anal sac tumors

BACKGROUND

Apocrine gland anal sac adenocarcinomas (AGACACs) are uncommon in dogs and very rare in cats. Hypercalcemia is a common paraneoplastic syndrome in dogs with AGACAC because of tumor cell production of parathyroid hormone-related protein. Metastasis to the sublumbar lymph nodes and lungs is also common.

DIAGNOSIS

Rectal examination is important for the diagnosis of an anal sac mass and assessment of enlarged sublumbar lymph nodes. Blood tests (including ionized or total calcium levels) are important for determining health status for surgery and prognostic information. Fine-needle aspirates can be used to differentiate AGACAC from other anal sac masses such as abscesses, but this is rarely necessary.

CLINICAL STAGING

CT scans of the abdomen and thorax are recommended for the evaluation of sublumbar lymph node metastasis and lung metastasis, respectively. CT imaging of both cavities also permits assessment of other potential sites of metastasis, such as the spleen and sternal lymph nodes. Abdominal ultrasonography can be used to assess the sublumbar lymph nodes and other intra-abdominal organs, but this has been found to be significantly inferior to advanced imaging because of poor ability to assess lymph nodes in the pelvic canal.

MRI - Sublumbar LN Metastasis

Ultrasound - Sublumbar LN Metastasis

CT - Bone Metastasis

TREATMENT

The treatment of dogs with AGACAC depends on whether the anal sac tumor is confined to the anal sac or metastatic to other sites. Surgery is recommended for dogs with non-metastatic AGASAC and AGASAC metastatic to the sublumbar lymph nodes (and possibly other intra-abdominal organs and the sternal lymph node). Chemotherapy is recommended for dogs with metastatic AGASAC, but not dogs with non-metastatic AGASAC. The role of radiation therapy has not been defined, but there is a strong potential postoperative radiation therapy may have some benefit in dogs with AGASAC metastatic to the sublumbar lymph nodes.

PROGNOSIS

The prognosis for dogs with AGACAC ranges from guarded to very good depending on the extent of disease. For dogs with non-metastatic AGASAC, the overall median survival time is > 1745 days with median survival times of 1612 days for surgery alone, > 1009 days for surgery and chemotherapy, and > 1163 days for surgery and radiation therapy. For dogs with AGASAC metastatic to the sublumbar lymph nodes, the overall median survival time is 551 days with median survival times of 358 days if treated with anal sacculectomy alone (i.e., sublumbar lymph nodes are not excised), 546 days if treated with anal sacculectomy and sublumbar lymph node excision, and 1927 days if treated with anal sacculectomy, sublumbar lymph node excision, and chemotherapy. Excision of the sublumbar lymph nodes results in a significant improvement in survival times over anal sacculectomy alone, and the addition of chemotherapy results in a significant improvement in survival times over surgery alone. The local tumor recurrence rate following anal sacculectomy is 18%. Poor prognostic factors for survival include preoperative hypercalcemia, AGASAC size (>2.5cm), sublumbar lymph node metastasis, and lung metastasis, and treatment protocols which do not include surgery.