All dogs and cats have a pair of glands on either side of their anus called anal sacs. The most common type of anal sac tumor is apocrine gland adenocarcinoma. Almost all nodules of the anal sac are cancerous. This type of cancer is rare in the cat. Anal sac adenocarcinoma metastasizes (cancer spread) commonly (50-90% of the time) and often spreads to local, draining lymph nodes (the nodes that sit within the abdominal cavity close to the spinal column), liver, lungs and bones. In approximately 30% of cases, dogs may develop high blood calcium. When this occurs, it may cause increased thirst and urination. Rarely it may cause decreased appetite, vomiting and tiredness/lethargy. Common signs you may notice due to the cancer include straining to defecate, pain or discomfort or blood in the stools.
A presumptive diagnosis of this cancer type is made with a fine-needle aspirate and cytology, which involves passing a small needle into the mass, aspirating the cancer cells onto a slide and documenting the characteristic cells. Once a presumptive diagnosis is made, most oncologists are comfortable recommending surgery to remove this cancer. This surgical biopsy will confirm the high likelihood that your dog has been diagnosed with the most common tumor in this region, anal sac adenocarcinoma. Rarely, other cancer types can be diagnosed. Therefore this biopsy is important before further therapy can be recommended.
Prior to surgery, we recommend ensuring that cancer spread has not already occurred with staging tests. Unfortunately, at the time of diagnosis, it is not uncommon for metastasis to already be present. The most direct and sensitive way to stage your pet is with a CT-scan of your pet’s abdomen and chest. Other options include chest and abdominal x-rays and abdominal ultrasound. A treatment plan will be tailored to your pet’s cancer depending on the information gathered from the biopsy report, staging tests and you and your pet’s needs.
Treatment and Prognosis:
Therapies that may be discussed with you and your family include surgery, radiation therapy and/or chemotherapy.
Due to the location of the cancer, surgical removal is often marginal (narrow margins). Therefore, following surgery, often further therapy is recommended to decrease the risk or delay the time until recurrence develops. Because of the high risk of metastasis, therapy directed at decreasing the risk or delaying the time until cancer spread (metastasis) is also important and would involve treatments that would reach all areas of your pet’s body, called chemotherapy. Chemotherapy may also help to kill the residual cancer cells likely present at the surgical margins and can be used alone or, preferably, in combination with radiation therapy. There are numerous chemotherapy options including oral and injectable chemotherapeutics.
Prognosis depends on many factors including the size of the tumor at the time of surgical removal, the presence of distant and local metastasis, the presence of high blood calcium, and type of treatment(s) pursued. Smaller tumors and a combination of therapies are associated the longest survival, many times years. Although anal sac adenocarcinomas are likely to recur with surgery alone, dogs that were able to have their cancer removed, in general, will live significantly longer.
Many dogs with this cancer type can live many months and even years with proper therapy. Most dogs, once their tumor is removed, can live a normal life. Our goal as veterinary oncologists is to provide a great quality of life while extending your pet’s life. Therefore, all therapies recommended have a small chance of complications or side effects. Your oncologist will discuss all of the uncommon risks and the probable benefits from each therapy before determining the plan going forward.