Anal Sac Adenocarcinoma
Tumours of the anal sacs are uncommon in dogs and very rare in cats. Anal sac adenocarcinoma (ASAC) arise from apocrine sweat glands which drain into the anal sacs. They occur at a relatively low frequency in the dog, representing 17% of all perianal tumours.
The most common presenting complaints for animals with anal sac tumours are licking the hind end, scooting or straining to defecate. Some owners may notice a mass under the tail, and some dogs may produce flat, ribbon-like stools. Occasionally, an anal sac mass may be detected during a routine physical examination. In some cases, dogs may not present for signs related to the mass, but may present with signs related to an elevated blood calcium level. Up to 50% of ASAC produce a hormone that raises blood calcium, leading to clinical signs of increased drinking and urinating, or vague symptoms such as poor appetite, weakness or vomiting.
As with most tumours in animals and people, we do not know what causes ASAC to occur. Spaniels may be over-represented. Males and females are equally affected, although this is the most common perianal tumour in females (perianal adenomas are common in entire males).
Initial Evaluation
When an anal sac mass is detected, one of the first steps is to obtain a fine needle aspirate (FNA) or biopsy. An FNA may often be performed with the patient awake, or using quick-acting injectable sedation, and the cytology of ASAC has quite a characteristic appearance. In addition, complete evaluation of a dog with an anal sac tumour involves a complete blood count (CBC), serum biochemistry profile, ionized calcium level and urinalysis; abdominal ultrasound to assess for involvement of organs such as the regional lymph nodes, the liver and spleen, and to rule out evidence of concurrent disease; and 3-view thoracic radiographs to rule out lung involvement (alternatively CT will provide greater detail). Not all tests are required for every patient. However, once we have these results, we are able to provide a more accurate prognosis with various types of treatment.
Treatment/Prognosis
In those cases where surgery is possible, and there is no evidence of any spread further than the local lymph nodes, surgery is the initial treatment of choice. If there is regional lymph node involvement, these lymph nodes should be removed at the time of removal of the primary mass. Removal of less than 50% of the anal sphincter should not result in chronic faecal incontinence.
Chemotherapy following surgery may improve the outcome in some dogs. Chemotherapy can also be used in cases where surgery is not possible (if the tumour is too large or invasive), or if there is spread beyond the regional lymph nodes. Alternatively, metronomic chemotherapy (low dose, daily oral medications) may delay tumour progression. The small molecule inhibitor, toceranib phosphate (Palladia) that is also an oral drug, is reported to be of benefit in treating dogs with ASAC. There is also evidence that radiation therapy may also play a role in certain cases. Some animals may need additional forms of treatment to address their other symptoms, such as bisphosphonates to help control their blood calcium level; others may need to eat a high-fibre diet and/or take stool softeners if they are having difficulty defecating.
The average survival time achieved with surgery alone is approximately 12-18 months. Increasing size of the primary tumour and affected nodes is associated with a poorer outcome. Adjuvant medical therapy may increase survival to around 2 years in many cases.
Dr Amy Lane BVSc (Hons) MVS FANZCVS
