ANAL SAC TUMORS IN DOGS
Overview of the Condition
Anal sacs are small pouches located on either side of the anus in dogs, cats, ferrets, and other animals. The lining in the sacs produces a smelly brown liquid that usually drains from the sac during defecation. The purpose of anal sacs is unknown; it's possible that the strong scent from these glands helps animals communicate information about themselves or their territories. Anal sacs can become inflamed, impacted (blocked), and infected, causing the animal to scoot its bottom on the carpet or chew at itself. Tumors of the anal sacs (apocrine gland adenocarcinomas) are uncommon in dogs but can become serious problems because they can invade local tissues, such as the rectum, or spread to other organs (metastasize), even when the primary tumor is still very small. Additionally, this tumor can cause increased blood calcium levels (hypercalcemia of malignancy) that can lead to kidney failure.
Anal sac tumors are found in male and female dogs and can occur in any breed. The average age at presentation is approximately 10 years. Clinical signs may be associated with the primary tumor or with kidney disease resulting from hypercalcemia. The veterinarian may also find a mass during rectal palpation when no clinical signs are present.
Figure 1a: A firm mass (arrows) is located in the area of the anal sacs in this dog.
Clinical signs associated with the primary tumor:
- Pain when defecating
- Straining to defecate
- Blood in stools
Clinical signs associated with kidney failure from high blood calcium:
- Lethargy (weak or tired)
- Loss of appetite
- Increased water intake
- Increased urinations
If a mass is felt in the area of the anal sac, it can be aspirated (cells removed with a needle and syringe) to determine if cancer cells are present. If anal sac carcinoma is confirmed, abdominal ultrasound and thoracic radiographs (chest x-rays) should be performed to search for evidence that the tumor has spread to the lymph nodes or lungs. Blood chemistries and urine are tested to check for high calcium and any evidence of kidney damage, since high blood calcium and subsequent kidney failure are associated with poor survival rates in animals with anal sac tumors. Hypercalcemia of malignancy (high calcium as a result of hormones secreted by the tumor) is detected in approximately 25% of dogs with anal sac carcinoma.
Figure 1: The anal sac adenocarcinoma has spread to the abdominal lymph nodes just under the spine (large arrows). Because the lymph nodes are so large, the colon (small arrows) is being pushed downward, increasing the risk of constipation.
Figure 2: Cancer cells from the anal sac tumor look large, dark blue, and foamy compared to the pale pink red blood cells.
Anal sacculitis (inflammation of the anal sacs) is very common in dogs and may occur as a single anal sac mass. Cytology (evaluation of the cells) of these masses usually reveals signs of inflammation only. Other tumors that may occur in the same area include benign perianal adenomas (common in old intact male dogs), rectal lymphoma, and rectal carcinomas. These can also be diagnosed with cytology or biopsy (tissue samples).
Although a variety of treatment combinations have been reported, surgery is the only method that has been proven to influence survival of dogs with anal sac carcinomas. Surgery treatment includes removal of the primary tumor and, in some cases, also the lymph nodes in the abdomen, which are affected by metastatic disease in half of dogs. In addition to surgery, most oncologists recommend radiation and chemotherapy in an attempt to prolong survival of affected dogs. Due to the highly invasive nature of anal sac carcinomas and the possibility that aggressive surgical removal may lead to fecal incontinence, radiation is commonly used when complete (clean) surgical margins cannot be achieved. Animals that present with hypercalcemia may need to be treated before surgery with intravenous fluid therapy, diuretics (to increase urine production), and corticosteroids or bisphosphonates to decrease blood calcium concentrations.
When to seek referral: Because of the invasive nature and location of these masses, referral to a board certified surgeon is recommended.
Potential Complications of Surgery
Surgical removal of large masses may lead to fecal incontinence. This may only be temporary, but clients need to be aware of this problem. Infections after surgery are uncommon. Dogs that have kidney damage from high calcium before surgery may continue to have kidney problems. The use of an Elizabethan collar for 10-14 days usually prevents suture dehiscence due to self-trauma.
Most animals recover very well and are discharged 1-2 days after surgery, unless complications occur. If non-absorbable sutures are used, they are removed in approximately 10-14 days. Some dogs will need stool softeners until tissue swelling resolves. Pain medications may be prescribed to make the dog more comfortable.
Prevention and Prognosis
No specific cause has been associated with anal sac carcinoma, which makes prevention difficult. Prognosis and survival depend on factors such as the type of treatment, size of the mass, and presence of hypercalcemia and metastatic disease.
In a recent study of dogs with anal sac adenocarcinoma, overall median survival was 584 days (meaning that half of the dogs were alive at 584 days). Dogs not treated surgically had shorter survival times (median, 402 days). Median survival is poorer in dogs with tumors larger than 4 inches (median survival, 292 days), hypercalcemia (median survival, 256 days), and spread to the lungs (median survival, 219 days). Interestingly, metastasis to the abdominal lymph nodes was not found to affect survival.
—Carlos Souza , DVM, MS
Small Animal Resident
Editor: Karen Tobias , DVM, MS
Reviewed 10/1/2011 by Mitchell A. Robbins, DVM, Diplomate ACVS