Anal sac adenocarcinoma 

Otherwise known as anal sac carcinoma or apocrine gland carcinoma

Overview

An anal sac adenocarcinoma is a malignant cancer of the anal gland. It represents approximately 2% of all skin tumours in dogs and is seen predominantly in breeds 10-11 years of age. It often appears as a firm, fixed lump around the anus region.

Breeds commonly affected

• German Shepherd
• English Cocker Spaniel
• Dachshund
• Alaskan Malamute
• English Springer Spaniel

Clinical Signs

• Mass on the anus
• Drinking more
• Increased thirst
• Enlarged lymph nodes
• Constipation
• Straining to toilet

Diagnosis

• Rectal examination to assess lymph nodes
• A blood test to check calcium levels and kidney function
• Fine needle aspirate of anal sac
• Radiographs or ultrasound of abdomen +/- chest
• CT or MRI to check lymph node size and evidence of spread (metastasis)

Paraneoplastic hypercalcemia

Paraneoplastic hypercalcemia is a condition where the blood calcium levels increase with the anal sac adenocarcinoma or metastases to the regional lymph nodes. Levels can decrease to normal range within 1-2 days of the surgical removal of the tumour. Recurrence of the hypercalcemia often indicates the recurrence of the tumour or the presence of metastasis.

Metastasis

Studies show that 56-79% of cases have already metastasised to the regional lymph nodes at the time of diagnosis. Other less common sites include the lungs, spleen, bone, pancreas, heart, and mediastinum.

Management

The hypercalcaemic state must be addressed first with fluid therapy.

Surgery involves a wide local removal of the mass. However, this is extremely complicated due to the close proximity to the anal sphincter and rectum. Greater than 50% removal of the anal sphincter can result in transient faecal incontinence. Some lymph nodes can be removed in 50% of the cases.

Unfortunately, 10% of cases may experience complications with anal sac resection:

• Haemorrhage
• Infection
• Incontinence
• Hypocalcaemia (low levels of blood calcium)
• Straining to toilet
• Scarring of the anal area

Chemotherapy

The effectiveness of chemotherapy for this cancer is not widely published. It can be used on its own or in combination with radiation therapy.

Prognosis

The prognosis for dogs treated for anal sac adenocarcinoma can range from fair to good depending on the level of disease and treatment undertaken. 

Local tumour recurrence occurs in 45-50% of cases within 10 months of surgery. 

Mean survival time

Factors that worsen the prognosis

• Lung and lymph node metastases
• Paraneoplastic hypercalcemia
• Tumour size greater than 10 cm