Anal Sac Carcinoma

What is anal sac carcinoma?

The anal sacs are located on each side of the anus. The sacs contain glands whose secretions typically malodorous and usually semi-pasty in texture. The secretions are considered by many to be part of the animals’ scent-marking behaviour, though their precise role is uncertain.

Tumours developing in the anal sac are an uncommon tumour. The tumour starts as a small mass on the wall of the sac, but can grow to completely envelop the gland.

How can I tell if my pet has anal sac carcinoma?

  • Mass beside the anus
  • Straining to defecate
  • Frequent licking or irritation about the anal region
  • Excessive thirst

Patients with anal sac tumours may show signs of discomfort around the anus, with frequent licking or difficulties with defecation. Faeces may be slightly flattened. In some cases a large mass beside the tail may be obvious. In 50% of cases, the tumour are very small and may only be evident on rectal examination. A small number of patients may present due to excessive thirst.

large anal sac mass on the left side of the anus

What is the cause of anal sac carcinoma?

Although originally considered to occur almost exclusively in the older (>10 years) female dog, larger studies have revealed no apparent sex bias. A possible higher breed incidence has been noted in the Cocker spaniel, and also the Labrador, German Shepherd dog, golden Retriever, cavalier King Charles spaniel, springer spaniel and border collie.

How is anal sac carcinoma diagnosed?

Diagnosis of anal sac adenocarcinoma is straightforward in many cases. The presence of a mass in the anal sac can be determined by rectal examination. Needle aspiration of the mass will usually provide a diagnosis.

A full evaluation requires accurate staging of the disease. Because anal sac cancer is known to be highly malignant, a CT of the chest, abdomen and pelvis is usually performed to allow a complete evaluation of the organs where tumour spread can occur. About 10% of dogs may have tumour spread beyond the regional nodal basin, with metastasis seen in the spleen, kidneys, liver, adrenal glands and lungs. 

CT showing spread of the anal sac cancer to lymph nodes under the spine (arrows)

How is anal sac carcinoma treated? 

A variety of treatment options exist for the management of anal sac adenocarcinoma, depending on the size and stage of the tumour, the physical status of the patient, and the availability of adjunctive therapeutic modalities. 

Surgical resection of the tumour, combined with removal of any secondary tumour deposits (metastatectomy) where necessary, has been associated with good long-term survival. Where the tumour is very large and may occupy up to 50% of the anal sphincter diameter, surgical management can be complicated and the case should be discussed with someone experienced with this type of surgery. Abdominal exploration and removal of secondary tumours should also be performed in all cases where metastases have been detected.

When surgery is not possible, we can often continue to control their disease with chemotherapy or radiation therapy.

What is the prognosis of anal sac carcinoma?

Previously, a grave prognosis was given for all dogs with this disease. However, recent work suggests that good tumour control is possible in many animals with appropriate therapy, with an overall mean survival time for all patients of 537 days. However, complete remission is achieved in about 30% of patients. 

It is accepted that achieving a complete cure in dogs with anal sac adenocarcinoma is not always possible, and even dogs with localised disease to one anal sac only will need to be monitored closely for the rest of their life to allow any secondary tumours to be detected early. 

While cure may not be possible, it is usually possible to greatly extend the life-span of affected dogs. In these patients, the cancer is treated as a chronic disease.  Some dogs may undergo repeated abdominal surgeries to allow any recurrent tumour to be removed. When surgery is no longer possible, then drug therapy can often help control the tumour for a further period.

Sadly, there are a small proportion of dogs whose tumour progresses rapidly, in spite of our efforts. At this time, we are unable to identify those patients who are likely to experience rapid relapse of their disease, which emphasises the need for continued monitoring and investigation.