Identifying and treating anal sac adenocarcinoma in dogs - Veterinary Medicine
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Identifying and treating anal sac adenocarcinoma in dogs
Because these aggressive tumors may be hard to spot, a routine rectal examination is recommended in all dogs to increase the likelihood of early detection. In addition, this tumor should be suspected in any dog with hypercalcemia.



1A. Cytologic examination of a fine-needle aspirate of an anal sac adenocarcinoma reveals clustering of the cuboidal cells and indistinct cellular borders. Anal sac adenocarcinoma cells typically lack overt criteria of malignancy (Wright's-Giemsa; 50x). (Photo courtesy of Joyce Knoll, VMD, PhD, DACVP.)
The recommended work-up for a dog with an anal sac mass includes a minimum database (complete blood count, serum chemistry profile, urinalysis) to assess the animal's overall health and identify whether paraneoplastic hypercalcemia is present. Hypercalcemia, caused by tumor secretion of parathyroid hormone-related protein (PTH-rP), is seen in 26% to 53% of cases.4,7,8 Anal sac adenocarcinoma is the second most common neoplastic cause of hypercalcemia, so a thorough digital rectal examination is indicated in all dogs in which hypercalcemia is noted.10

1B. Histologic section of adenocarcinoma of the apocrine gland of the anal sac. The lumen of the anal sac is in the center (black arrow), immediately surrounded by lymphocytic inflammation and clusters of normal apocrine glands (thin white arrow). Normal architecture to the left and far right is effaced by the lobulated adenocarcinoma (thick white arrow) (hematoxylin-eosin; 20x). (Photo courtesy of John H. Keating, DVM, DACVP.)
A fine-needle aspirate and cytologic examination of an anal sac mass can differentiate anal sac adenocarcinoma from other tumor types that may occur in this area (i.e. mast cell tumor, melanoma, lymphoma). Typically, sheets of clumped cuboidal epithelial cells with indistinct cellular borders are seen (Figure 1A). Despite this tumor's aggressive biologic behavior, the cytologic appearance of the cells can be benign, with few criteria of malignancy apparent. Suppurative inflammation or necrosis identified in cytologic specimens can be misleading since tumors can be secondarily infected. In such cases, obtain a biopsy after appropriate antibiotic therapy.1 Biopsy may also be needed to distinguish between epithelial tumors involving the circumanal glands and apocrine glands of the anal sacs (Figure 1B). However, a lesion at the site of an anal sac often increases clinical suspicion of the tumor type.

2. The ultrasonographic appearance of iliac lymphadenopathy due to metastatic anal sac adenocarcinoma in a dog.
Staging tests include abdominal ultrasonographic and three-view thoracic radiographic examinations. Evidence of local spread to the iliac lymph nodes is common, occurring in 40% to 72% of cases at presentation.3-8 This metastasis to the iliac lymph nodes can occur early in the course of disease while the anal sac tumor is still quite small.8 Lateral abdominal radiographic findings can suggest a soft tissue opacity in the sublumbar region, causing ventral deviation of the colon. Abdominal ultrasonography has higher sensitivity than does radiography in detecting iliac lymphadenopathy and can better characterize the number and size of lymph nodes and evaluate the remainder of the abdomen for evidence of distant metastatic disease (Figure 2).1 Pulmonary metastatic disease occurs less commonly and can be identified as a nodular or diffuse pattern on thoracic radiographs.1 Other uncommon sites of metastasis include the spleen, liver, and lumbar spine.4 The information gathered during staging will help guide treatment and formulate a prognosis.


Because anal sac adenocarcinoma has a high rate of local metastasis and a locally invasive nature, a multimodal approach to treatment is typically recommended. The benefit of surgery and radiation has been shown in retrospective studies, while the role of chemotherapy is less clearly defined.

Medical management of hypercalcemia

The clinical significance of hypercalcemia related to anal sac adenocarcinoma is not well-characterized. In most cases, this paraneoplastic syndrome resolves once the tumor is surgically removed.3 However, long-term complications of persistent hypercalcemia in dogs with unresectable disease are not well-established, and, in our experience, these dogs have not developed evidence of renal compromise. Nevertheless, judicious monitoring of blood urea nitrogen and creatinine concentrations and urine specific gravity in hypercalcemic dogs with anal sac adenocarcinoma is warranted, and treatment of hypercalcemia should be instituted at the discretion of the attending veterinarian. Immediate care may include sodium chloride diuresis along with furosemide. Bisphosphonate therapy may also be considered. Corticosteroids may be used once lymphoma has been ruled out as a cause of hypercalcemia.1


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