Anal Sac Gland Tumors

Supported by the Savannah and Barry French Poodle Memorial Fund
University of Pennsylvania School of Veterinary Medicine
Last Modified: August 21, 2005

Michael H. Goldschmidt, MSc, BVMS, MRCVS, Diplomate ACVP Professor and Head, Laboratory of Pathology and Toxicology Chief, Surgical Pathology Department of Pathobiology
Frances S. Shofer, PhD, Adjunct Associate Professor of Epidemiology and Biostatistics


The anal sacs in the dog are paired structures found on the ventrolateral aspect of the anus. They are located between the external and internal anal sphincter muscles. The normal anal sac is between 0.5cm to 1.0cm in diameter. There is a short excretory duct that lies between the anal sac and the glabrous skin of the anal area. The anal sac and the duct are both lined by keratinizing stratified squamous epithelium. Several large sebaceous glands lie at the opening of the duct into the anal sac. In the wall of the anal sac are many apocrine glands which communicate with the anal sac via a duct. The anal sac contents contain desquamated keratin, as well as sebaceous and apocrine secretions. Anal sac gland tumors arise from the apocrine glands. It is important to note that these tumors are of apocrine and NOT hepatoid or perianal glands, which are also found in this area.

Anal Sac Gland Adenoma

Definition: a benign tumor arising within the wall of the anal sac with differentiation to apocrine secretory epithelium

Synonyms: adenoma of the apocrine glands of the anal sac, perianal adenoma of the apocrine gland

This benign tumor is very rare. It cannot be differentiated on gross appearance from the malignant anal sac carcinoma. On histopathology, the tumor is well demarcated, has little cellular pleomorphism, a low mitotic activity, and consists of multiple small islands of glandular tissue which are separated by fibrovascular connective tissue.

Anal Sac Gland Carcinoma

Definition: a simple malignant tumor arising in the wall of the anal sac with differentiation to apocrine secretory epithelium

Synonyms: carcinoma of the apocrine glands of the anal sac, anal sac gland adenocarcinoma, adenocarcinoma of the apocrine gland of the anal sac, perirectal adenocarcinoma


Sex N Percent
Female 155 15% (21%)
Female Spayed 342 33% (33%)
Male 186 18% (25%)
Male Castrated 361 35% (21%)
(Normal Population %)

Breeds at
Increased Risk
N Probability OR 95%
English Cocker Spaniel 14 <0.0001 8.7 5.1 14.8
Alaskan Malamute 6 0.0411 2.4 1.1 5.4
German Shepherd 96 <0.0001 2.4 1.9 2.9
English Springer Spaniel 38 <0.0001 2.4 1.7 3.3
Cairn Terrier 10 0.0348 2.1 1.1 3.9
Husky 30 <0.0001 2.0 1.4 2.9
Mixed Breed 380 <0.0001 1.8 1.6 2.1
Basset Hound 18 0.0266 1.8 1.1 2.8
Dachshund 21 0.0500 1.6 1.0 2.4

Breeds at
Decreased Risk
N Probability OR 95%
Golden Retriever 38 <0.0001 0.49 0.35 0.67
Shetland Sheepdog 9 0.0089 0.45 0.23 0.86
Yorkshire Terrier 5 0.0231 0.39 0.16 0.93
Boxer 7 <0.0001 0.23 0.11 0.48
Collie 2 0.0155 0.23 0.06 0.91
Scottish Terrier 1 0.0089 0.13 0.02 0.94
West Highland White Terrier 1 0.0011 0.10 0.01 0.71
Rottweiler 1 <0.0001 0.04 0.01 0.30
Doberman Pinscher 1 <0.0001 0.03 0.00 0.24

OncoLink Veterinary Cancers

Clinical Presentation/Physical Exam Findings

  • These tumors may be seen bulging outward from the ventrolateral aspect of the anus.
    • The area may be reddened and have some degree of hair loss.
  • Many of the tumors are 2-4cm in size.
  • Smaller sized tumors may be felt as hard masses during anal gland expression or on digital rectal examination.
  • Most masses will not be ulcerated unless they’ve been traumatized.
  • Some animals may present with polyuria, polydipsia, weakness, and hypercalcemia.
    • This is due to the tumor’s ability to produce a parathyroid hormone related protein and thus cause pseudohyperparathyroidism.
  • On rectal palpation, anal sac gland tumors will feel firm.
  • If small, they may be well demarcated and confined to the wall of the anal sac.
    • However as they continue to grow there will be invasion of local tissues.
  • Anal sac gland carcinomas metastasize most frequently to the sacral and sub-lumbar lymph nodes.
    • These may also be enlarged during rectal palpation.
  • As the tumor continues to grow, it may impinge on the rectum and cause clinical signs including straining and difficulty defecating.
  • Carcinomas also tend to invoke a desmoplastic response that decreases the diameter of the rectum.

Tumor Pathology

Gross Findings

  • During surgical removal, anal sac gland carcinomas appear multi-lobulated and grey-white to pink
  • The anal sac may be identified as the tumor tends to lie adjacent to the anal sac and extends into the perirectal tissue
  • An invasive tumor can infiltrate the perirectal tissue as well as the anal sphincter but does not typically extend into the rectal wall

Microscopic Findings

There are three different histopathological patterns found with this tumor.

  • Solid type
  • consists of sheets of tumor cells subdivided by thin bands of fibrous tissue 
  • cells have round or oval euchromatic to hyperchromatic nuclei with a prominent nucleolus and a small amount of eosinophilic cytoplasm
  • Rosette type
  • nuclei are peripherally located
  • eosinophilic cytoplasm radiates from a small central focus of eosinophilic secretion
  • Tubular type
  • accumulation of eosinophilic secretion within the tubular lumina
  • neoplastic cells often have an abundant eosinophilic cytoplasm, hyperchromatic nuclei and variable mitotic activity

Anal sac gland carcinomas can present in any one of the three forms listed above. Some carcinomas may actually contain different combinations of the cell patterns.

Clinical Behavior

Anal sac gland carcinomas often metastasize via the lymphatics to the sub lumbar and sacral lymph nodes. Occasionally wide dissemination or metastasis to the lung and spleen may also be seen.


  • Goldschmidt, M.H., & Hendrick, M.J. (2002). Tumors of the skin and soft tissue. In D.J. Meuten (Ed.), Tumors in domestic animals 4 th ed (pp. 45-119). Iowa: Iowa State Press
  • Goldschmidt, M.H., & Shofer, F.S. (1998). Skin tumors of the dog and cat. Woburn, MA: Butterworth-Heinemann
  • Gross, T.L., Ihrke, P.J., & Walder, E.J. (1992). Veterinary dermatopathology: A macroscopic and microscopic evaluation of canine and feline skin disease. (pp. 327-485). St. Louis, Missouri: Mosby Year Book
  • World Health Organization (1998). Histological classification of epithelial and melanocytic tumors of the skin of domestic animals (2 nd series, vol 3). Washington, DC: Armed Forces Institute of Pathology
  • Yager, J.A. & Wilcock, B.P. (1994). Color atlas and text of surgical pathology of the dog and cat. Ontario, Canada: Mosby Year Book

Frequently Asked Questions