Anal Sac Tumors

Anal sac tumors may be externalized and raw, or hidden. Either way, they are dangerous.

Anal sac tumors are insidious.

Buried in the tissue between the internal and external anal sphincters, they are often not detected until they are large. By then, they have often metastasized. For anal sac masses detected small, speed is essential, as these carcinomas spread early and aggressively. Primary tumors tend to grow slowly, so metastasis may have already occurred, even if the anal sac mass is tiny.


Because of their tendency to early dissemination, patients ideally should be evaluated prior to surgery to determine the exact status of:
• local invasion

metastasis to sublumbar lymph nodes (inside the abdomen, under the lower spine)
• lumbar spine, where sublumbar lymph node metastasis can spread to bone
• other lymph nodes around the body
• lungs, a less-common site of metastasis, but important for prognosis

I say “ideally,” because examination and X-rays alone cannot completely evaluate all of these areas. For example, magnetic resonance imaging (MRI) is the preferred means of determining the size and shape of the sublumbar lymph nodes. Access to MRI may have geographic and financial limitations. In that case, if the primary mass is of an operable size and location, and X-rays of the chest and abdomen are normal, surgery may still be an option.

Ultrasound may be more readily available and less expensive than MRI, and is efficient in evaluating lymph nodes, as well as the liver and spleen, additional organs to which anal sac tumors sometimes metastasize.

How would one know his dog has an anal sac tumor? Kim L. Cronin, DVM, Dipl. ACVIM, recommends a rectal examination be a routine part of every physical examination. Masses may also be palpated when emptying anal sacs (to read the basics on anal sacs, click here) because of scooting, dragging the behind, or licking the perineum. Some dogs will experience a sense of pressure in the perineal area, and may strain to defecate. This straining is called tenesmus, and increases with increase in tumor size and invasiveness. If spread has occurred to the sublumbar lymph nodes, large tumors may impinge on the colon, adding to tenesmus and even restricting flow of stool through the colon.

Also, laboratory testing on anal sac tumor patients may reveal hypercalcemia.  After lymphoma, it is the second most likely tumor type to produce elevated calcium levels in the bloodstream. Persistent elevation of serum calcium especially damages kidneys. Approximately 25% of anal sac tumor patients will experience hypercalcemia. This interruption of calcium homeostasis occurs because of the tumor’s effect on the parathyroid glands. Even very small tumors may cause significant hypercalcemia.

Complete Blood Count (CBC), Chemistry Profile  and Urinalysis are routine parts of any pre-treatment workup, and serum calcium level is a routine part of the chemistry profile.

In perhaps the most complete retrospective (case-review) study of anal sac tumors to date, Dr. L. E. Williams, et al, in 2003*, wrote in the Journal of the American Veterinary Medical Association (JAVMA), “Carcinoma of the apocrine glands of the anal sac in dogs, (1985-1995).” Therein, he wrote that, in patients with tumors less than 10 cm (4 inches), the mean survival time was 584 days. However, in dogs with tumors over 10 cm, mean survival time was only 285 days.
In the same study, it was found that dogs with normal serum calcium levels survived an average of 19 months while those with hypercalcemia lived only 9.6 months.

As would be expected, the presence of pulmonary (lung) metastasis reduced the survival time from 18 months to seven months.

Dr. Cronin, referencing the study, says, “Interestingly, the presence of iliac [part of the sublumbar group of nodes] lymph-node metastasis did not affect the overall survival time in this study. This supports the recommendation to treat dogs with regional lymph-node metastasis aggressively.”


While treatment is tailored to each patient’s needs, with consideration for each owner’s budget, it can include surgery, radiation and chemotherapy.

Dogs treated with surgery alone have been reported to have survival times of 180 to 360 days.

Surgery to remove masses from the anal sacs is the starting point. However, nestled between two muscle groups responsible for fecal continence, masses that invade the sphincters, colon, and/or pelvic canal may be impossible to remove completely. Even making the masses smaller, however, can help reduce hypercalcemia.

Anal sac carcinoma is unusual in that even when certain metastases occur, surgery may still be an option. The sublumbar lymph nodes can be removed surgically in many cases. However, blood vessels to the nodes can be problematic, and hemorrhage is the most common complication of this aspect of treatment. Ultrasound of the area may not accurately predict the size and quantity of blood vessels to the nodes.

Radiation can be used as a followup to surgery, or as stand-alone treatment. The best outcomes from radiotherapy occur when only microscopic tumor cells persist, and can be curative in some cases. The sublumbar lymph nodes should also be irradiated, even if no detectable neoplasia is present there. To repeat, anal sac carcinoma spreads early, and these are the nodes it metastasizes to first. It only makes logical sense to eliminate any possible cancer there.

Radiation is also helpful in patients whose tumors are too large to excise surgically. Tumor size can be reduced by radiation, although no cure is expected. Tenesmus and discomfort may be alleviated or improved by radiation therapy.
Radiation treatment can be costly and logistically challenging, as it is available only at select facilities. On the other hand, it can substantially increase survival time.

Chemotherapy can also help to improve survival time. As microscopic metastases probably exist in every patient by the time of diagnosis, chemotherapy can go everywhere in the body, zapping cancer cells where it finds them. In the Williams case-review study cited above, survival times averaged 212 days in patients receiving chemotherapy alone, whereas surgery and chemotherapy together yielded an average of 589 days’ survival.

Anal sac tumors comprise only two percent of skin-type tumors of dogs. However, as you have seen, the complexity of the disease causes them to be a common cause of referral to specialists.
See you next week, Dr. Randolph.
*Yes, 2003 was ten years ago, as of this writing. It is important to recognize that anal sac tumor behavior has not changed since 1985, and retrospective studies such as Dr. Williams’ require a huge investment of time, money and man-hours. Therefore, his findings are still valid, and it will likely be a long, long time before another study that size is performed on anal sac carcinoma.


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