|Anal Sac (Apocrine Gland) Tumors in Cats and Dogs
|What are anal sac (apocrine gland) tumors?
Anal sacs are paired structures, one sac on each side of the anus, which are lined by many glands. These glands
produce 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,
frequently producing a hormone which 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 has often already metastasizes (spread) to other sites such regional lymph
nodes, lumbar spine or the liver, spleen, or lungs.
Anal sac adenocarcinoma
How common are anal sac tumors in cats and dogs?
Anal sac adenocarcinomas make up ~2% of skin tumors in dogs and less than 1% of skin tumors in cats. The tumors
typically occur in older dogs and some studies suggest that females may be at higher risk of anal sac cancer.
What are the symptoms of anal sac tumors in cats and dogs?
Signs may vary from observing a lump in the anal sac to signs secondary to increased calcium. Common symptoms
include difficult or painful bowel movements, scooting, constipation, swelling of the area around the anus, licking of the
area , ribbon-like stool and bleeding as a result of local irritation. If the pets have high calcium levels as a result of anal
sac tumors, other symptoms may be present such as increased thirst, increased urination, decreased appetite, weight
loss, vomiting, muscle weakness, and low heart rate. If the tumor has spread to the spine, the pets may experience lower
back pain or stilted gait.
How is the diagnosis of anal sac tumors made?
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 actually 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. The samples are then examined under the microscope by a
veterinary pathologist. To test whether the 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 more complete assessment.
Does cancer cause pain in pets?
Pain is common in pets with cancer, with some tumors causing more pain than others. In addition to pain caused by the
actual tumors, pets will also experience pain associated with cancer treatments such as surgery, radiation therapy or
chemotherapy. Untreated pain decreases the pet's quality of life, and prolongs recovery from the illness, treatment or
injury. It is, therefore, essential that veterinary teams taking care of pets with cancer should also play a vital role in
educating pet owners about recognizing and managing pain in their pets. The best way to manage cancer pain in pets is
to prevent it, a term referred to as preemptive pain management. This strategy anticipates pain ahead of time and
administers pain medication before the pet actually experiences pain, thus ensuring the pet's maximum comfort.
To learn more about how to recognize pain in pets with cancer and what cancer pain management options are available
for your pet, please visit the Cancer Pain Management section.
How important is nutritional support for pets with cancer?
Cancer cachexia (a term referring to progressive severe weight loss) is frequently observed in pets with cancer. Pets with
cancer lose weight partly because of lack of appetite and partly because of cancer-induced altered metabolism. Some of
the causes for decreased appetite are related to the cancer itself (for example, tumors may physically interfere with food
chewing, swallowing, and digestion process) and some may be related to the side effects of cancer treatment (for
example, some chemotherapy drugs cause nausea and vomiting, and radiation therapy can cause mouth inflammation).
Proper nutrition while undergoing cancer treatment is essential to maintain your pet's strength, improve survival times,
quality of life and maximize response to therapy. Adequate nutritional support was shown to decrease the duration of
hospitalization, reduce post-surgery complications and enhance the healing process. Additionally, pets with cancer need
to be fed diets specifically designed to provide maximum benefit and nutritional support for the patient. To learn more,
please visit the Cancer Nutrition section.
What are the treatment options for anal sac tumors in cats and dogs?
The recommended 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 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. If non-absorbable
sutures are used, they are removed in approximately 10-14 days. Pets may need to take stool softeners until tissue
swelling resolves, and pain medications should be prescribed to make the pet more comfortable given the invasive nature
Metastasis 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.
In addition to surgery, chemotherapy may be used to kill any remaining cancer cells left behind after the surgery. Even is
there is no evidence of metastases 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 all of the tumor. While unlikely to cure the cancer, it can 'buy' the pet more time.
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.
What are the treatment associated risks?
Any surgical procedure has the rare risk of anesthetic death but the use of modern anesthetic protocols and careful
monitoring have largely minimized this 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 post-operative 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 to empty the bowel, and discomfort.
Are there any clinical trials investigating new treatments for cats or dogs with anal sac tumors?
UC Davis is currently conducting a clinical trial to compare two chemotherapy regimens (carboplatin plus piroxicam versus
mitoxantrone plus piroxicam) in dogs with anal sac tumors after they underwent surgery to see which of the two treatments
is more effective. This trial can be conducted at any clinic provided that the clinic is able to administer chemotherapy and
can report results to UC Davis. If you would like to participate in this study, have your veterinarian call UC Davis at
530.752.1393. Although there is no financial incentive for this trial, participating will allow veterinary oncologists evaluate
which of the two regimens works better so that other dogs with anal sac tumors are given the superior regimen in the
Additionally, there are several clinical trials available for cats and dogs with any tumor type for which your pet may qualify.
To learn more these trials (which are partially or fully funded by the institutions), please visit the Dog Clinical Trials (any
tumor type) or Cat Clinical Trials (any tumor type) section.
To learn more about veterinary clinical trials in general, please visit the Pet Clinical Trials section.
What is the prognosis for anal sac tumors in cats and dogs?
The prognosis will largely depend on the extent of the disease at the time of diagnosis. Pets with local disease (one that
has not spread to other organs) whose tumor was completely removed by surgery face much better prognosis than pets
whose cancer has already spread, or whose tumor could not be completely removed.
There is only 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
(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% 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 a median survival after surgery of 15.5 months (range from 3 to 35
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 combination. The
results showed that 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 (>10 cm2), the median survival was 292 days compared to 584 days for those with smaller tumors (<10
cm2). 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)
How do I find a qualified veterinary oncologist?
To locate a qualified veterinary oncologist worldwide who can discuss with you appropriate cancer treatment plan for your
pet's cancer condition or to provide you with a second opinion, please visit the "Locate a veterinary oncologist" section.
Additional online sources about anal sac cancer in pets
1) Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
2) Morrison Wallace B. Cancer in Dogs and Cats: Medical and Surgical Management. Baltimore: Williams&Wilkins, 1998.
3) Williams LE, Gliatto JM, Dodge RK, et al. Carcinoma of the apocrine glands of the anal sac in dogs: 113 cases (1985-
1995). J Am Vet Med Assoc, 223(6): 825-31, 2003.
|PET CANCER CENTER
Comprehensive guide to cancer diagnosis and treatment in cats and dogs