THE most common tumour types encountered around the anus are tumours of the perianal (or circumanal) sebaceous glands, either benign (adenomas) or malignant (adenocarcinomas), and tumours of the apocrine glands of the anal sacs. Any other cutaneous or subcutaneous tumours (e.g. mast cell tumours, melanoma) occasionally occur in the vicinity of the anus.
The anal sacs, or paranal sinuses, are spherical sinuses located on each side of the anus, approximately in the 4 o’clock and 8 o’clock positions. Specific apocrine sweat glands, the paranal sinus glands produce and release secretions in the sacs. Tumours originating from these glands are very rarely benign, with adenocarcinoma being by far most common.
Epidemiology and behaviour
Anal sac adenocarcinomas (ASCs) are uncommon tumours, thought to represent 2% of cutaneous and subcutaneous tumours and about 17% of perianal tumours.
Geographical differences in breed predilections may exist: in studies conducted in the UK, spaniels appeared to be at a higher risk than the rest of the canine population, followed by Labradors/Golden retrievers and German Shepherds.1,2 In US-based studies, the same breeds have been reported to be at risk in variable order. Historically, it was thought that female dogs were predisposed to ASCs, but larger and more recent studies have proved this wrong 1,3 and no marked gender predilection is now thought to exist. In addition, ASCs appear to be non-hormone-dependent, possibly even more prevalent in neutered animals, especially in males.1
Anal sac carcinomas are highly metastatic tumours. Metastasis is estimated to be present at the time of diagnosis in 46% to 100% of cases, with locoregional lymph nodes being the preferred sites for distant spreading of the disease. Less frequently, invasion of the spleen, liver and uncommonly lungs can also be observed at presentation.
Hypercalcaemia of malignancy (HM) is a common paraneoplastic syndrome associated with ASC, estimated to occur in 27% to 53% of affected dogs. It is caused by the production of a peptide structurally close to parathormone (PTH) by the tumour. This peptide, referred to as parathormone-related peptide (PTHrP), therefore has the ability to link with PTH receptors and mimic its actions, thereby causing hypercalcaemia. Very rarely, squamous cell carcinomas will develop from the anal sacs.4,5
Clinical presentation and diagnosis
Affected dogs are most often presented for one or several of the following signs: presence of a perianal mass, constipation/ obstipation, caused either by the anal sac mass or by the obstruction of the pelvic canal by enlarged lymph nodes, or signs associated with hypercalcaemia (polyuria/polydipsia, weakness, lethargy, vomiting, anorexia, bradycardia, etc.).
Up to half of ASCs are, however, either occult (causing no clinical signs) and only found incidentally, or causing signs unrelated to anal disease.6 This stresses the importance of routine rectal palpation in older animals, especially in the breeds at risk.
Whenever an ASC is a differential (e.g. suspicion of HM, marked pelvic and lumbar lymphadenomegaly), the anal sacs must be very carefully palpated. If the patient is not perfectly relaxed or co-operative, sedation or a short general anaesthesia may be preferable to facilitate precise palpation by avoiding contraction of the anal sphincter. The anal sacs are first completely emptied, then palpated. It is important to keep in mind that ASC may be extremely small (<1mm in diameter), even in the face of extremely enlarged locoregional lymph nodes (Figure 1). Therefore, the slightest granule-like sensation on palpation of an anal sac must raise the suspicion of ASC in patients with a compatible history and clinical signs. Uncommonly, bilateral disease can be encountered, either concomitantly on first presentation (6%-10% of cases) or separately in time.7
Rectal palpation is also the opportunity to evaluate the presence of enlarged lymph nodes in the pelvic cavity. The hypogastric lymph node, located immediately caudally to the aortic quadrifurcation, is the lymph node most frequently palpated as being enlarged on rectal palpation. It is palpated dorsally to the rectum and feels like a firm mass, attached to the ventral aspect of the sacrum. Occasionally, the lateral sacral lymph nodes are present and enlarged, and can be palpated on either or both side(s) of the rectum, medially to the ilium.
Further staging consists of the assessment of the other locoregional (retroperitoneal) lymph nodes, liver, spleen and lungs. Although abdominal radiographs may reveal marked enlargement of medial iliac lymph nodes, it is not sensitive nor precise, and abdominal ultrasound is much more useful for evaluation of lymph nodes, liver and spleen. It also allows guidance for fine-needle aspirations of any of them. Radiographs are, however, useful for evaluation of the skeleton for bone metastases. Chest x-rays can be used to screen for pulmonary metastases. Alternatively, advanced imaging (computed tomography particularly) can be used for complete staging.
When possible, fine-needle aspirates of the primary tumour, enlarged lymph nodes or splenic and hepatic nodules should be obtained. Cytology should confirm the clinical suspicion of ASC, showing characteristic epithelial neoplastic cells (Figure 2).
Malignancy, however, is not always obvious cytologically, but should be assumed by default as benign tumours of the anal sac are quite rare. Alternatively, core-needle or incisional biopsies can be taken for pathological diagnosis.
References
- Polton, G. A., Mowat, V., Lee, H. C. et al (2006) Breed, gender and neutering status of British dogs with anal sac gland carcinoma. Veterinary and Comparative Oncology 4: 125-131.
- Polton, G. A. and Brearley, M. J. (2007) Clinical stage, therapy, and prognosis in canine anal sac gland carcinoma. Journal of Veterinary Internal Medicine 21: 274-280.
- Williams, L. E., Gliatto, J. M., Dodge, R. K. et al (2003) Carcinoma of the apocrine glands of the anal sac in dogs: 113 cases (1985-1995). Journal of the American Veterinary Medical Association 223: 825-831.
- Mellett, S., Verganti, S., Murphy, S. et al (2015) Squamous cell carcinoma of the anal sacs in three dogs. Journal of Small Animal Practice 56: 223-225.
- Esplin, D. G., Wilson, S. R. and Hullinger, G. A. (2003) Squamous cell carcinoma of the anal sac in five dogs. Veterinary Pathology 40: 332-334.
- Bennett, P. F., DeNicola, D. B., Bonney, P. et al (2002) Canine anal sac adenocarcinomas: clinical presentation and response to therapy. Journal of Veterinary Internal Medicine 16: 100-104.
- Bowlt, K. L., Friend, E. J., Delisser, P. et al (2013) Temporally separated bilateral anal sac gland carcinomas in four dogs. Journal of Small Animal Practice 54: 432-436.
