+ General Considerations

  • Apocrine gland anal sac ADC accounts for 2% of all skin tumors
  • Anal sac ADC is more common in dogs and very rare in cats
  • Other anal sac tumors include adenoma and SCC
  • Metastasis and hypercalcemia is rare in cats with anal sac ADC
  • Site: firm and fixed mass with local invasion at either 4 o'clock or 8 o'clock
  • Anal sac mass may be small and hypercalcemia or metastasis may be detected prior to anal sac mass

+ Paraneoplastic Hypercalcemia

  • Paraneoplastic hypercalcemia is common and present in 27%-90% cases
  • Hypercalcemia is associated either with the anal sac ADC or, more commonly, with regional lymph node metastasis
  • Hypercalcemia of malignancy or pseudohyperparathyroidism is due to the production of PTH-related protein which is consistently and markedly increased compared to normal dogs and decreases to normal range following surgical resection
  • Linear relationship between total serum calcium and PTH-related protein in dogs with anal sac ADC
  • Parathyroid glands are atrophied in dogs with anal sac ADC and hypercalcemia
  • Complete or near complete removal of tumor burden will often reverse hypercalcemia within 1-2 days
  • Recurrence of hypercalcemia is indicative of local tumor recurrence or metastasis

+ Metastasis

  • 56%-79% metastatic rate at the time of diagnosis with regional lymph node very common (47%-72%)
  • Metastatic sites: regional lymph nodes (sublumbar), lungs, spleen, bone (appendicular and axial skeleton), pancreas, heart, and mediastinum


+ Signalment

  • Breed predisposition: GSD, English Cocker Spaniel, Dachshund, Alaskan Malamute, and English Springer Spaniel
  • Median age 10-11 years
  • Sex predilection for spayed female was initially reported, but majority of studies do not support this finding

+ Clinical Signs

  • Perianal mass or incidental finding
  • Clinical signs usually caused by either hypercalcemia (i.e., polyuria, polydipsia, and urinary incontinence) or obstruction of pelvic canal secondary to metastatic lymphadenopathy (i.e., constipation and tenesmus)
  • Pain, lameness ± neurologic disease occasionally seen due to regional bone metastasis ± direct extension from sublumbar lymph nodes to lumbar vertebra

+ Diagnosis

  • Rectal examination to assess sublumbar lymph node size and mobility
  • Serum biochemistry: ionized calcium and renal profile
  • FNA of anal sac mass may differentiate perianal tumors from other tumor types but will rarely differentiate benign from malignant perianal tumors (benign anal sac tumors are very rare)
  • Caudal abdominal radiographs or, preferably, ultrasound, CT, or MRI to assess sublumbar lymph node size
  • FNA of sublumbar lymph node may differentiate metastatic lymph node from other causes of lymphadenomegaly
  • Chest radiographs: pulmonary metastasis is uncommon without sublumbar lymph node metastasis


+ Surgery

  • Aggressive saline diuresis ± diuretic therapy may be required prior to surgery if hypercalcemic with renal failure
  • Wide local resection:
  • Resection of < 50% of anal sphincter will cause transient but not permanent fecal incontinence
  • Complete resection often difficult due to proximity to rectum and poor definition of perianal area
  • Recurrent disease difficult to resect
  • Exploratory celiotomy and sublumbar lymphadenectomy:
  • Sublumbar lymph nodes can be resected in approximately 50% of cases
  • Resectability cannot be determined preoperatively and large sublumbar nodes do not preclude resection
  • Sublumbar lymph nodes can either be invasive or easily removed
  • Castration has no benefit
  • Omentalization of the sublumbar lymph nodes has been described in 1 dog for palliation of tenesmus and dysuria caused by metastatic cystic sublumbar lymph nodes
  • 10% complication rate associated with anal sac resection including intraoperative hemorrhage, infection, fecal or urinary incontinence, hypocalcemia, tenesmus, and perianal fistula formation

+ Radiation Therapy

  • Indications: inoperable anal sac ADC and metastatic ± inoperable sublumbar lymph node
  • Radiation therapy can be used either intraoperative (10-15 Gy) for treatment of the sublumbar lymph nodes or postoperative for both the anal sac ADC and sublumbar lymph nodes

+ Chemotherapy

  • Effect of chemotherapy is unknown
  • Platinum drugs have shown 30%-33% PR in 2 studies
  • Doxorubicin ± cyclophosphamide has been associated with short-term PR
  • Mitoxantrone has been combined with radiation therapy
  • Piroxicam has no proven effect but antitumor and antiangiogenic effects may be beneficial

+ Prognosis

  • Fair to good prognosis
  • Anal sacculectomy ± radiation therapy provides good control of local disease
  • However, anal sacculectomy alone will often result in failure at sublumbar lymph nodes
  • Surgery ± intraoperative ± adjunctive radiation therapy may be effective for control of sublumbar lymph nodes and is associated with mild to marked colitis and proctitis
  • Local tumor recurrence in 45%-50% with a median DFI 10 months
  • MST 544 days, with 2-year survival rate 37%
  • MST 500 days for surgery alone, with 1-year survival rate 65% and 2-year survival rate 29%
  • MST 657 days for radiation therapy alone, with 1-year survival rate 79% and 2-year survival rate 38%
  • MST 540 days for surgery and chemotherapy, with 1-year survival rate 69% and 2-year survival rate 36%
  • MST 742-956 days for surgery, radiation therapy, and chemotherapy, with 1-year survival rate 80%, 2-year survival rate 56%, and 3-year survival rate 35%
  • Poor prognostic signs include lung ± lymph node metastasis, hypercalcemia, tumor size, and treatment, with MST significantly shorter with:
  • Lung metastasis (219 days v 584 days) ± any metastasis (16 months v 6 months)
  • Hypercalcemia (256 days v 584 days)
  • Tumors ≥ 10 cm 2 (292 days v 584 days)
  • Dogs not treated with surgery (402 days v 548 days)
  • Dogs treated with chemotherapy alone (212 days v 584 days)
  • No significant difference in survival for dogs with surgically resected sublumbar lymph nodes and dogs without sublumbar lymph node metastasis
  • Cause of death renal failure secondary to hypercalcemia or local or distant metastasis