Identifying and treating anal sac adenocarcinoma in dogs
Anal sac adenocarcinoma is reported to represent 2% of all skin tumors and 17% of perianal tumors in dogs.1,2 So although it is an uncommon tumor, it is locally invasive and has a high rate of metastasis to the draining iliac lymph nodes, making early diagnosis and treatment essential for long-term control. Hypercalcemia of malignancy is seen in some cases as a paraneoplastic syndrome and may contribute to presenting clinical signs.
Early detection and aggressive treatment are essential in the successful management of this tumor, which emphasizes the importance of a rectal examination as part of a routine physical examination in all animals. Treatment typically requires a multimodal approach involving surgery, radiation, and, potentially, chemotherapy.
SIGNALMENT AND CLINICAL PRESENTATIONAnal sac adenocarcinoma occurs primarily in older dogs, with an average age of 10.5 years.2 Historically, female dogs were reported to be predisposed to anal sac adenocarcinoma.3 However, most recent retrospective studies have shown an equal sex distribution between spayed females and castrated males.2,4-8 Interestingly, a study of 810 British dogs with anal sac adenocarcinoma found that intact males were less likely to develop this tumor, suggesting a possible protective effect of testosterone.9 Several breeds have been reported as overrepresented, including cocker spaniels, springer spaniels, German shepherds, dachshunds, and Alaskan malamutes.2
Owners or groomers may be the first to notice perineal swelling or discomfort. The primary anal sac mass or enlarged iliac lymph nodes may cause constipation, obstipation, tenesmus, dyschezia, or ribbonlike stools. Earlier clinical signs are not always noticed by owners but can include scooting and excessive licking of the perineal area. Polyuria, polydipsia, anorexia, and lethargy can be seen secondary to hypercalcemia of malignancy, a common sequela of anal sac adenocarcinoma, although not all dogs with hypercalcemia will display such clinical signs. Anal sac adenocarcinoma can also be an incidental finding on rectal examination in a high proportion of affected dogs.8 On occasion, the anal sac mass may be small compared with the size of the metastatic iliac lymph nodes, so an abdominal ultrasonographic examination with careful attention to the sublumbar area is recommended when an anal sac mass is palpated or in cases of hypercalcemia of unknown origin.8
In most cases, a physical examination will reveal a perianal mass associated with one or both anal sacs. Occasionally, a mass may not be appreciated externally, so a thorough digital rectal examination is advised in all dogs. Enlarged iliac lymph nodes may be palpable on rectal examination in cases of metastatic or reactive lymphadenopathy, but this enlargement can be difficult to detect in larger dogs or in cases in which the enlargement is subtle.