Anal Sac Adenocarcinoma in Dogs

Tumours of the anal glands are common in dogs, especially from middle age. Here is a summary of the signs, treatment and prognosis that should in no way replace the opinion of your trusted vet.

Signs Of An Anal Sac Tumour

Tumours are a relatively rare cause of problems in the anal sac. Anal glands in dogs more commonly cause problems due to impaction or infection that you can read about at the link.

Often the signs of either are the same. These are:

  • scooting (rubbing the bottom on the ground)
  • licking the bottom
  • an unusual smell or discharge

Additionally, a tumour may be first noticed when a swelling appears at the side of the anus. In 25-50% of cases, anal sac adenocarcinoma may cause dogs to drink excessively due to high blood calcium levels caused by the tumour.

Diagnosis Of Anal Sac Adenocarcinoma

A tentative diagnosis is made by a rectal examination. Often the vet does so intending to treat an impacted anal gland only to discover a firm mass or nodule instead. Other times, an infected gland responds poorly to standard treatment and is then surgically investigated.

Many anal sac adenocarcinomas are found during a routine health exam in which there is no previous suspicion.

A mass or lump in the position of the anal glands is almost always an anal sac adenocarcinoma. Therefore, it is reasonable to plan surgical removal immediately, with the aim of having the removed tissue analysed. This will confirm the diagnosis.

Tumour staging is then performed via blood and urine tests, multiple chest xrays and abdominal ultrasound or CT scan.

Prognosis

Anal sac adenocarcinoma is a highly aggressive tumour which metastasises to the lumbar lymph nodes and abdominal organs in 50 to 90% of cases. The prognosis for anal sac adenocarcinoma depends on many factors, but especially the size and extent of the tumour.

Masses under 3.2cm with no visible spread treated by surgical removal have the best prognosis. A recent study gave a median survival time (MST) of 1237 days.

As tumour size increases, survival time generally reduces. The presence of spread to lymph nodes does not necessarily reduce MST. On average, dogs can expect to live at least one year with surgery.

Treatment of Anal Sac Adenocarcinoma

Treatment starts with curative-intent surgical removal, followed by staging as outlined above. Most vets then advise repeat surgery to remove any abnormal lymph nodes as these are usually easy to access. However, it is likely that some microscopic tumour remains.

Even if this is the case, survival times with surgery alone are long enough to make surgery a reasonable alternative on its own.

Results for adding chemotherapy have been variable, and therefore I do not recommend it. However, toceranib, a new tyrosine kinase inhibitor has recently shown promise in extending disease-free times for some dogs.

The drug is well-tolerated in most cases other than a percentage with diarrhoea or poor appetite. However, it is extremely expensive and out of the reach of many dog owners. Results, though often good, are also highly unpredictable.

In a recent study of 29 dogs with visible metastasis, clinical benefit was seen in 20 (6 with a partial response had an MST of 1031 days and 14 with stable disease had 350 days). The remaining 9 had progressive growth of the tumour (MST 76 days).

It is worth bearing in mind that the MSTs in this study were calculated from the start of treatment, not diagnosis. As many of these dogs had had other treatments tried first, the true figure is somewhat higher.

In conclusion, if your dog has an anal sac tumour, see your vet for advice. They may send you to a dog cancer specialist if available, or be able to help you themselves. At the end of the day, the decision on what to do is yours, as long as your dog is not in pain or discomfort.

By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. These blogs are from a series regularly posted on email and Twitter. Subscribe via email here to never miss a story!
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Andrew

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