Anal Sac Adenocarcinoma in Dogs

Updated November 28th, 2020

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 disease of the anal sac. Most problems caused by anal glands in dogs are due to impaction or infection. Follow the link to learn more.

The signs of anal gland issues are much the same, regardless of cause. These are:

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

A tumour may be noticed by an owner when a swelling appears at the side of the anus. High blood calcium levels caused by the tumour may also cause dogs to drink excessively, although this is only documented in 25-50% of cases.

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 can then be performed via blood and urine tests, 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. Spread to local 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. Some vets then advise repeat surgery to remove any abnormal lymph nodes, however these are not always easy to access. Regardless, 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. Toceranib, a new tyrosine kinase inhibitor has recently shown signs of 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 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).

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!
Have something to add? Comments are welcome below and will appear within 24 hours.

Andrew

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