Although anal-sac tumors make up only 2 percent of all cutaneous neoplasms in dogs, they comprise a significant portion of
the referrals to veterinary surgeons and oncologists.
Because of the large size of these tumors at the time of diagnosis and the early development of metastasis, the majority are
found in more advanced stages, making successful treatment difficult. Treatment often is a multi-step process involving surgery,
radiation therapy and chemotherapy. With aggressive treatment, even patients with advanced disease can experience prolonged
control of these tumors with a good quality of life.
Initially, it was believed that these tumors were more commonly found in older, female dogs, but recent studies found there
is no gender predilection (Moore, 2006). Breed predilections include German Shorthair Pointers, English Cocker Spaniels, Dachshunds
and Alaskan Malamutes. In breeds that are predisposed to anal-sac tumors, bilateral tumors can be seen. In our clinic, English
Cocker Spaniels frequently present with bilateral anal-sac tumors.
Photo 1: Pulmonary metastasis in a dog with an anal-sac tumor. (Photographs courtesy of Dr. Kim L. Cronin)
Anal-sac tumors are considered highly malignant. Not only do they infiltrate the surrounding tissues, they metastasize early
to the regional lymph nodes. Metastasis to the regional lymph nodes has been reported in 46 percent to 96 percent of patients
(Withrow, 2007). Other sites of metastasis include the lungs, spleen, liver, kidneys and lumbar vertebrae (Photos 1 and 2).
Photo 2: Metastasis to the sublumbar lymph nodes on CT scan.
Mapping the clinical course
Metastasis to distant sites typically occurs in later stages of the disease.
The clinical course of anal-sac tumors can be insidious. The tumors can be an incidental finding during a routine examination.
It is not uncommon for dogs referred to our clinic for another malignancy to be found with a previously undetected anal-sac
mass. This underscores the importance of a rectal examination as part of a routine physical. Early detection will provide
the best chance of achieving long-term control.
The slow progression of these tumors allows patients to adapt to enlarging tumors with few clinical signs. It is not until
these tumors reach a critical volume that clinical signs develop.
The two most significant clinical symptoms are tenesmus and hypercalcemia. Tenesmus is associated with large primary tumor
size and/or the presence of metastasis to the sublumbar lymph nodes.
Anal-sac tumors are the second most frequent malignant cause of hypercalcemia in dogs, next to lymphoma. The most effective
method of controlling the hypercalcemia is by treatment of the primary tumor and/or metastatic disease. Dogs that present
with hypercalcemia of an unknown origin, particularly older dogs, should have a thorough evaluation of the anal sacs, because
even small tumors can cause significant hypercalcemia.
Hypercalemia can be found in up to 25 percent of dogs with anal-sac tumors (Moore, 2006). Not all dogs that are hypercalcemic
will have clinical signs associated with hypercalcemia. Anal-sac tumors can produce a protein called PTH-rp (parathyroid hormone-related
protein) that is capable of binding to PTH receptors, leading to alterations in calcium homeostasis and subsequent hypercalcemia
and hypophosphotemia. PTH levels are low in these patients due to negative feedback.
Staging and diagnosis