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Mast Cell Tumors in Dogs

Dogs have two anal sacs, one on either side of the anus. These small, pouch-like sacs — which are located between the external and internal anal sphincter muscles — are lined with sweat glands that produce a smelly, brownish-gray fluid. When a dog defecates, a small amount of this fluid is excreted from the glands. It is believed that this is one of the ways a dog marks his or her territory. Although not common, tumors of the anal sac can occur. These are often aggressive tumors that can grow quickly and tend to invade the surrounding tissue.

Canine anal tumors have many names, including anal gland anal sac adenocarcinoma (AGASACA), anal gland carcinoma, anal gland adenocarcinoma, anal sac carcinoma, and anal sac adenocarcinoma. These terms all refer to the same type of cancer.

The symptoms of anal sac tumors in dogs may or may not be noticeable. For example, you might see a swelling or a growth in the perianal area. You might also notice your dog straining while trying to defecate, or that his or her stool suddenly appears different. However, it’s also possible that you’ll never know your dog has a tumor until it is discovered during a routine rectal examination performed by your veterinarian.

Types of Anal Gland Cancer in Dogs

Cancer of the anal sacs (also known as canine anal gland (or sac) adenocarcinoma or apocrine gland adenocarcinoma) is the most common malignant tumor found in a canine’s perianal region. Anal gland cancer, in fact, represents approximately 16.5%1 of all malignant rectal tumors in dogs. These tumors are typically found only in one anal sac but, on occasion, both sides may be affected.

Anal gland tumors tend to metastasize early and spread to surrounding body parts. Regional lymph nodes and the liver are most commonly affected, but anal gland tumors in dogs can also spread to the lungs. In addition, one of the side effects of anal gland adenocarcinoma in dogs is high blood calcium, which can damage the kidneys.

Some reports state that female dogs may be more likely to develop anal sac tumors, while other reports have concluded that neither sex is more prone to developing this type of cancer. While anal sac tumors can occur in any breed, Cockers, Springers, German Shepherds and Cavalier King Charles spaniels may be at a higher risk of developing this type of cancer. Age is a clear factor, as these tumors are more likely to occur in older canines.

Signs and Symptoms of Anal Gland Tumors in Dogs 

Your canine may not exhibit any noticeable symptoms of anal gland cancer. However, the following are some of the typical signs and symptoms for which to watch:

  • Problems with defecating: Including constipation, straining and painful bowel movements
  • Changes in your dog’s stool: Stool may be flatter, thinner or appear ribbon-like
  • Increased urination and thirst: You may notice your dog drinking and urinating more frequently due to increased calcium2 levels caused by an anal sac tumor
  • Issues with the anal sacs: Your dog may spend a lot of time licking this area, or you might notice a swelling or a growth in the perianal region 

Diagnosis and Staging 

  • Digital palpation: A thorough digital rectal examination of your canine can help your veterinarian or oncologist learn the size and extent of the tumor
  • Aspiration cytology: A fine needle is inserted into the mass and the cells are examined to rule out other possible issues, such as infections or inflammation
  • Biopsy: A biopsy is performed after the mass is removed to determine if it is malignant and to confirm the diagnosis
  • Blood and urinalysis: These tests are performed to get a general assessment of your dog’s health, organ function and to check for infections, signs of disease, and other issues
  • Three-view chest radiographs and abdominal ultrasound: These imaging studies are performed to determine whether the cancer has spread to other organs or to the lymph nodes

Treatment Options for Anal Gland Cancer

If the anal gland cancer has not spread, your veterinarian has several treatment options.

Surgery

A local treatment for anal sac tumors in dogs is surgery3. If the cancer has spread to other areas, such as the lymph nodes, those may also be removed.

Surgery With Chemotherapy

Chemotherapy may also be recommended after surgical excision if there is evidence that the cancer has spread.

Stereotactic Radiation Therapy (Concurrent With Chemotherapy)

PetCure Oncology’s stereotactic radiation therapy (SRT/SRS) delivers high doses of radiation with extreme precision to the cancerous tumor. In doing this form of radiation therapy, no surgery is needed.  SRT/SRS results in less damage to the healthy tissue surrounding the tumor compared to other forms of RT. Recovery time with SRT is also fast, with few to no side effects.

If the cancer has spread: 

Your veterinarian may try to treat your dog with a combination of RT/chemo and possibly surgery. The prognosis with this treatment is still unknown.

If the mass is not treated, the tumor will grow larger, which will make it increasingly more difficult for your dog to push feces through his or her anus. If your dog is not a candidate for surgery, veterinarians recommend feeding your canine stool softeners and soft food, which should make it easier for him or her to have a bowel movement.

Life Expectancy, Survival and Prognosis 

When it comes to anal gland cancer in dogs, life expectancy depends on several factors, including the type of treatment your pet receives:

  • After surgery, if the margins of the tumor are not clean: The cancer often returns in six months
  • After surgery and chemotherapy: Life expectancy is about 18 months
  • After SRT, concurrent with chemotherapy: Life expectancy is about 18 months to two years
  • Without treatment: Life expectancy is typically measured in months. Quality of life issues should be discussed. 

Other factors can affect your dog’s life expectancy. For example, a dog with lung metastasis or high calcium levels will typically have a shorter life span.

For more information on anal sac tumors or treatment, please contact your local PetCure Oncology Center. We have understanding and caring team members who are ready to answer your questions.

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