Dog Anal Gland Cancer (Adenocarcinoma): Signs to Know

Anal sac adenocarcinoma is a malignant tumor of the anal glands in dogs that is often discovered late — either incidentally during a routine rectal exam or after spreading to regional lymph nodes. It can cause life-threatening hypercalcemia even before the primary tumor is noticed.

Last reviewed: June 2026

What Is Anal Sac Adenocarcinoma in Dogs?

Anal sac adenocarcinoma (AGASACA — apocrine gland adenocarcinoma of the anal sac) is a malignant tumor arising from the secretory glandular cells lining the anal sacs. It is the most common perianal tumor in dogs and one of the more biologically aggressive tumors in veterinary oncology. Unlike benign perianal adenomas (which are androgen-dependent and common in intact male dogs), AGASACA affects both sexes and all neuter statuses, with female dogs — particularly spayed females — actually having slightly higher reported rates in some studies.

The tumor has two features that make it medically complex:

1. Regional lymph node metastasis is extremely common — the sublumbar (internal iliac) lymph nodes receive lymphatic drainage from the anal sacs, and metastasis to these nodes is present in approximately 50% of dogs at diagnosis and in up to 96% at necropsy. Massively enlarged sublumbar nodes can compress the colon or rectum, causing constipation or obstruction.

2. Paraneoplastic hypercalcemia — many anal sac adenocarcinomas secrete a parathyroid hormone-related protein (PTHrP) that causes calcium levels to rise, independent of bone metastasis. Hypercalcemia (high blood calcium) causes kidney damage, excessive thirst and urination, lethargy, muscle weakness, and cardiac arrhythmias, and can be the presenting complaint before the tumor itself is detected.

As described in Withrow & MacEwen's Small Animal Clinical Oncology, AGASACA is considered a locally aggressive tumor with a high metastatic rate, but well-managed dogs frequently survive 1–2 years post-diagnosis with multimodal treatment.

Signs to Know

Many dogs with anal sac adenocarcinoma initially show minimal or no signs directly referable to the tumor. Signs arise either from the primary tumor, its lymph node metastases, or the systemic effects of hypercalcemia:

Signs from the primary tumor:

• Scooting or excessive licking of the perianal area
• Visible or palpable mass near the anal sac (lateral to the anus) — often firm, not fluctuant like an abscess
• Straining to defecate (tenesmus) — particularly if the tumor has grown large
• Ribbon-shaped or flattened stool — from colonic compression by enlarged lymph nodes
• Perianal discomfort

Signs from hypercalcemia (often the presenting complaint):

• Marked increase in thirst and urination — polyuria/polydipsia (PU/PD)
• Lethargy and weakness
• Decreased appetite or vomiting
• Constipation or decreased bowel activity
• Muscle tremors or weakness
• Cardiac arrhythmias in severe cases

A key pattern: any dog presenting with unexplained hypercalcemia should have both anal sacs examined — even if no anal signs are present. This is a mandatory part of the hypercalcemia workup (AAHA Canine Life Stage Guidelines, 2019).

Diagnosis and Staging

Diagnosis requires a combination of rectal/perianal examination, imaging, and histopathology:

• Rectal examination — the most important physical exam finding; the primary tumor is typically a firm, discrete mass palpable in the lateral perineum adjacent to the anus; many are initially missed because they are small and owners notice no external swelling
• Fine needle aspirate of the primary mass — cytology can suggest adenocarcinoma and guides biopsy planning
• Biopsy with histopathology — definitive diagnosis requires tissue; incisional or surgical excision biopsy
• Abdominal radiographs or ultrasound — to screen for sublumbar lymphadenopathy and abdominal organ involvement
• Chest radiographs — lung metastasis, though less common than lymph node spread at initial presentation, should be documented
• CT scan of the abdomen and pelvis — provides the most accurate assessment of primary tumor size, sublumbar node involvement, and surgical planning; standard at specialty oncology centers
• Serum total calcium and ionized calcium — assess for hypercalcemia; PTHrP assay if available confirms paraneoplastic etiology

Treatment

Multimodal treatment — combining surgery, radiation, and/or chemotherapy — offers the best outcomes:

• Surgery (anal sacculectomy) — removal of the primary tumor is the cornerstone of treatment; sublumbar lymphadenectomy (removal of affected regional nodes) is recommended when technically feasible and improves survival
• Radiation therapy — effective for residual or inoperable primary tumor and for sublumbar lymph node disease; improves local control
• Chemotherapy — platinum-based agents (carboplatin, cisplatin) and melphalan have reported response rates; recommended as adjuvant therapy given the high metastatic rate
• Management of hypercalcemia — IV fluid diuresis, bisphosphonates (pamidronate), and in some cases steroids; hypercalcemia resolves when the primary tumor is removed but recurs with disease progression

Prognosis is meaningfully better when the primary tumor is small and lymph nodes are not yet metastatic at the time of surgery. Dogs with controlled primary disease and lymph node disease managed surgically have reported median survival times of 12–24 months in published case series.

When to See a Vet

Call your vet today if:

• Your dog is scooting persistently, especially if not recently expressed
• Your dog is straining to defecate or producing ribbon-shaped stool
• Your dog is drinking and urinating dramatically more than usual
• You feel a firm lump near your dog's anus during routine grooming or bathing

Go to the ER immediately if:

• Your dog has severe lethargy, weakness, or vomiting alongside increased thirst — these can signal dangerous hypercalcemia
• Your dog is completely unable to defecate and appears in pain
• Cardiac arrhythmia or collapse occurs in a dog with known high calcium

Frequently Asked Questions

Can you feel anal sac adenocarcinoma from the outside? Sometimes — but not always. Small tumors may only be detectable by rectal palpation during a veterinary exam. Larger tumors may produce a visible or palpable firm lump lateral to the anus. Because early tumors are often small and internal, this is one reason rectal examination is performed at routine wellness visits in middle-aged and older dogs, and any new perianal firmness should be evaluated immediately rather than assumed to be a cyst or anal gland issue.

How is anal sac adenocarcinoma different from a regular anal gland infection? An anal gland abscess or impaction is soft, painful, and often causes obvious discomfort. AGASACA is typically firm, often not acutely painful initially, and may be discovered incidentally. Infected or impacted anal glands are common and usually resolve with expression and antibiotics. A firm perianal mass that does not resolve like a typical abscess, or a mass discovered on rectal exam, should be aspirated and evaluated cytologically rather than treated empirically.

How much does diagnosing and treating anal sac adenocarcinoma cost? Initial workup including physical exam, radiographs, ultrasound, and fine needle aspirate typically costs $400–900. CT scan for staging adds $1,200–2,500. Surgery (anal sacculectomy with or without lymph node removal) runs $1,500–4,000 at specialty centers. Radiation therapy for residual disease costs $4,000–8,000 for a course. Chemotherapy adds $300–800 per cycle. Hypercalcemia treatment during hospitalization typically adds $500–1,500 per episode.

Is anal sac adenocarcinoma common in certain dog breeds? English Cocker Spaniels have the highest documented predisposition in multiple studies. Other breeds with increased representation include Springer Spaniels, Dachshunds, and German Shepherd Dogs. However, any breed can be affected, and the condition should be considered in any middle-aged or older dog with a perianal mass or unexplained hypercalcemia regardless of breed.

What happens if anal sac adenocarcinoma is not treated? Without treatment, the primary tumor grows locally and metastatic lymph nodes enlarge, eventually causing progressive obstruction of the colon and rectum. Hypercalcemia worsens, damaging the kidneys and heart over time. Most untreated dogs deteriorate significantly within 3–6 months of diagnosis. Treatment, even palliative, substantially extends comfortable survival.

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