Anal Sac Adenocarcinoma in Dogs

The anal sacs are two small pouches that are located on each side of the anal opening. These sacs contain modified sweat glands that produce foul-smelling, brownish secretions that are eliminated during defecation, most likely to mark your dog’s territory. Tumors can grow on both anal sacs, but they more commonly only affect one, and often spread to other organs, such as the liver and lungs.

Anal sac adenocarcinoma is an uncommon condition affecting older dogs that can cause tumors to grow on the anal sacs. These tumors are malignant and usually spread to other areas of the body, including the lymph nodes. Tumor growth can produce hormones that cause hypercalcemia, or an elevated level of calcium in the blood that can weaken bones, create kidney stones, and even lead to kidney failure.

The tumors of this type of disease often grow inward, and though a lump may be visible, the actual tumor may be quite larger. These tumors can cause your dog’s anal sacs to become infected or blocked, resulting in activities of licking, scooting, or problems with elimination. Since hypercalcemia is so common with this condition, symptoms such as excessive drinking and urination can often point to tumor growth. Signs include:

• Constipation
• Swelling or lump in the anal region 
• Rectal pain
• Blood in the stool
• Scooting rear end along ground 
• Excessive licking of anal area
• Straining or difficulty passing feces
• Thin, ribbon-like stools
• Hypercalcemia, or elevated blood calcium
• Loss of appetit
• Increased urination
• Increased thirst 
• Lethargy 
• Vomiting
• Muscle weakness
• Arching of the back
• Decreased activity
• Slow heart rate

When mutations occur to the DNA within cells, they can cause cells to grow uncontrollably. While it is not yet known what causes the mutations to occur, research has suggested that a hormone imbalance involving testosterone may play a role in this specific type of cancer. 

Anal sac adenocarcinoma has been diagnosed in both male and female dogs. There are breeds who may be predisposed to developing these types of tumors, and can include:

• Cocker Spaniels
• English Springer Spaniels 
• German Shepherds
• Dachshunds
• Alaskan Malamutes

Your veterinarian will begin with a physical exam which can reveal a mass in the anal area. A digital rectal examination is often given. The lymph nodes may also be palpated to see if they are enlarged. 

Diagnostic testing can include taking a blood sample for blood and serum tests, and a urinalysis to assess the health of the kidneys and other internal organs. These can reveal elevated calcium levels in hypercalcemic dogs. This type of cancer has a high rate of metastasis, and imaging techniques, such as chest X-rays and abdominal ultrasounds, can help to determine if the tumors have spread to the lymph nodes or internal organs, such as the spleen or liver. 

Once the tumors have been located, a biopsy is taken through fine-needle aspiration to determine if the tumor is indeed cancerous. Other testing methods can include a parathyroid hormone test, or an exploratory laparotomy if the iliac lymph nodes are enlarged.

Treatments can vary, but almost always begin with surgery to remove the tumor and a wide area of surrounding tissue. This often gives the best results, relieving any symptoms related to defecating. If the cancer has spread to the lymph nodes, those tumors are excised as well. Your dog may need multiple surgeries to fully alleviate the tumors as they spread and grow fast. A possible complication of the surgery to remove the primary tumor in the anal sacs is fecal incontinence.

Other treatments can include chemotherapy, which can kill any remaining cancer cells left after surgery and can improve recovery up to 50%. Radiation therapy can be used in combination with other treatments, or when the tumor cannot be surgically removed. Complications associated with radiation treatments can include chronic diarrhea and rectal pain and may not appear for many weeks or months.

Supportive treatments may be needed, and can include medication to control hypercalcemia, stool softeners if your dog is having trouble passing feces, and possibly the addition of a high fiber diet. Other drugs that may be prescribed include corticosteroids, antibiotics, opiates for post-surgery pain management, and anti-inflammatories. Topical treatments may also be helpful, such as vitamin E and aloe.

After surgery, your dog may be given an Elizabethan collar to prevent him from interfering with his anal area. You may need to change dressings and monitor recovery of the surgery site, as well as administer medications, which often includes a stool softener to be administered for many weeks. Usually, your dog will be checked by your veterinarian after two weeks, then will be scheduled for routine check-ups every three months for up to two years. 

Recovery can vary. Often, by the time symptoms are present, the cancer has spread to other tissues of the body. Survival time can be from 12 to 18 months past treatment, and in some cases, kidney failure is permanent.

While it is impossible to predict when and where cancer can develop, having your dog undergo a rectal examination during routine exams can increase your chances of detecting this condition early. Any time your dog shows elevated calcium levels, he should also be checked for anal sac adenocarcinoma.