Chemotherapy

4B. A lateral caudal abdominal radiograph taken of the same dog as in Figure 4A three months after palliative radiation therapy, showing a partial reduction in the size of the iliac lymph nodes and less compression of the colon as it enters the pelvic canal. (The staples were placed during surgery to aid in radiation planning.)

In one study, two of four dogs with unresectable anal sac gland carcinomas that received carboplatin exhibited decreased mass size sufficient to permit surgical resection.⁶ In the same study, 12 of 40 dogs with iliac lymphadenopathy had relief of fecal obstipation after carboplatin treatment.⁶ A different study reported a partial reduction in tumor size in four out of 13 dogs with anal sac adenocarcinoma treated with cisplatin alone and in one out of three dogs treated with carboplatin alone.⁴ Favorable survival times have been reported in dogs treated with radiation, surgery, and mitoxantrone (n = 15) and surgery with oral daily melphalan (n = 19); however, the individual role of chemotherapy in those studies cannot be determined as there were no control groups.^5,7

PROGNOSIS

The survival time for dogs with anal sac adenocarcinoma varies, and retrospective studies have attempted to identify significant prognostic factors. The overall median survival time in two larger studies evaluating 80 and 113 dogs was 479 and 544 days, respectively.^6,8 The dogs in these studies received a wide variation in treatments, including surgery, radiation therapy, and chemotherapy, making it difficult to draw conclusions about the effect of individual treatment modalities or effective chemotherapy protocols. In the larger study, dogs that underwent surgery as a part of their treatment had an improved survival time compared with dogs that received chemotherapy alone.⁸ Larger tumors, the presence of distant metastatic disease, and hypercalcemia have been variably associated with a significantly decreased survival time.^3,6,8 The median survival time was higher at 956 days in a study of 15 dogs that were all treated in the same manner with surgery, radiation, and mitoxantrone chemotherapy.⁷ None of the tumors in the dogs in this study had spread beyond the iliac lymph nodes, which may have contributed to the lengthy survival time.⁷

Because of the prognostic significance of tumor size at diagnosis and the presence of metastatic disease, the most favorable prognosis can be expected with early detection and aggressive treatment. Although hypercalcemia has been associated with a decreased survival time in some studies, it has not been a consistent finding.^4,7,11 The presence of hypercalcemia should not warrant a more conservative approach, given that favorable responses and prolonged survival times with appropriate treatment have been reported.^7,11

SUMMARY

Anal sac adenocarcinoma is a locally invasive tumor with a high rate of metastasis to the local lymph nodes. Clinical signs may be associated with partial obstruction of the colon or rectum by the primary tumor or enlarged metastatic lymph nodes. In many cases, the tumor is an incidental finding on routine physical examination. Hypercalcemia is present in up to half of cases and may contribute to the presenting clinical signs; it typically resolves with surgical removal of the tumor. Routine staging includes a complete blood count, a serum chemistry profile, a urinalysis, thoracic radiography, and abdominal ultrasonography. A multimodal treatment approach is recommended, with surgery and radiation therapy providing local control. The ideal chemotherapy protocol for preventing or delaying recurrence or the development of metastatic disease is still undefined. Early detection and treatment are imperative to achieving the most favorable outcome.

Meredith Gauthier, DVM, DACVIM (oncology)*
Lisa G. Barber, DVM, DACVIM (oncology)
Kristine E. Burgess, MS, DVM, DACVIM (oncology)
Department of Clinical Sciences
Cummings School of Veterinary Medicine
Tufts University
North Grafton, MA 01536

*Current address:
Mississauga Oakville Veterinary Emergency Hospital and Referral Group
2285 Bristol Circle
Oakville, ON L6H 6P8

REFERENCES

1. Turek MM, Withrow SJ. Tumors of the gastrointestinal tract. H. Perianal tumors. In: Small animal clinical oncology. 4th ed. St. Louis, Mo: Saunders Elsevier, 2007;503-510.

2. Goldschmidt MH, Shofer FS. Anal sac gland tumors. In: Skin tumors of the dog and cat. 1st ed. Oxford, UK: Pergamon Press, 1992;103-108.

3. Ross JT, Scavelli TD, Matthiesen DT, et al. Adenocarcinoma of the apocrine glands of the anal sac in dogs: a review of 32 cases. J Am Anim Hosp Assoc 1991;27:349-355.

4. Bennett PF, DeNicola DB, Bonney P, et al. Canine anal sac adenocarcinomas: clinical presentation and response to therapy. J Vet Intern Med 2002;16(1):100-104.

5. Emms SG. Anal sac tumours of the dog and their response to cytoreductive surgery and chemotherapy. Aust Vet J 2005;83(6):340-343.

6. Polton GA, Brearly MJ. Clinical stage, therapy, and prognosis in canine anal sac gland carcinoma. J Vet Intern Med 2007;21(2):274-280.

7. Turek MM, Forrest LJ, Adams WM, et al. Postoperative radiotherapy and mitoxantrone for anal sac adenocarcinoma in the dog: 15 cases (1991–2001). Vet Comp Oncol 2003;1(2):94-104.

8. Williams LE, Gliatto JM, Dodge RK, et al. Carcinoma of the apocrine glands of the anal sac in dogs: 113 cases (1985-1995). J Am Vet Med Assoc 2003;223(6):825-831.

9. Polton GA, Mowat V, Lee HC, et al. Breed, gender and neutering status of British dogs with anal sac gland carcinoma. Vet Comp Oncol 2006;4(3):125-131.

10. Elliott J, Dobson JM, Dunn JK, et al. Hypercalcaemia in the dog: a study of 40 cases. J Small Anim Pract 1991;32:564-571.

11. Hobson HP, Brown MR, Rogers KS. Surgery of metastatic anal sac adenocarcinoma in five dogs. Vet Surg 2006;35(3):267-270.

12. Jeffery N, Phillips SM, Brearly MJ. Surgical management of metastasis from anal sac apocrine gland adenocarcinoma of dogs. J Small Anim Pract 2000;41:390.

13. Anderson CR, McNiel EA, Gillette EL, et al. Late complications of pelvic irradiation in 16 dogs. Vet Radiol Ultrasound 2002;43(2):187-192.