Anal Sac Cancer in Dogs

Lili Duda, VMD
Last Modified: June 23, 2006

Dear OncoLink "Ask the Experts,"
My dog has been diagnosed with anal sac cancer. What should I expect from his treatment?  
Thank you,

Lili Duda, VMD, Editor of the OncoLink Veterinary Oncology Section, responds:

Dear D.,
Anal sac adenocarcinomas are tumors arising from the apocrine glands present on either side of the rectum. They occur primarily in older (average 10 years) female dogs. Dogs might have symptoms such as straining to have bowel movements (either due to the tumor itself or due to enlarged lymph nodes in the pelvic area), or the tumors might be found incidentally on a routine physical examination. Sometimes dogs may have symptoms such as increased drinking and urinating which results from very high calcium levels in the blood (due to hormones produced by the tumor) which in turn can affect the kidneys.

Initial evaluation of a dog that is suspected to have an anal sac tumor should include a blood cell count and blood chemistries (particularly to evaluate blood calcium and phosphorus levels and kidney function) and ultrasound or X-ray examination of the abdomen to look for evidence of tumor spread to the pelvic lymph nodes. Dogs can have fairly small tumors in the anal sac and still have very enlarged lymph nodes. Chest X-rays might also be helpful because these tumors can also spread to the lungs.

Surgery is the first treatment approach. If the primary tumor is amenable to surgery, it should be removed. While this is unlikely to result in cure because these tumors tend to spread early in the course of disease, it might provide a significant disease-free interval with a good quality of life. The goals of surgery are to minimize symptoms and to return the blood calcium to normal if it is elevated. If the lymph nodes are enlarged, they can be removed as well. Surgery in these areas can be technically challenging and might require a surgical specialist. The benefits of surgery must be weighed against the risk of complications.

Radiation therapy can be used to control both the primary tumor and associated lymph nodes. It can be used alone or in combination with surgery, depending on the particular dog in question. Similarly, chemotherapy might be useful (either alone or in combination with other treatments) in causing regression of the tumor or delaying the progression of the disease. However, more studies need to be done to more fully define the best treatment protocols for this tumor. Treatment decisions are based on a thorough evaluation of each individual case and the clinical judgment of the oncologist and/or surgeon.

Unfortunately, regardless of treatment, most dogs will still eventually succumb to this tumor. For dogs treated with surgery alone, average survival times are about 9 months, with a range of several weeks to over three years. Some early anecdotal reports suggest that an aggressive combination of surgery, radiation and chemotherapy might improve on these results, but this remains to be confirmed.

If you or your veterinarian have not already done so, please consult a qualified veterinary oncologist to further explore the treatment options for your pet.

From the National Cancer Institute