Anal sac adenocarcinoma in dogs might be something you’ve heard of, but it’s important to know about this uncommon yet serious type of cancer. It affects the anal sacs, which are those small glands on either side of your dog’s bum. If your dog starts having difficulty defecating, loses its appetite, or has high calcium levels in its blood, it could be a sign of this condition. These symptoms might indicate something called paraneoplastic hypercalcemia. Medical management is important in treating hypercalcemia to prevent renal injury and manage the condition effectively. Catching it early is key to giving your dog a better chance at recovery and a longer life. Treatment can involve surgical removal of the main tumor, chemotherapy, or radiation therapy. In this article, I’ll walk you through the signs to watch for, how we diagnose it, and the treatment options available.

What are anal sacs anyways?

Before we discuss the specifics of cancer, let's quickly cover what anal sacs (or anal glands) normally do. Every dog has two small sacs located just inside the anus, one on each side (think 4 o'clock and 8 o'clock positions). These sacs produce a distinct, oily, smelly fluid.

Their main job is scent marking – leaving a unique chemical signature when your dog poops. This fluid can also sometimes help lubricate hard stools. Usually, these sacs empty naturally during defecation, but sometimes they can become impacted or infected, which is a separate issue from cancer.

A brief synopsis of anal sac adenocarcinoma

Anal sac adenocarcinoma originates from specific glands (apocrine glands) found in the walls of your dog’s anal sacs. This cancer, sometimes called Apocrine Gland Anal Sac Adenocarcinoma (AGASACA), tends to be aggressive right where it starts (locally invasive). Female dogs have historically been predisposed to this type of cancer, although recent studies show an equal prevalence between spayed females and castrated males.

Unfortunately, this type of cancer also has a high chance of spreading (metastasizing). It typically spreads first to the nearby lymph nodes around the pelvis before potentially moving to more distant organs like the lungs, liver, or spleen later on. Enlarged sublumbar lymph nodes are significant during abdominal imaging, as nearly 50% of dogs with AGASAC can have these lymph nodes identified as metastatic evidence.

Some dogs with these tumors develop elevated calcium levels in their blood (hypercalcemia), which can cause clinical signs like increased thirst and urination, and lethargy.

While anal gland cancer in dogs is considered rare overall, certain breeds seem to be more predisposed, including English Cocker Spaniels, English Springer Spaniels, German Shepherds, Dachshunds, and Alaskan Malamutes. However, it can occur in any breed, and the average age is 10 years. Older dogs are particularly at risk, highlighting the importance of routine check-ups and rectal examinations.

What are the symptoms of anal gland tumors in dogs?

It’s important to know that some dogs with anal sac adenocarcinoma might not show any obvious signs, especially early on. However, there are several symptoms you should be aware of. If you notice any of these in affected dogs, it’s best to see your veterinarian:

Changes related to defecating:

Straining or difficulty pooping: Your dog might seem uncomfortable or take a long time. Decreased appetite can also be a symptom.
Constipation: Less frequent or difficult bowel movements.
Thin or ribbon-like stools: The shape of the poop might change.

Signs around the rear end:

Scooting: Dragging their bottom on the floor.
Excessive licking: Frequently licking or chewing at the anal area.
Swelling: A noticeable lump or anal sac mass near the anus (though this isn’t always visible externally).
Discharge: Any unusual fluid coming from the anal/perianal area.

In the more aggressive forms of cancer, the blood calcium can elevate, called hypercalcemia, and this can result in excessive thirst and lethargy.

If there are consistent signs of anal gland cancer and your dog is coughing, there is the possibility of metastasis or spread to the lungs.

Please see your veterinarian if you notice any of the above signs or have concerns.

How are anal sac tumors diagnosed?

If you bring your dog in with concerning symptoms, or if something is noted during a routine check-up, your veterinarian will start with a thorough examination to figure out what’s going on. Here are the steps typically involved:

Physical examination (including a rectal exam): The first step is a complete physical check-up. Your vet will gently feel the anal sacs and surrounding area for any masses, swelling, or abnormalities.
Fine Needle Aspirate (FNA): If a mass is found or suspected, your vet may use a small needle to collect a small sample of cells directly from the area. Looking at these cells under a microscope can often suggest cancer, but it’s usually not enough for a definitive diagnosis.
Blood tests: Standard blood work, including a complete blood count (CBC) and a serum chemistry panel, is essential. These tests check your dog’s overall health, look for problems with organs, and importantly, measure the calcium level in the blood, which can be high with this type of cancer.
Checking for spread (staging): Because AGASACA can spread, your vet will need to check other parts of the body. This usually involves:

Abdominal ultrasound: To look closely at lymph nodes near the tumor and organs like the liver and spleen.
Thoracic radiographs: To check if the cancer has spread to the lungs.
Computed tomography: Sometimes, a whole-body CT scan might be recommended for a more detailed look at potential spread.

Lymph node sampling: Your vet may take samples (using FNA or biopsy) from the lymph nodes near the tumor (regional lymph nodes) to see if cancer cells have spread there, which is common with AGASACA.
Biopsy (histopathology): While the tests above provide strong clues, the only way to get a definitive, confirmed diagnosis of anal sac adenocarcinoma is by taking a small piece of the tumor tissue (a biopsy). This tissue is sent to a pathologist who examines it under a microscope (histopathology). This is often done when the tumor is surgically removed.

These steps help us not only confirm the diagnosis but also understand how advanced the cancer is (staging), which is key for planning the best treatment.

Treatment for anal sac adenocarcinoma

Once diagnosed and staged, your veterinarian will discuss the best treatment plan for your dog. The main goal is often to control the tumor where it started and address any spread. Here are the common approaches:

Surgical management is often the main treatment, especially if the tumor is small enough to be removed.

Anal sacculectomy: This involves surgically removing the affected anal sac(s) along with the tumor, aiming for complete excision.
Potential complications: As with any surgery, there are risks at the surgical site. For anal sacculectomy, these can include trouble controlling bowel movements (fecal incontinence, possibly temporary or permanent), narrowing of the anal opening, infection, wound healing problems, nerve damage, or temporary low calcium levels after surgery.
Lymph node removal: If cancer has spread to nearby lymph nodes, removing them via abdominal surgery may be recommended as part of primary tumor removal. This is complex and carries risks like bleeding or nerve damage affecting urination. Referral to a board-certified veterinary surgeon is often best for this procedure.
When surgery might not be curative: If the cancer has already spread widely, surgery alone is unlikely to cure it, but might still help with diagnosis or temporary symptom relief.

Besides surgery, radiation therapy (RT) and chemotherapy might be part of the treatment plan:

Radiation therapy (RT): Uses targeted radiation beams to kill cancer cells. It might be used after surgery if the tumor wasn’t fully removed, or instead of surgery if the tumor is inoperable. RT often targets the tumor site and nearby lymph nodes. It can have significant side effects, both short-term (like skin irritation and colitis) and potentially long-term (like chronic bowel issues or tissue damage), which your vet will discuss.
Chemotherapy: Uses drugs to fight cancer cells throughout the body. It’s often recommended alongside surgery or RT to help manage potential spread. While commonly used, its exact impact on survival time isn’t fully established, but it’s generally considered most helpful when the main tumor is also treated locally. Adjuvant chemotherapy is sometimes considered, though its benefits are still debated.

Your veterinarian or a veterinary oncologist will discuss which combination of these treatments is most appropriate for your dog’s specific situation, considering the tumor stage, your dog’s overall health, and potential risks versus benefits.

Intravenous (IV) fluids are the main treatment to help lower calcium before surgery or if surgery isn’t possible, with fluid therapy being a critical component.

Managing pain is essential, especially right after surgery or if radiation causes significant skin irritation. Pain medications, including non-steroidal anti-inflammatory drugs and tramadol, are all key for managing pain associated with symptoms such as difficulty defecating.

Supportive therapies that should be considered

Alongside treatments aimed directly at the cancer, supportive care helps manage complications and side effects, keeping your dog comfortable:

Managing calcium levels: High blood calcium (hypercalcemia) is common with this type of cancer. Intravenous fluids are the main treatment to help lower calcium before surgery or if surgery isn’t possible. Other medications like corticosteroids such as prednisone, and diuretics might be used if fluids aren’t enough. Interestingly, some dogs develop temporary low calcium after surgery (if it was high before), which may require short-term treatment.
Pain control: Managing pain is essential, especially right after surgery or if radiation causes significant skin irritation. Your vet will prescribe appropriate pain relief.
Bowel management: Stool softeners might be given for a few weeks after surgery to make defecation easier. If radiation causes diarrhea (colitis), treatments like probiotics, dietary changes, fiber, or anti-diarrheal medications can help.

What is the prognosis for dogs with anal sac adenocarcinoma?

The prognosis, or expected outcome, for a dog with anal sac adenocarcinoma can vary quite a bit, and survival times can be significantly influenced by various factors. It really depends on the individual dog and several key factors:

Tumor size and spread (stage): This is often the most important factor. Smaller tumors that haven’t spread (metastasized) generally have a better outlook. Unfortunately, this cancer often spreads early, commonly to the lymph nodes near the tumor. Spread to more distant organs like the lungs or liver makes the situation more serious.
Treatment: Dogs who receive treatment, particularly surgery to remove the main tumor (and affected lymph nodes, if feasible), tend to do better than those who don’t. Combining therapies like surgery, radiation, and/or chemotherapy often offers the best chance at controlling the cancer for longer. These combined therapies can lead to prolonged survival times, especially when initiated early.
High calcium levels: Having high blood calcium (hypercalcemia) associated with the tumor can indicate a more challenging situation, but this often improves once the tumor is treated.
Recurrence: Even with successful treatment, there’s a possibility the cancer can come back later (local recurrence) or spread, so ongoing monitoring is important to detect any disease progression.

While this is a serious cancer, advancements in surgery, radiation, and medical therapies are helping dogs live longer and maintain a good quality of life.

Anal Sac Adenocarcinoma in Dogs