ADENOCARCINOMA OF THE APOCRINE GLANDS OF THE ANAL SAC IN DOGS

Background

Anal sac adenocarcinoma overwhelmingly occurs in old, female dogs, whether they are intact or spayed. There appears to be no breed predisposi­tion; however, one European study reported that five of eight affected dogs were long-haired and shorthaired German pointers.

A characteristic feature of this tumor is production of a parathyroid hormone-related protein that causes hyper­calcemia and hypophosphatemia.

Clinical Parameters

Affected dogs are often presented to veterinarians because of an unrelated problem, because the owner notices a swelling in the perineum, or because there is dyschezia, tenesmus, or a ribbonlike stool shape. Tenesmus may be due either to the primary tumor or to sublumbar lymphadenopathy, which may be palpable rectally. Signs may be present for up to 1 year before presentation. In 40% to 60% of dogs in several reported series the tumor was an incidental finding on rectal examination or was found only after hypercalcemia had been identified. This empha­sizes the importance of including a rectal palpation in rou­tine physical examinations. The tumor mass is usually between 1 and 10 cm in diameter; smaller primary masses that are difficult to palpate may be present. Because the tumor may be bilateral, it is important to palpate both anal sacs.

Other clinical signs are pruritus, ulceration and bleeding, decreased appetite, weight loss, and weakness or paresis. Polydipsia and polyuria are common in hypercalcemic dogs. The identification of hypercalcemia on a biochemical profile warrants careful palpation of the anal sacs.

Clinical Work-up

If anal sac adenocarcinoma is suspected in a dog, a routine blood chemistry profile should be per­formed to identify hypercalcemia and any second­ary renal damage. Definitive diagnosis is made by surgical biopsy, although a high index of suspicion for this disease should follow detection of a peri­anal mass in an old, female dog with hypercalcemia.

Paraneoplastic hypercalcemia is common in dogs with apocrine gland adenocarcinoma of the anal sacs and may occur in both males and females. In one study, serum calcium was elevated in 25% of the affected dogs to an average of 14.6 mg/dl. In another study 90% of dogs with anal sac adeno­carcinoma had elevated serum calcium levels, to an average of 16.1 mg/dl. Hypophosphatemia occurred concurrently with hypercalcemia in some but not all of the affected dogs.

This neoplasm is highly malignant and metasta­sizes early in the course of the disease to the sub- lumbar and iliac lymph nodes. In one study approximately 50% of the dogs developed lymph node metastases; in two other studies 94% of the dogs had metastases to the above-mentioned sites. Abdominal radiographs are useful in identifying sublumbar lymphadenopathy, but ultrasonography may be more accurate than radiographs or digital rectal palpation in disclosing the extent of lymph node involvement.

Less frequent sites of metastasis are the lungs, which may show a nodular or diffuse pattern ra­diographically, and (rarely) the lumbar vertebrae, liver, and kidneys. Metastasis may occur when the primary tumor is very small, and clinical signs relating to the primary tumor may not be obvious.

Prognostic Factors

In one study, hypercalcemic dogs had a median survival of 6 months after surgical excision of the tumor, compared with 11.5 months for normocal- cemic dogs. Dogs with metastases detected at sur­gery predictably had shorter median survival times (6 months) than did dogs without metastases (15.5 months).

Therapeutic Approach

Ideally, hypercalcemia should be controlled before and during definitive therapy. Surgical excision of the primary tumor is often difficult because of the large size of these tumors and their invasive growth characteristics.

Complications of surgery reflect the difficulties encountered in any surgical procedure involving the perineal area. Fecal incontinence can follow surgery in up to 20% of dogs and may be perma­nent. Wound infection can occur and cause sepsis.

Local recurrence is a problem with some dogs, and others develop recurrence in the regional lymph nodes. If the sublumbar nodes are enlarged at diagnosis, it may be possible to remove them surgically; however, tumor-invaded nodes are fre­quently friable and invade around the vessels and nerves in this area.The nodes were well-encapsulated in 80% of dogs treated surgically in one study, but they were also well vascularized; thus the surgeon should be prepared to encounter bleeding. In this study, complications during lymph node surgery caused the death of one third of the dogs; almost one third of the survivors developed transient uri­nary incontinence, presumably as a result of neu­rologic trauma. Overall, 6 of 27 dogs died within 2 weeks after undergoing surgery for removal of either the primary tumor or its metastases. Median survival for the remaining dogs was 8.3 months; the range of survival was 6 weeks to 39 months. Five dogs were still alive at 14 months after sur­gery. This moderate success rate was corroborated in another study in which 50% of the dogs died between 2 and 22 months after surgery, with an average survival of 8.8 months. In another report the median survival of dogs with this disease treated with surgery alone was 295 days.

Chemotherapy might be promising as adjuvant therapy for this tumor, but relatively little has been reported.Three dogs treated with doxorubicin and cyclophosphamide, either alone or in combination with prednisolone, vincristine, and L-asparaginase (for concurrent lymphoma), had survival times of 1, 2, and 14 months.

Dogs with anal sac adenocarcinoma should be treated surgically in an attempt to achieve complete excision of the primary mass. Sublumbar lymph nodes should be removed if they are enlarged, although this is a technically demanding surgery. Adjuvant radiation therapy and chemotherapy using cisplatin should be considered, although the roles of adjuvant therapies are still being defined.