Clinical Rounds: Anal sac adenocarcinoma - Veterinary Medicine
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Clinical Rounds: Anal sac adenocarcinoma
Make rounds with these veterinary specialists and residents for a complete picture of this neoplasia in dogs.


Karen M. Tobias, DVM, MS, DACVS
Surgical perspective
Karen M. Tobias, DVM, MS, DACVS

4A. A preoperative view of a large anal sac adenocarcinoma in a mixed-breed male dog.
Mass resection, including the entire anal sac, is the treatment of choice for anal sac adenocarcinoma. It is not necessary to remove the other anal sac if it is unaffected.

4B. An intraoperative view of the same dog as in Figure 4A. Masses of this size require resection of the anal sphincter and a portion of the rectal wall.
With large masses, a portion of the anus will be included in the resection; in some dogs, a small amount of rectum must also be excised (Figures 4A & 4B). Potential complications of surgery include infection, dehiscence, incontinence, tenesmus, rectocutaneous fistula, and recurrence.1,3 Concurrent removal of infiltrated sublumbar lymph nodes is controversial; improved prognosis was correlated with lymph node resection in one study2 of 80 dogs but not in another study3 of 32 dogs. Potential complications of lymph node removal include severe hemorrhage, urinary incontinence, and death. Postoperative monitoring should include physical and digital rectal examinations, thoracic radiography, and abdominal ultrasonography at one, two, three, and six months after surgery. If recurrence or metastases are not noted by six months after surgery, rechecks are often extended to twice yearly.

Medical oncology perspective
Olya Smrkovski, DVM, DACVIM (oncology)

Olya Smrkovski, DVM, DACVIM (oncology)
Anal sac adenocarcinoma is an aggressive neoplasm, with metastatic disease reported in more than 50% of dogs at the time of diagnosis.1-4 The most common site of metastasis is the sublumbar lymph nodes, but the tumor may also spread to the lungs, liver, spleen, and bone. The underlying etiology of the tumor is unknown, although breed predisposition has been reported in cocker spaniels.1,2

Negative prognostic indicators include the presence of metastases, a primary tumor size > 10 cm2, and a lack of surgical therapy. In one study, hypercalcemia at the time of diagnosis was also associated with shorter median survival time.4 With treatment, survival of dogs with small masses (< 2.5 cm) and no lymph node enlargement or metastatic disease is more than three years.2 Dogs with larger masses but no lymph node enlargement have a median survival time of about two years, and dogs with enlarged lymph nodes have a median survival time of one to one and a half years.2 If evidence of pulmonary metastasis is found, the median survival time decreases to 219 days.

While surgery is the most appropriate treatment for dogs with no metastasis on presentation, a multimodal approach, including radiation therapy and chemotherapy, is likely beneficial for dogs with nodal metastasis.8 The role of chemotherapy in prolonging the overall survival time of dogs with anal sac adenocarcinoma remains undefined. Doxorubicin, mitoxantrone, and platinum agents have documented antitumor activity and should be considered, especially when surgery and radiation are not options. The median survival time of dogs treated with chemotherapy alone is 212 days.4 The effectiveness of toceranib for treating anal sac adenocarcinoma has not been reported; however, short-term resolution of hypercalcemia and reduction in local recurrence and metastasis of anal sac adenocarcinoma have been noted anectdotally.9 Toceranib's mechanism of action against anal sac adenocarcinomas has not been elucidated.


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