Swelling Around Your Dog’s Anus — Symptoms, Diagnosis, and Treatment of Anal Sac Adenocarcinoma

Oncology/CancerDiseasesMeongsiljang Veterinary Advisory Board

Anal sac adenocarcinoma in dogs is a malignant tumor that arises from the apocrine glands of the anal sacs. If detected early, it can be cured with surgery. However, as the disease progresses, it may lead to complications such as lymph node metastasis and hypercalcemia.

What Is Anal Sac Adenocarcinoma in Dogs?

A veterinarian performing a palpation exam around a puppy’s anus

Anal sac adenocarcinoma is a malignant tumor that arises from the apocrine glands within the anal sacs, which are located on either side of the anus. Although tumors in this area are relatively rare, they are often malignant, invasive to surrounding tissues, and prone to metastasizing to nearby lymph nodes. The primary treatment is surgical removal, and when the tumor is small and has not spread, surgery can lead to a favorable prognosis. A critical point to remember is that these tumors are often not visible externally in their early stages, leading to delayed diagnosis. They typically occur in older dogs and are reported more frequently in females. Additionally, because they can cause hypercalcemia—a complication that elevates blood calcium levels—a comprehensive systemic evaluation is essential.

Causes and Risk Factors of Anal Sac Adenocarcinoma

The exact cause of anal sac adenocarcinoma remains unknown, but certain tendencies have been identified.

Age: This cancer primarily affects older dogs. Reported cases typically occur around 10 years of age, and it is rare in young dogs.

Sex: It is known to be more frequently reported in female dogs.

Tumor Characteristics: Although anal sac tumors are rare, they are often malignant once they develop. They tend to be locally invasive and frequently metastasize to nearby lymph nodes, particularly those in the iliac region.

Tendency for Hypercalcemia: Cancer cells may secrete parathyroid hormone-related protein (PTHrP), which can lead to hypercalcemia (elevated blood calcium levels) in a significant proportion of cases. This condition can affect the kidneys, so careful monitoring is essential.

Early Symptom Checklist — See a Vet If You Notice These Behaviors

If any of the following symptoms recur, please have your pet examined by a veterinarian.

Scooting or dragging the hindquarters: This behavior is an attempt to relieve discomfort from the anal sacs.

Swelling or lumps around the anus: A small lump may be visible or palpable.

Difficulty defecating or narrowed stools: These symptoms occur when a tumor presses on the rectum.

Excessive licking of the anus: This is often caused by pain or discharge.

Sudden increase in water intake: This may be a sign of complications from hypercalcemia.

Lethargy and decreased appetite: These can indicate a progressive tumor or systemic complications.

A puppy dragging its bottom along the floor due to anal discomfort

Red Flags Requiring Immediate Veterinary Care

Visit a veterinary clinic within 24 hours if you notice any of the following symptoms: • A lump around the anus is growing rapidly or bleeding • Your pet has been unable to defecate for more than 2–3 days • Vomiting, seizures, and severe lethargy are occurring simultaneously • Water intake has suddenly increased significantly while urination is minimal — this may indicate kidney damage due to hypercalcemia

How Is It Diagnosed?

Diagnosing anal sac adenocarcinoma involves several steps.

Palpation and visual examination: We check the size, location, and firmness of any lumps around the anus.

Fine-needle aspiration cytology: Cells are collected from the lump to quickly determine if it is malignant.

Biopsy: Tissue is sampled for pathological examination to confirm the diagnosis.

Abdominal ultrasound and CT scans: These help assess whether the cancer has spread to the iliac lymph nodes or within the abdominal cavity.

Blood tests: These evaluate the overall health status, including blood calcium levels.

A veterinarian performing an abdominal ultrasound on a puppy.

Treatment Options — Stage-Specific Approaches

The treatment approach depends on the tumor size and whether it has metastasized.

Stage 1 (localized tumor, no lymph node metastasis): The anal sacs and tumor are completely removed via surgery. This stage has the highest cure rate.

Stage 2 (regional lymph node metastasis): The tumor and affected lymph nodes are surgically removed, followed by additional radiation therapy.

Stage 3 (distant metastasis): A combination of surgery, radiation, and chemotherapy is used. The goal is to extend survival time.

For the first year after surgery, regular check-ups every three months are essential to detect any recurrence early.

Our veterinary surgery team performing tumor removal surgery on a dog

Post-Surgery Home Care Tips

Here are some home care tips to help your pet recover after surgery.

Wound care: Keep the e-collar on to prevent your pet from licking the incision around the anus, and check the area daily for redness, swelling, or discharge.

Dietary management: Following your veterinarian’s recommendation, a high-fiber diet can help maintain soft stools and reduce pain during bowel movements.

Activity restriction: Avoid strenuous exercise for two weeks after surgery and keep outdoor walks short.

Follow-up exams: Be sure to keep all scheduled follow-up blood tests, including calcium level checks.

A puppy wearing a neck collar after surgery, resting at home.

Prognosis and Recurrence — Key Points to Know

Anal sac adenocarcinoma can recur even after surgery, so it’s essential to continue regular check-ups. • If a small tumor (less than approximately 3 cm) without metastasis is surgically removed, the prognosis is generally favorable, with reported median survival times reaching about 1,200 days (over 3 years). • However, if the tumor is large, or if there are adverse factors such as invasiveness, necrosis, or lymphovascular invasion, or if lymph node or distant metastasis is present, the prognosis worsens and the risk of recurrence increases. • If hypercalcemia is present, it can cause irreversible damage to kidney function. • The earlier the disease is detected—while still small and without metastasis—the better the expected outcome. Early screening is the best approach.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Is anal sac adenocarcinoma contagious to other dogs?

No, cancer is not contagious. It cannot be transmitted to other pets living in the same household or to people.

Does constant butt-scratching always mean cancer?

Not necessarily. More common causes include anal gland impaction, allergic skin conditions, and parasites. However, if your pet repeatedly scratches or drags its rear, if you feel a lump, or if the perineal area becomes swollen, a veterinary examination is necessary.

Can it be treated without surgery?

Surgical removal is the primary treatment. If surgery isn’t feasible, radiation or chemotherapy can help slow tumor growth. Consult with a veterinary oncologist to determine the best course of action for your pet.

What causes hypercalcemia?

This is a paraneoplastic syndrome caused by tumors secreting substances that raise blood calcium levels (parathyroid hormone-related protein, PTHrP). Symptoms include vomiting, increased water intake, and weakness, and in severe cases, it can lead to irreversible kidney damage.

At what age should regular check-ups begin?

Anal gland tumors are most common in older dogs, so we recommend an annual comprehensive health checkup—including a physical exam of the perianal area and a digital rectal exam—once your dog reaches senior age. If a lump is felt in the anal gland region, we also advise checking blood ionized calcium levels.

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References

[1] Withrow SJ, Vail DM, Page RL. Withrow & MacEwen's Small Animal Clinical Oncology, 5th Ed. Elsevier, 2013.

[2] Dobson JM, Lascelles BDX. BSAVA Manual of Canine and Feline Oncology, 3rd Ed. BSAVA, 2011.

[3] Turek MM, Withrow SJ. Perianal tumors. In: Withrow & MacEwen's Small Animal Clinical Oncology. 2013. Chapter 23.

[4] Noli C, Foster A, Rosenkrantz W. BSAVA Manual of Canine and Feline Dermatology, 4th Ed. BSAVA, 2022.

[5] Meuten DJ. Tumors in Domestic Animals, 5th Ed. Wiley-Blackwell, 2017.