Anal Sac Adenocarcinomas

A 10 year 6 month old, FS, German shepherd, initially presented to her primary veterinarian for vomiting yellow liquid 2-3 times per week for the past 1-2 months. Also, she was straining to defecate with no production when going on walks but was having normal bowel movements in the yard prior to going on walks.  On physical examination she was noted to have a 1-2 cm mass in the left anal sac.  Abdominal radiographs (not available for review) and a FNA of the anal sac mass were performed.  Cytology results revealed an anal sac adenocarcinoma and referral to an oncologist was recommended.

She was evaluated by the oncology service at Hope Veterinary specialists 10 days later.  Complete blood work was performed which was unremarkable.  Three view thoracic radiographs revealed no evidence of metastasis and abdominal ultrasound showed an enlarged left sublumbar lymph node that measured 1.75 x 4.19 cms (Figure 1) and was normoechoic with diffusely heterogenous parenchyma.

Treatment options were discussed including the most aggressive option of surgery followed by adjuvant treatment such as radiation +/- chemotherapy vs. a palliative approach including radiation to the mass and lymph nodes with systemic therapy; either Palladia or chemotherapy.

The owners opted to pursue surgery and a left anal sacculectomy/mass resection and left sublumbar lymph node excision were performed without complications.  Histopathology results revealed an anal sac adenocarcinoma with lymphatic invasion and narrow excision.  The left medial iliac lymph node showed metastatic anal sac adenocarcinoma.

A follow-up appointment with the surgery service for suture removal and with the oncology service was performed at 16  days postop.  The sutures were removed and chemotherapy (carboplatin IV q 3 weeks x 4-6 cycles) +/- radiation to the draining lymph nodes was discussed.  The owner opted for chemotherapy alone and carboplatin was started that day.  A recheck CBC in one week had no concerning findings.  A recheck examination was performed by the oncology service 3 weeks after receiving her first dose of carboplatin and the owner reported that the first dose had affected her appetite slightly for a few days but had since resolved.  A CBC had no concerning findings and the second dose of carboplatin was administered and maropitant citrate was prescribed.  A recheck ultrasound to evaluate her lymph nodes is to be performed after two cycles of Carboplatin.

Tumors of the anal sac are uncommon and 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 disease. There does not appear to be a breed or sex predilection for this tumor and no consistent etiology has 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 at the perianal region, and bleeding were most commonly seen and are a result of a large anal sac tumor or severely enlarged regional lymph nodes. In other cases the increased drinking and urination was noted as a result of high calcium in the blood. This paraneoplastic syndrome is associated with release of the parathyroid related protein (PTHrp) which elevates the blood calcium, in some cases leading to significant kidney damage. 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 anal sac adenocarcinoma includes; bloodwork (CBC, serum chemistry), urinalysis, chest radiographs, abdominal ultrasound and aspirates of the tumor or regional lymph nodes (if enlarged). A definitive diagnosis is obtained either by aspirates of the tumor or lymph node or biopsy of the tumor. Metastasis is relatively common with this tumor and may be present in 36-96% of the 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 a cure and involves removing the anal sac and regional lymph nodes if affected.  In cases where surgery is not complete, then radiation therapy offers good local control. At the same time radiation therapy is also directed at the regional lymph nodes (sublumbar) prophylactically.

The benefits of chemotherapy for this cancer are unknown, however, chemotherapy is added to most protocols due to the high chance of metastasis by the time therapy is instituted. Commonly used chemotherapy agents include Carboplatin

In some cases multimodality therapy are used (surgery, chemotherapy, radiation therapy) in the treatment of dogs with anal sac carcinoma and in one study of dogs treated with surgery, radiation therapy (15 treatments) and chemotherapy (mitoxantrone) a median survival of  > 900 days was noted. Although the survival was quite favorable, significant radiation therapy induced side effects were noted in majority of dogs.

Lastly, a novel class of drugs exist called “signal blockers ” which have been shown to have efficacy against this cancer (Palladia).  It inhibits signals used by the tumor to create new blood vessels and thus can be used against a variety of tumors, including anal sac adenocarcinomas. In a study of dogs receiving Palladia nearly a 70% response rate was noted and the response duration was ~23 weeks.

Submitted by Dr Lauren May VMD, DACVS


mayFigure 1

Ultrasound image showing an enlarged left medial iliac lymph node (1.75 x 4.2 cms).

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