Overview

• Malignant neoplasm derived from apocrine glands of the anal sac.
• Locally invasive.
• High metastatic rate, often to sublumbar lymph nodes.
• Frequently associated with hypercalcemia, secondary to parathyroid hormone–related peptide secretion.
• Prognosis guarded to fair.

Signalment

• Older dogs; extremely rare in cats.
• Females overrepresented in some studies.
• English cocker spaniels significantly overrepresented, springer and Cavalier King Charles spaniels also overrepresented.

Signs

Causes & Risk Factors

None definitively identified

Differential Diagnosis

• Anal sac abscess
• Perianal adenoma/adenocarcinoma
• Mast cell tumor
• Lymphoma
• Squamous cell carcinoma
• Perineal hernias

CBC/Biochemistry/Urinalysis

• Hypercalcemia-25–50% of cases.
• Secondary renal failure may develop.

Other Laboratory Tests

If hypercalcemia is present, and tumor cannot be identified, parathyroid hormone and PTHrP levels can be assessed-high PTHrP support neoplasia as the cause of hypercalcemia.

Imaging

• Abdominal radiography-to evaluate sublumbar lymph nodes and lumbar and pelvic bones.
• Thoracic radiography-to evaluate for pulmonary metastasis.
• Abdominal ultrasonography-may identify mildly enlarged sublumbar lymph nodes not visible radiographically, also nodules in liver/spleen.
• MRI-recently shown to identify lymphadenopathy with greater sensitivity than ultrasound.

Diagnostic Procedures

• Fine-needle aspiration of anal sac mass to rule out conditions other than adenocarcinoma; while differentiation of benign versus malignant neoplasm of perianal masses is difficult, apocrine gland adenocarcinoma of the anal sac will have a neuroendocrine appearance and can be differentiated from perianal gland tumors.
• Fine-needle aspiration of enlarged lymph nodes, liver, or splenic nodules to confirm metastasis.
• Incisional biopsy for histopathology required for definitive diagnosis, although excisional biopsy may be appropriate if location of mass and cytology are supportive of anal gland neoplasia.

Drug(s)

• Limited reports of partial responses to platinum compounds in dogs-cisplatin (70 mg/m² IV with 6-hour saline diuresis-18.3 mL/kg/h), carboplatin (300 mg/m² IV as a slow bolus) every 3 weeks.
• Mitoxantrone (5 mg/m² IV every 3 weeks for five treatments) in combination with radiation therapy used in one small case series.
• Possible role for melphalan after debulking surgery (7 mg/m² PO q24h for 5 days every 3 weeks).
• Toceranib phosphate reported to have some benefit (partial response or stable disease) in 28 dogs with measurable tumor.

Contraindications/Possible Interactions

• Avoid platinum chemotherapeutic agents in dogs with renal insufficiency.
• Do not use cisplatin in cats.

Patient Monitoring

• Complete resection-physical examination, thoracic radiography, abdominal ultrasonography, and serum biochemistry at 1, 3, 6, 9, and 12 months postoperatively, then every 6 months thereafter.
• Incomplete resection-monitor tumor size and blood calcium and renal values.

Expected Course and Prognosis

• Guarded prognosis with both local progression and metastasis occurring.
• Cures may occur if tumor is found early and treated aggressively.
• Growth of the tumor may be slow and debulking lymph node metastatic disease may significantly prolong survival.
• Hypercalcemia is variably associated with a poor prognosis.
• Four papers (involving 200 dogs) showed median survival times of 6 to 20 months, depending on stage and treatment.
• A recent report on 16 dogs without metastasis showed a median survival time not met with a follow-up of 33 months.
• Dogs with lymph node metastasis lived significantly longer if the nodes were extirpated.
• Ultimately, dogs that cannot have their tumors excised completely succumb to hypercalcemia-related complications or mass effect from primary tumor or sublumbar nodal metastases.

Associated Conditions

Hypercalcemia as a paraneoplastic syndrome

Abbreviation

PTHrP = parathyroid hormone-related peptide

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