Canine Anal Sac/Gland Adenocarcinoma

Tumors of the anal sac are uncommon a represent a small percentage of all tumors in dogs (<1%). The most common malignant tumor of the perianal region is the anal sac (gland) carcinoma accounting for 16.5% of all perianal tumors. These tumors are locally invasive and metastasize early in the course of the disease. There does not appear to be a breed or sex predilection for this tumor and no consistent causative factors have been noted.

In many cases, these tumors are noted as an incidental finding on a routine rectal examination and can range in size from very small to very large before clinical signs occur. In dogs with clinical signs, perianal swelling, straining to defecate, licking the perianal region, and bleeding, were most commonly seen and resulted from a large anal sac tumor or severely enlarged regional lymph nodes pressing on the colon. In other cases, increased drinking and urination was noted as a result of high calcium in the blood. High blood calcium may be seen in some cases secondary to the cancer and is associated with a blood factor released by the tumor. High blood calcium can cause significant kidney damage in some cases. It has been shown that up to 25% of dogs with anal sac tumors will have elevated calcium levels in the blood.

A diagnostic workup for this cancer includes bloodwork (CBC, serum chemistry), chest radiographs, abdominal ultrasound, and in some cases aspirates of the tumor or regional lymph nodes (if enlarged). A definitive diagnosis is obtained either by aspirates or biopsy of the tumor. This workup will also stage the patient and help determine where the tumor may have spread. Metastasis is relatively common with this tumor and may be present in 50% or more of dogs.

Once a diagnosis has been obtained, therapy will be determined based upon the size of the tumor and ability of the surgeon to remove the tumor without complications. Surgery offers the only chance for long-term control (a cure is sometimes possible) and involves removing the anal sac and regional lymph nodes if enlarged. In cases where surgery is not complete or “narrow” and some residual cancer cells exist at the surgery site, then radiation therapy may offer good local control. Radiation therapy is directed at the scar and also the regional lymph nodes (sublumbar). Acute side effects of radiation therapy may be noted and result in colitis and rectal irritation. During this time period, an e-collar must be worn to prevent licking of the area. Despite the side effects, radiation therapy is the best post-op treatment option that may provide long-term local tumor control in dogs with residual cancer.

The complete benefits of chemotherapy for this cancer are uncertain, however, chemotherapy is added to most protocols due to high chance of this cancer spreading by the surgery is instituted. Invasion of the tumor cells into regional blood and lymphatic vessels may be noted on the biopsy and will increase the potential for tumor spread/metastasis. The most commonly used chemotherapy agents include mitoxantrone, adriamycin, carboplatin, and gemcitabine. Other agents used as part of a treatment protocol include NSAIDs such as Deramaxx, Rimadyl, or Piroxicam. These drugs have been shown to have anticancer effects by directly killing cancer cells, increasing their sensitivity to radiation therapy or chemotherapy, and by killing blood vessels that feed tumors.

In most cases all three types of therapy are used (surgery, radiation therapy, and chemotherapy) in the treatment of dogs with anal sac carcinoma. A recent study in which dogs were treated with surgery, radiation therapy and chemotherapy (mitoxantrone), found that half the dogs lived for >900 days.

Prognostic factors help us to determine the likely outcome of a patient. Previously identified factors associated with this tumor include:

  • Dogs with high calcium levels have a poor prognosis (250 days vs 580 days)
  • Dogs with lung metastasis have a poor prognosis (219 days vs 548 days)
  • Dogs treated with surgery as part of therapy had a better prognosis than those without surgery (548 days vs 402 days)
  • Dogs with large tumors (>1 inch) have a poor prognosis (292 days vs 584 days)

In cases in which the tumor cannot be removed with surgery (large, bulky tumors or cases with distant tumor spread), treatment with palliative radiation therapy and various types of chemotherapy can be implemented. This type of radiation therapy (+/-chemotherapy) involves administering weekly treatments and is designed to shrink the tumor or prevent it from growing while alleviating any clinical signs associated with the tumor (ex: difficulty in urination/defecation, increased calcium level) and is not intended to cure dogs of their cancer. In one study, 75% of the dogs treated experience improvement of their clinical signs, 43% experienced a reduction in the size of the tumor and ~ 30% had disease stabilization. The overall duration of response was 6-9 months and the overall survival time was 11 months. Factors that significantly affected survival were: lymph node metastasis (8 months), resolution of clinical signs post treatment (15 months), and cases that experienced complete or partial regression of the tumor (15 months).

Newer chemotherapy agents such as Palladia (Toceranib) may also be considered to incorporate into the treatment protocol for dogs with anal sac tumors. This newer chemo agent has shown efficacy in shrinking and/or stabilizing tumors. Additionally, it can be considered as a cross-over (“rescue”) chemo in cases where indicated.