Anal Sac Tumors in Cats and Dogs

  • Anal sac (apocrine gland) tumors can be found by rectal examination.
  • Common symptoms include difficult or painful bowel movements, scooting, swelling around the anus, ribbon-like stool, and bleeding as a result of local irritation.
  • Anal sac tumors can result in high calcium levels, which will cause increased thirst increased urination, decreased appetite, weight loss, vomiting, muscle weakness, and low heart rate.
  • Surgical removal of anal sac tumors is the treatment of choice whenever possible.
  • It is estimated that cancer has spread in 50-80% of cases at the time of diagnosis.
  • Dogs whose cancer spread to other organs have a median survival of 6 months after surgery compared to 15.5 months for dogs without metastases (spread of cancer).
  • The tumors typically occur in older dogs and some studies suggest that females may be at higher risk of anal sac cancer.

Anal sacs are paired structures, one sac on each side of the anus, which are lined by many glands. These glands produce a liquid that is expelled with each bowel movement as a form of territorial marking. Anal sac tumors arise from the glands of the anal sac, and may be either benign (known as anal sac adenomas) or malignant (known as anal sac adenocarcinomas). Anal sac adenocarcinoma is very rare in cats but has been reported. The tumor itself usually affects only one of the two anal sacs; however, some pets may have tumors in both. The tumor can be very small or quite large, sometimes producing a hormone that causes blood calcium levels to rise above normal levels. The high level of calcium is called hypercalcemia and can cause problems by damaging the kidneys. Unfortunately, by the time the diagnosis of anal sac adenocarcinoma is made, the tumor may have already metastasized (spread) to other sites such as regional lymph nodes, lumbar spine or the liver, spleen, or lungs.


Anal sac adenocarcinomas are usually identified during physical examination of the anus, although some tumors are not always easily felt. Some tumors are found during routine rectal examination for impacted anal glands or when taking the animal’s rectal temperature. If a mass is detected veterinarians will typically perform several follow-up tests. Complete blood count (CBC), serum chemistry profile, and urinalysis will identify possible hypercalcemia, evaluate the pet’s overall health, and help identify any other abnormalities. To confirm whether the mass is benign or malignant, a fine needle aspirate or tissue biopsy is performed. Incisional biopsy (when a small piece of the tumor is taken) is usually performed on large tumors under sedation and local anesthesia whereas excisional biopsy (when the entire tumor is removed) is usually performed on small tumors under general anesthesia. Additionally, cells can also be isolated from the sublumbar lymph nodes (lymph nodes that are close to the anal sac and the first site to which the tumor will spread) to determine if the tumor has already spread to this region. To test whether cancer has spread to other organs, abdominal X-rays, chest X-rays, and abdominal ultrasound are performed. Advanced imaging techniques such as CT scans can be used to get a more precise and complete assessment.

  • Surgery: The initial treatment for anal sac tumors is complete surgical excision. Whenever possible, the surgery will remove the anal sac tumor as well as a wide margin of normal tissue around and under it in order to maximize the likelihood that no tumor cells are left behind. Depending on the tumor’s size, its surgical removal may very infrequently result in fecal incontinence and you should discuss this with your veterinary surgeon to get a clearer idea of its severity given your pet’s condition. If recovering well without complications, most pets are discharged 1-2 days after surgery. Pets may need to take stool softeners until tissue swelling is resolved, and pain medications should be prescribed to make the pet more comfortable given the invasive nature of surgery. Metastasis (spread of cancer) to the nearby sublumbar lymph nodes occurs in more than 50% of cases, therefore, their surgical removal may be required if the lymph nodes are enlarged. If the pets showed increased levels of calcium (hypercalcemia), it will usually resolve on its own 24-96 hours after the surgery, however, it is recommended to periodically measure the pet’s calcium levels to monitor possible cancer recurrence or metastasis. Pets with persistently high calcium levels may be given medication in order to prevent damage to the kidneys.
  • Chemotherapy: In addition to surgery, chemotherapy may be used to kill any remaining cancer cells left behind after the surgery. Even if there is no evidence of metastasis at the time of diagnosis, it is possible that some tumor cells are already circulating throughout the pet’s body, getting ready to establish new tumors in distant organs. Chemotherapy can also be used in pets if the tumor could not be removed with surgery, has already metastasized at the time of diagnosis, or if the surgery was not able to remove the entirety of the tumor. While unlikely to cure the cancer, it can offer the pet more quality time.  Many patients experience successful management of their disease for months to up to two years.
  • Radiation Therapy: Radiation therapy is typically used to treat anal sac tumors that could not be removed by surgery or if the surgery was not able to remove the entire tumor.

Any surgical procedure has the rare risk of anesthetic death but the use of modern anesthetic protocols and careful monitoring have largely minimized the risk. Because the surgery is performed near the anus, there is an increased likelihood of developing an infection but the administration of antibiotics after the procedure should control this potential complication. If the sublumbar lymph nodes are also being removed, there is a risk of bleeding during the surgery because the tumor-invaded lymph nodes are often closely associated with blood vessels. These lymph nodes are also very close to nerves, especially those connecting with the bladder, and their removal may cause nerve damage leading to temporary or in some cases permanent postoperative urinary incontinence (inability to control urination). Fecal incontinence (inability to control bowel movement) can occur in a small percentage of animals after the surgery. For pets having difficulty defecating, high-fiber diet and/or stool softeners are usually prescribed to alleviate this problem. Pets undergoing radiation treatment may experience radiation complications such as mild to severe moist desquamation (shedding of skin cells that were exposed to radiation), colitis, difficulty emptying the bowel, and discomfort.


The prognosis will largely depend on the extent of the disease at the time of diagnosis. Pets with local disease (has not spread to other organs) whose tumor was entirely removed by surgery have a much better prognosis than pets whose cancer has already spread, or whose tumor could not be completely removed.

There is only a limited number of published studies regarding outcomes of anal sac tumors in pets. One study of 32 dogs showed that female dogs had a worse prognosis compared to male dogs, and had a 50% chance of cancer recurrence (cancer coming back) after surgery. It is estimated that by the time diagnosis of anal sac tumors is made, the cancer has spread in 50-80% of cases. Dogs whose cancer metastasized had a median survival after surgery of 6 months (range from 1.5 to 39 months) whereas dogs without metastasis had median survival after surgery of 15.5 months (range from 3to 35 months).

The largest published study involved 113 dogs with varying stages of anal sac cancer treated with different therapies. Of these 113 dogs, 104 underwent treatment consisting of surgery, radiation therapy, chemotherapy, or a combination. The results showed that the median survival for the treated dogs was 544 days (meaning that at 544 days, 50% of the dogs were alive). The study also showed that dogs treated with chemotherapy alone had shorter survival (median of 212 days) compared to those who received other treatments (median of 584 days). Dogs who did NOT receive surgery experienced shorter survival (median of 402 days) compared to those who did undergo surgery (median of 548 days). For dogs with large tumors (>10cm2), the median survival was 292 days compared to 584 days for those with smaller tumors (<10cm2). Dogs whose blood tests show increased calcium levels faced shorter survival (median of 256 days) compared to those with normal calcium levels (median of 584 days). Dogs with metastases to the lungs had significantly shorter median survival (219 days) compared to those without metastases (548 days) (Williams, J Am Vet Med Assoc, 2003)