Anal Sac Adenocarcinoma

Description

A highly malignant neoplasia that readily metastasizes to the medial iliac lymph nodes. Around 50% may lead to paraneoplastic hypercalcaemia due to parathyroid hormone-related peptide (PTH-rP). The tumours can be between 1 and 10cm in size and 50% of them will metastasize.

Signalment

Have only been confirmed in dogs although they may occur in cats. Typically older bitches (greater than 10 years) are affected (90% of cases) however male dogs are occasionally affected. Predisposed dog breeds include the German shepherd dog, cocker spaniel, welsh spaniel and standard dachshund.

Diagnosis

History and Clinical Signs

See Anal Sac Disease - General

Also: Obstipation due to enlarged internal iliac lymph nodes and Perineal swelling. Additionally signs of hypercalcaemia including polydipsia, polyuria, anorexia and weight loss.

Rectal examination

Tumours of the anal sac can be detected by digital rectal palpation of the anal sac and associated structures, this must be performed with care as frequently the tumours are small and unilateral. Best carried out under general anaesthetic with manual compression of the caudal abdomen towards the rectum.

Laboratory Tests

Serum Biochemistry: Hypercalcaemia is present in 50%-90% of dogs with anal sac adenocarcinoma. This is thought to occur due to the expression of a gene for parathyroid hormone-related peptide by the tumour. Parathyroid hormone-related peptide has actions similar to parathyroid hormone, it acts to increase calcium levels in the blood. In severe cases of hypercalcaemia organ function can become compromised changing specific parameters on the biochemistry panel.

Urinalysis: Patients may be dehydrated with a pre-renal azotaemia due but have a relatively low urine specific gravity.

Radiography

Thoracic Radiography

At least two views to check for thoracic metastases.

Abdominal imaging

Caudal lateral abdominal radiographs or abdominal ultrasound to assess the medial iliac lymph nodes for metastases. When enlarged they displace the rectum ventrally.

Sites of metastasis include the liver, spleen, abdominal lymph nodes and the lungs.

Biopsy

Ultrasound guided needle biopsy and histopathology of the effected lymph nodes.

Treatment

The treatment of choice is surgical excision if the patient is normocalcaemic without any metastases. The success will depend upon the size of the mass.

Adjuvant radiotherapy

Can reduce the local recurrence and used if excision is incomplete. Radiotherapy, however may lead to radiation-induced colitis.

Palliative chemotherapy

Using platinum or anthracycline can lead to the primary tumour shrinking before surgical excision.

Treatment of Hypercalcaemia

High fluid rates of 0.9% saline to diurese calcium. Furosemide should be administered once the patient in normocalcaemic.

Prognosis

Potential postoperative complications include infection, wound dehiscence, faecal incontinence and in 25% of cases, local recurrence. Hypercalcaemia reoccurs in 35-50% of cases due to metastases. Serum calcium levels should therefore be regularly checked along with evidence of mass regrowth. Post-op survival ranges between 2 and 39 months with the average being 8 months.

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References

Hall, E.J, Simpson, J.W. and Williams, D.A. (2005) BSAVA Manual of Canine and Feline Gastroenterology (2nd Edition) BSAVA

Merck & Co (2008) The Merck Veterinary Manual

Dobson, J.M. and Lascelles, B.D.X. (2003) BSAVA Manual of Canine and Feline Oncology (2nd Edition) BSAVA

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