Anal Sac Adenocarcinoma - Everything A Dog Owner Should Know

This article covers all of the basics that a dog owner should know about apocrine gland anal sac adenocarcinoma (AGASACA or anal sac adenocarcinoma for short).

Please keep in mind that every dog is different, every dog's cancer is different, and medicine is not black and white.

If your dog has anal sac adenocarcinoma, consider a consultation with a veterinary oncologist for all of the latest information as well as treatment options that apply specifically to your dog and your dog's cancer.

What It Is

A malignant tumor (carcinoma) located in the anal sacs arising from apocrine cells. If you think of the dog's anus as a clock face (strange, I know) the anal sacs are located at 4 and 8 o'clock.

Benign tumors of the anal sac are very rare, however, dogs may have perianal (around the anus but not involving the anal sac) tumors, which are frequently benign. Talk to your vet or an oncologist if your dog has a perianal or anal sac tumor.

Even though AGASACA is the most common anal sac tumor, we can also see melanoma, lymphoma, mast cell tumor, etc. in this region, so an accurate diagnosis is important.

Common Signs

Many times, this tumor is an incidental finding (up to 39% of cases). This means that a small nodule (or large mass) involving the anal sac is noted on routine physical exam when a rectal is performed. This is why all physical exams should include a rectal exam with expression of the anal glands.

If the tumor is large enough it may cause discomfort. If your dog is experiencing discomfort they may lick the anal region or scoot (drag their bottom along the floor). If you observe any of these signs, have a rectal exam performed. Possibilities include very full anal sacs, an abscess in the region or a tumor.

In 25-50% of cases, the tumor produces a hormone (PTHrP) which causes an elevated ionized calcium level. The elevated calcium commonly causes nausea (decreased appetite, possibly vomiting) excessive thirst and frequent urination.

Sometimes, an elevated calcium is the first clue that a dog might have AGASACA, which prompts a rectal exam.


Obtaining a diagnosis is fairly simple. The anal region is a sensitive area. I typically apply topical numbing cream and give the patient a small amount of sedation. This will make them nice and calm and ensure that they are comfortable.

I use a small needle to sample (aspirate) the mass, and smear cells on glass slides. This tumor is typically quite easy to diagnose using this method. I STAT (rush) the sample and have a cytology diagnosis in 1 business day.

Recommended Work-up and Staging Tests

At the time of diagnosis metastasis (spread of cancer to other organs) has occurred in 50-80% of cases, so an appropriate work-up is essential - don't just jump to surgery without completing the tests below.

(1) Aspirate mass and obtain cytologic diagnosis - Why? Other types of cancer can occur in this area. Without a diagnosis you will not know the appropriate tests to perform before surgery, if surgery is even recommended, the best treatments, or the prognosis with various treatment options.

(2) Full blood work -Complete blood count, chemistry panel

(3) Urinalysis

(4) Ionized calcium - If calcium is elevated, it should normalize when all cancer is removed (this is a simple blood test)

(5) 3-view thoracic radiographs - This test ensures that cancer has not spread to the lungs and that the heart appears normal in size and shape. Images should be reviewed by a board-certified radiologist. If cancer has spread to the lungs, surgery is not usually recommended.

(6) Abdominal ultrasound - This test is commonly skipped (due to a lack of knowledge of best practices and of the 50-80% metastatic rate) but it is so important. AGASACA will usually spread to the sublumbar lymph nodes first; these lymph nodes are located in the abdomen. You cannot adequately evaluate these lymph nodes with radiographs (x-ray). Ultrasound is needed to measure these lymph nodes and assess all other abdominal organs for evidence of cancer (metastasis as well as other possible cancers).

If someone performs ultrasound sporadically (a few times per week), their skills are unlikely to be proficient enough to assess these lymph nodes and sample them if they are abnormal. This ultrasound should be done by someone that specializes in ultrasound (ultrasound is all that they do) or who is a board certified radiologist or internist. If metastatic (affected by cancer) lymph nodes are present but not removed, the patient is expected to have a shorter survival (not live as long).

What if the tumor is small? Does your dog still need an ultrasound?

Yes, yes and yes. A 5 mm (diameter of pencil eraser) anal sac tumor can spread to a lymph node causing it to be very large (4 cm, diameter of golf ball) lymph node that causes straining and difficulty defecating, decreased quality of life, and ultimately prompts euthanasia. That's why it's so important to have an excellent ultrasound if you're considering surgery.

It is considered inappropriate to remove the anal sac tumor and leave behind metastatic (affected by cancer) lymph nodes, as it is often the lymph nodes that actually affect quality of life before the anal sac tumor itself.

So many dogs have surgery to remove the anal sac tumor but never have an ultrasound. Since 50-80% of dogs will have spread of cancer at diagnosis (usually to abdominal lymph nodes), ultrasound is essential if your goal is to give your dog the best care and best outcome.

Why is it wrong to skip the ultrasound?

(1) If the lymph nodes are affected, they may cause pain and difficulty defecating long before the anal sac tumor itself ever does.

(2) If a dog has a high calcium, removing the anal sac tumor alone will not reduce the calcium. Both the affected lymph nodes AND the anal sac tumor must be removed to normalize calcium.

Why do we want a normal calcium? Elevated calcium causes nausea, excessive thirst/urination, and stresses the kidneys (can lead to kidney failure eventually).

(3) If ultrasound is performed AFTER surgery and metastatic lymph nodes are only noted AFTER the anal sac tumor has been removed, a second surgery is recommended to remove the lymph nodes.

Two surgeries are unfair for your dog (and your wallet) when only one was needed (with appropriate planning).

Treatment Options

Below I have listed the basic treatment options for the more common disease scenarios. It would be impossible to list all treatment options and all scenarios. Remember, every dog is different and every dog's situation is different.

If your dog has anal sac adenocarcinoma and you want a list of the most appropriate options for your dog, see your nearest oncologist.

Treatment Options - No Metastasis

Tumor smaller than 2.5 - 3 cm diameter - Recent data shows that patients with small tumors that received surgery alone are likely to live years in most cases. There are exceptions, so please consult an oncologist to determine if chemotherapy is indicated for your dog's tumor (or if surgery alone may be enough to allow for "longterm survival").

Tumor larger than 3 cm diameter - For large tumors that have not metastasized, we typically recommend surgery followed by chemotherapy. Chemotherapy usually involves six treatments with carboplatin followed by monitoring. If radiation is added to the treatment protocol, lifespan is expected to be longer.

When the tumor returns, Palladia oral chemotherapy can be added to help patients live longer. If the tumor recurs in a single lymph node, the lymph node can be resected; additional chemotherapy would be recommended at that point (perhaps mitoxantrone or Palladia).

Treatment Options - Metastasis to lymph node

In this scenario, dogs will live the longest if treated with surgery (removal of anal sac tumor and affected lymph node) followed by carboplatin chemotherapy. We typically monitor with chest x-rays and abdominal ultrasound once all six chemotherapy treatments are finished. We expect the tumor to return at some point. At that time, we typically resume chemo with a different type of treatment.

If more than two lymph nodes are affected, surgery may not be the best option; moving straight to chemotherapy such as Palladia (oral chemo) might make the most sense. In this scenario, Palladia would be given indefinitely, for as long as it is effective (controlling cancer) and well tolerated (no concerning side effects)

Treatment Options - Metastasis to distant organ (liver, lung, etc)

If distant metastasis has occurred to the liver, lung or to another distant organ (lymph nodes are a regional organ, not distant), we usually don't recommend surgery. In this case, the two main options are Palladia oral chemotherapy or to treat palliatively.

Palladia is given indefinitely, for as long as we feel that it is effective (shrinking the tumor or keeping it stable) and well-tolerated (patient feels well on it, no worrisome blood work changes). We typically know within the first month whether it is effective or not. In my experience, sometimes Palladia doesn't help, sometimes it helps for a few months, six months, 12 months, or longer.

Palliative care involves treatment aimed at helping the patient feel better, but does not involve actively killing cancer cells. In the case of AGASACA, this typically involves pain medication to keep the patient comfortable, it might involve stool softeners if enlarged lymph nodes are making defecation difficult/painful, and it could involve medications to help reduce calcium if it's elevated (bisphosphonates, steroids).

What if no treatment is elected?

Anal sac adenocarcinoma is usually a slow growing tumor. If your dog is diagnosed and the tumor is not yet causing discomfort, is not terribly large, and he does not have enlarged sublumbar lymph nodes, it is possible that he can live for 6-12 months before euthanasia will be indicated due to declining quality of life. Remember every case is different and some dogs have more aggressive tumors than others.


*Note that the survival times listed below are median survival times (MST) - this means that in the study from which the data was obtained, half of the dogs lived longer and half did not live as long.

It seems that dogs treated with surgery live significantly longer than those not treated with surgery (18.2 months vs 13.4 months). Dogs with lung metastasis had significantly shorter survival than those without lung metastasis (7.2 months vs 18.3 months). Dogs with large tumors (greater than 10 cm2, or just over 3 cm diameter) lived shorter than those with smaller tumors (9.7 months vs 19.4 months). Dogs with hypercalcemia (elevated calcium) lived shorter than those with normal calcium (8.5 months vs 19.4 months). I will say that if a dog has an elevated calcium, but the disease is localized (anal sac tumor +/- one affected lymph node), the elevated calcium should not shorten survival if appropriate surgery is performed and the patient receives chemotherapy.

In the majority of cases cases, provided that the anal sac adenocarcinoma is caught early (no distant metastasis and the tumor is not huge) with appropriate care (surgery, chemotherapy, and routine monitoring with ultrasound and x-ray) patients can do quite well despite this being a highly metastatic disease.

Dr. Lori Cesario

Board Certified Veterinary Oncologist

PS: I'm happy to now offer online oncology consultations. Learn more about how this service can help by visiting the Vet Cancer Consultants site.

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(1) Dog Breeds and Their Associated Cancers

(2) Mast Cell Tumors - The Great Imitator in Canine Cancer

(3) Canine Splenic Tumors - What You Need To Know

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