymal tumors include lipoma, 
fibrosarcoma and hemangiosarcoma.

Lipomas are benign fatty tumors that are common in dogs. Often dogs that have one lipoma will develop multiple tumors. It is important to aspirate and confirm that it is a lipoma as opposed to a possible mast cell tumor. Surgical resection is only indicated when the location and size of the tumor is causing a problem, e.g., an axillary lipoma that is causing problems with the mobility of the forelimb. Sometimes a tumor that appears to be a lipoma is what is known as an infiltrative lipoma, a benign tumor that looks like normal fat but that infiltrates surrounding normal tissue most commonly muscle. Treatment for this situation may require a combination of surgery and radiation therapy.

Fibrosarcoma
Fibrosarcoma is a type of cancer that arises from the fibrous connective tissues of the skull, spine, pelvis and ribs but can arise from any bone. Fibrosarcomas are typically locally aggressive with infiltration into the surrounding normal tissues but less likely to metastasize to other parts of the body. Tumors are often subcutaneous but can grow and extend through the skin resulting in ulceration of the overlying skin. Tumors can be a solitary mass effect, multi-nodular and often will have obvious attachments to the surrounding normal tissues. This cancer is seen most commonly in older male dogs, except for a certain variety that is seen in the mouths of younger dogs.Treatment options include wide surgical resection, or combination approaches with surgery and radiation, and with or without chemotherapy.

Hemangiosarcoma (dermal or subcutaneous forms) - Hemangiosarcoma tumors that arise in the skin are different than other types of hemangiosarcoma. The skin form of hemangiosarcoma is the most easily removed surgically, and thus have the greatest potential for complete cure.

The skin forms of hemangiosarcoma are classified as either dermal or subcutaneous. The dermal or cutaneous skin form looks like a rosy red or even black growth on the skin. This form is associated with sun exposure and thus tends to form on non-haired or sparsely haired skin (such as on the abdomen or inner thigh) or on areas with white fur. Dogs with short white haired fur (such as dalmatians, whippets, and pit bull terriers) are predisposed to the development of this tumor. Approximately 1/3 of cases will spread internally in the malignant usually associate with cancer so it is important to remove such growths promptly.

Subcutaneous (hypodermal or beneath the skin) hemangiosarcoma can occur anywhere on the body. Lesions appear as soft, fluctuant masses or firm, infiltrative structures, commonly with hemorrhagic discoloration and ulceration and can appear as bruise-like lesions. Staging is important because lesions can be primary or metastatic from another location. Surgical removal with complete margins is the treatment of choice but complete surgical excision can sometimes be difficult. Radiation and/or chemotherapy Round cell tumors. These tumors are named because of their appearance under the microscope and include mast cell tumors,  lymphosarcoma, histiocytomas, plasmacytomas, and transmissible venereal tumors.

Mast cell tumors (MCTs)
Mast cells are specialized cells that are normally found throughout the body and help animals respond to inflammation and allergies. Mast cells can release several biologically active chemicals when stimulated which include histamine, heparin, seratonin, prostaglandins and proteolytic enzymes. Although these chemicals are vital to normal bodily function, especially immune response, they can be very damaging to the body when released in chronic excess.

Mast cell tumors (MCTs) (also referred to as histiocytic mastocytoma, mast cell sarcoma, mastocystosis (when there is systemic involvement)) are cancerous proliferations of mast cells that can spread throughout the body.  The most significant danger from mast cell tumors arises from the secondary damage caused by the release of chemicals that they produce: gastric ulcers, internal bleeding, and a range of allergic manifestations.

These tumors are the most frequently recognized malignant or potentially malignant neoplasms of dogs. MCTs may be seen in dogs of any age, but the average age is 8-10 years. There is no way to definitively identify MCTs without a biopsy and pathology report.. It can be difficult not only to recognize mast cell tumors but to predict their course. They may be relatively innocent or aggressively malignant.

These tumors may develop anywhere on the body surface as well as in internal organs, but the limbs, especially the posterior upper thigh, ventral abdomen, and thorax are the most common sites. Location on mucocutaneous junctions or on the ventral surface of the body is associated with a more aggressive behavior. Many breeds appear to be predisposed, especially boxers, pugs, Rhodesian ridgebacks, and Boston terriers. The tumors vary significantly in size, shape, appearance and texture, but most commonly, they appear as raised, nodular masses that may be soft or solid which usually have dark granules in them. The granules contain substances which, when released, cause swelling, itching, and redness. Infrequently, when a large number of granules discharge their chemical contents into the bloodstream resulting in vomiting, stomach ulcers, shock and even death.

Treatment options are based on the type and grade of the tumor, surgical feasibility, and the presence or absence of the spread of malignant mast cells throughout the body. These options may include surgery, radiation, chemotherapy or other drugs such as prednisone.

Cutaneous Histiocytomas are common in dogs, particularly those younger than 3 years. Cocker spaniels, Doberman pinschers, boxers, Labrador retrievers and Scottish terriers are more frequently afflicted.  These tumors are typically solitary, fast growing, dome-shaped dermal masses with a hairless or ulcerated surface. These are typically benign tumors that may spontaneously regress in approximately 3 months and are cured with surgical removal as they do not metastasize.  These should not be confused with the more serious diseases of cutaneous histiocytosis, systematic histiocytosis or Histiocytic sarcoma and malignant histiocytosis.

Plasmacytomas
Also referred to as extramedullary plasmacytomas. Tumors can occur anywhere on the body involving haired skin, and have been identified at mucocutaneous junctions as well as in the oral cavity and other mucosal sites. They are usually raised, smooth, solitary nodules. They are typically benign and can be treated effectively with surgical resection. They are also responsive to radiation therapy. Alternatively, medical management is possible.

Transmissible venereal tumor (TVTs) typically involve mucosal membranes (genital tract or nasal cavity) and are sexually transmitted. They are typically red, proliferative growths that can potentially metastasize to the regional lymph node. Treatment is most commonly chemotherapy with vincristine administered weekly until the complete resolution of the lesion. Surgical resection is typically not performed as this tumor is quite responsive to chemotherapy, as well as radiation therapy.

Melanomas
These tumors demand immediate attention. They are more common in dogs with heavily pigments coats (Airedale's, Scottish terriers, spaniels and schnauzers) because melanoma is derived from melanocytes – the cells that provide pigment in the skin.  Sometimes melanomas are categorized as a round cell tumor because they are typically round, raised, and darkly pigmented. Tumors can be benign or malignant and can be located anywhere on the body.

Benign cutaneous melanomas of dogs are usually seen as round, firm, raised, darkly pigmented masses from one-quarter to 2 inches in diameter.  They occur most often on
the head, digits or back and generally do not metastasize.

Malignant melanomas can metastasize to any area of the body and often spread to the lymph nodes and lungs, which present very challenging and dangerous prospects for the dog. Specific locations are typically associated with more aggressive behavior with a higher metastatic potential such as in a digit, scrotum or oral cavity. The presence of malignant melanoma may be first discovered in the lungs where diffuse pulmonary infiltration of tumors will be displayed throughout the lung tissue on a radiograph (x-ray). Lymph node swelling or enlargement may be a sign of malignant spread of a melanoma. Some melanomas do not display the characteristic darkly pigmented color of most melanomas.  

Treatment of melanomas is best provided by surgical removal of the tumor and nearby surrounding tissue or amputation if it occurs in a digit. Melanoma involving the oral cavity often necessitates radical mandibulectomy, or removal of the a part of the associated lower jaw,  Localized tumors may be completely removed and the patient cured.  

However, if a malignant melanoma has had the opportunity to spread to distant areas of the body, long term survival of the dog is more difficult.  Radiation therapy or immunotherapy to regulate the immune system may be of benefit in selected cases. Chemotherapy has been used with marginal success.  Complete remissions of metastatic melanoma cases are rare.  Fortunately most cutaneous (skin) melanomas are benign, but individual growths need to be evaluated as unique and unpredictable since any given melanoma may become malignant.

If you discover a darkly pigmented, raised, thickened growth anywhere on your dog, be sure to have your veterinarian evaluate it.  Pigmented (black) areas of the skin are common in dogs , especially in the tongue, gum and eyelid tissues and these darkened areas may be completely normal for that individual.  However, if any darkly pigmented areas are actually raised above the normal surface or seem thickened or ulcerated or inflamed, an exam is indicated.      

Causes

There are many types of skin cancer in dogs.  The types included in this section include: Mast cell, squamous cell, and melanoma. The cause of most skin cancers in dogs is unknown. Exposure to the sun has been shown to cause a higher incidence of three types of skin cancer: squamous cell carcinoma, basal cell carcinoma and hemangioma. There may be a genetic basis for the development of certain cancers as certain breeds of dogs have been found to have a higher incidence of skin cancer. These include boxers, Scottish terriers, bull mastiffs, basset hounds, weimaraners, Kerry blue terriers and Norwegian elkhounds.  

Light-colored dogs with thin haircoats that spend time in the sun have a higher risk of developing certain forms of skin cancer.  Some breeds of dogs (boxers, Scottish terriers, bull mastiffs, basset hounds, weimaraners, Kerry blue terriers and Norwegian elkhounds) have been found to have a higher incidence of skin cancer thereby raising the distinct possibility of a genetic link.  Male dogs who have not been neutered have a higher incidence of perianal tumors.  Generally, the age of a dog also plays a role as middle age to older dogs are more likely to develop cancerous growths.

Symptoms

Skin cancer frequently presents as a lump or bump that may or may not be ulcerated. The surrounding skin may be red and/or flaky. In addition, the site may be itchy which can cause your dog to lick, bite or scratch the affected area frequently.

Systemic symptoms depend on the location and type of tumor, how aggressive it is and whether it has metastasized.  

Systemic symptoms include:

  • Appetite loss
  • Vomiting (maybe bloody)
  • Diarrhea
  • Lethargy
  • Coughing
  • Wounds that cannot heal
  • Enlarged lymph nodes

Skin cancer in dogs has a better chance of cure if detected and treated early so it’s critical to examine your dog at least on a monthly basis.  If your dog has a thick or long coat, it may be more difficult to detect skin cancer so be sure to separate the hairs to get a close look at the skin. Also look inside the mouth and  under the tail.  If you notice any of the following, contact your veterinarian immediately.  A delay in treatment can negatively affect outcome.

  • The presence of any lumps and bumps
  • Scaly or crusty lesions, or areas of skin that have changed color;
  • Lesions or tumors that do seem to heal or bleed easily (e.g. when being scratched);
  • Swelling in the breast tissue or discharge from a nipple

Systemic symptoms are variable, depending on the location of the tumor and the degree to which is has developed and/or spread. Signs of systemic involvement may include: loss of appetite, vomiting, bloody vomit, diarrhea, abdominal pain, dark or black feces, itchiness, lethargy, anorexia, irregular heart rhythm and blood pressure, coughing, labored breathing, various bleeding disorders, delayed wound healing, enlarged lymph nodes.

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 will need to be ordered by your veterinarian to determine if your dog has cancer, what form of cancer it is, and at what stage it is.  

In order to make a definitive diagnosis, your veterinarian may take a fine-needle aspirate from the growth to submit a sample for preliminary biopsy. The entire tumor will then need to be fully removed, if possible, and submitted for biopsy. Blood tests may include a complete blood count, serum chemistry profile, and buffy coat. The CBC may reflect low or high white blood cell count, low platelet count, elevated mast cell counts. The buffy coat is diagnostic (although subject to false-positives) and reflects mast cells circulating in the bloodstream where they are ordinarily not found in large numbers. A positive buffy coat suggests bone marrow involvement. Other tests may include urinalysis, lymph node aspirate, bone marrow aspirate, x-rays, and ultrasound. The pathologist assigns a histiologic "grade" to the tumor, a somewhat subjective assessment of how well differentiated the cells are and therefore how aggressively malignant the cancer appears to be. The veterinarian assigns a "stage" to the cancer, as well.

Grade
The pathologist will apply specialized stains to the tumor sample and microscopically examine it in order to determine its grade. The majority of MCT are Grade 1 tumors and are well differentiated and appear to have a very good prognosis with no treatment beyond complete surgical removal. Grade 2 tumors are moderately differentiated, and the prognosis and treatment options are perhaps most complicated and difficult to predict. Grade 3 tumors are poorly differentiated, very aggressive, and most likely to rapidly metastasize. They carry the poorest prognosis but are fortunately the least common. Histiologic grade is most predictive of prognosis.

Stage
Staging refers to degree to which the cancer has already spread at the time of diagnosis.
Tumors caught early - before they have invaded other tissues or caused signs of systemic disease have the most optimistic prognosis. MCT most commonly metastasizes to lymph nodes, bone marrow, liver and spleen, so much of the lab work focuses on these areas to detect and assess abnormalities. It is unusual for MCT to spread to the lungs, as is so common with many cancers. In simplest terms, Stage 1 refers to a single tumor with clean margins and no signs of spreading. Stage 2 and Stage 3 show progressively greater signs of invasion, perhaps to local lymph nodes, demonstrating dirty margins, or presenting as multiple tumors. Stage 4 involves systemic metastasis and carries a poor prognosis. 

Conventional treatment for skin cancer depends on the type of tumor and how advanced the disease is at the time of diagnosis. Treatment options include: Surgery may be performed if the tumor has not spread and the entire tumor can be removed without compromising function of the associated tissues. Radiation may be 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.

Like the treatment for skin cancer, the prognosis varies significantly depending on the type of tumor and how advanced the disease is. Some tumors are easily cured by surgically removing them, while others require radiation and/or chemotherapy treatments. There are situations where the only option available to a dog is supportive (palliative) care because the cancer is too advanced, the dog has other health complications or the owners may not want to pursue aggressive treatment.