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
CLINICAL FEATURES
+ 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
TREATMENT
+ 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 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
