Carcinoma of the Apocrine Glands of the Anal Sac in Dogs
What is Apocrine Gland Carcinoma of the Anal Sac?
Apocrine gland carcinoma of the anal sac is detected as an incidental finding during annual examination in approximately 50% of dogs diagnosed with this tumor, and rectal palpation should therefore be part of any complete physical examination. Tumors are locally invasive and have a moderate rate of spread to other places in the body (called metastasis). Apocrine gland carcinoma of the anal sac can also produce a pseudohormone leading to marked elevations in blood calcium levels (paraneoplastic hypercalcemia).
Diagnostic tests are recommended to determine which sites in the body are involved and to determine whether paraneoplastic hypercalcemia is present. These tests include:
- Complete blood count (CBC)
- Serum biochemistry panel
- Urinalysis (U/A)
- +/- ionized calcium
- Thoracic radiographs
- Abdominal sonogram
- +/- Contrast urethrocystogram
Treatment for apocrine gland carcinoma of the anal sac is aimed both at controlling the tumor in the site where it originates and delaying or preventing metastasis. Controlling the tumor in the site where it originates is best accomplished with either wide surgical excision (which may be difficult for large tumors in this area) or the combination of surgery to remove obvious gross disease and radiation therapy to destroy residual microscopic disease. In conjunction with local treatment, we recommend systemic carboplatin chemotherapy to delay or prevent the onset of metastasis. Carboplatin is given at 3-week intervals for a total of four treatments. Doxorubicin is also believed to have activity against this cancer. Chemotherapy drugs work by targeting fast-growing cells; cancer cells divide faster and abnormally compared to normal cells. Because of this, chemotherapy preferentially destroys cancer and spares normal tissues. There is the potential for some normal tissues with rapid growth rates (intestinal lining, bone marrow, hair cells) to be transiently damaged by chemotherapy. However, our goal in treating cancer in animals is quality of life, so the dosages are lower than those used in people. We do not normally see significant toxicity with chemotherapy. There is always some risk when a drug is given for the first time, and the oncologist will inform the owner what types of side effects may develop and what to watch for. If the pet has no problems the first time they receive a drug, they should have no problems with subsequent treatments of that drug. If the pet experiences any side effects, we address them as needed and lower the dosage of the causative drug for future treatments to prevent recurrent problems.
We also recommend piroxicam, an oral Non-Steroidal Anti-Inflammatory Drug (NSAID) which has anti-inflammatory and analgesic properties as well as possible anti-tumor activity against carcinomas. This medication should be given with food and the pet owner instructed to watch for signs of gastrointestinal upset (vomiting, change in stool, etc.). If such signs develop, piroxicam should be discontinued for 3-5 days. Restarting of therapy can then be considered in conjunction with the stomach-protectant misoprostil.
If definitive treatment is not an option, palliative radiation therapy can be considered. Palliative radiation therapy consists of two consecutive twice-daily doses of radiation therapy and is aimed at alleviating pain, inflammation, and swelling associated with the tumor. We recommend that the treatment be repeated in one month if no disease or symptom progression is noted in hopes of extending the duration of response.
The reported median survival time for dogs with apocrine gland adenocarcinoma of the anal sac treated with surgery alone is approximately 12 months. This may be improved upon with the addition of radiation therapy and chemotherapy, and a more recent study based at North Carolina State University reported an overall median survival time of 18 months for dogs treated with a variety of treatment combinations. Four negative prognostic factors have been identified: tumor size, pulmonary metastasis, sublumbar lymph node metastasis, and hypercalcemia; each of the latter two have been associated with a significantly shorter survival time of approximately 6 months.
- Williams LE, JM Gliatto, RK Dodge, et al. Canine carcinoma of the apocrine glands of the anal sac: 113 cases (1985-1995). J Am Vet Med Assoc 2003;223(6):825-831.
- Ross JT, Scavelli TD, Matthiesen DT, et al. Adenocarcinoma of the apocrine glands of the anal sac in dogs: a review of 32 cases. J Am Anim Hosp Assoc 1991;27:349-355.