Identifying and treating anal sac adenocarcinoma in dogs - Veterinary Medicine
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Identifying and treating anal sac adenocarcinoma in dogs
Because these aggressive tumors may be hard to spot, a routine rectal examination is recommended in all dogs to increase the likelihood of early detection. In addition, this tumor should be suspected in any dog with hypercalcemia.



When technically feasible, surgery is the initial treatment of choice for anal sac adenocarcinoma limited to the primary tumor and local lymph nodes. Surgery may also be indicated in dogs with distant metastatic disease as a palliative measure if the tumor is causing clinical signs. The primary mass should be surgically resected with the intent of complete excision. Because of the tumor's local infiltration, location, and large size at presentation, an aggressive surgery with wide margins often is not possible. Fecal incontinence is a potential complication, which can be temporary or permanent. Other postoperative complications include wound infection, which can result in dehiscence or sepsis, hypocalcemia, and perianal fistula formation.3,8 Early surgical intervention is important, as the masses can be fast-growing, and a window of opportunity may be missed if the mass becomes inoperable. In dogs with hypercalcemia, even incomplete resection can lead to normalization of calcium concentrations postoperatively.3

If the iliac lymph nodes are enlarged, an exploratory laparotomy should be performed for assessment and resection if possible. In cases of hypercalcemia, the tumor cells within the lymph node can secrete PTH-rP, so primary tumor resection alone may not result in normocalcemia. Additionally, enlarged lymph nodes that are not surgically removed will continue to grow and eventually will likely result in obstipation and tenesmus.

Two small case series reported lymphadenectomy and excision of the primary tumor in four and five dogs, with a mean survival of three years in the first series and a median survival of 20.6 months in the second series; two dogs were still alive at 19 and 54 months.11,12 No surgical complications were noted in any of the cases. Another retrospective study found a significant survival advantage in dogs with anal sac gland carcinoma that underwent primary tumor removal and lymph node removal compared with dogs whose metastatic lymph nodes were not removed.6 The lymph nodes may be easily resected in some cases, but they can also be friable and invade the large blood vessels that surround them. Referral to a board-certified surgeon is advised for lymphadenectomy because of the difficult location and potential complications. Fatal hemorrhage is possible, and neurologic damage can lead to transient or irreversible urinary incontinence.3,4 In a few cases in which complete resection is possible, adjuvant treatment may not be required to attain long-term survival, but more data are needed to determine whether recurrence or metastasis develops in dogs treated with surgery alone.6

Routine postoperative management includes patient-appropriate analgesia with opiates and nonsteroidal anti-inflammatory drugs. Hypercalcemia usually resolves within 24 hours. Food can be introduced within eight to 12 hours of surgery, and a stool softener can be added to the food for two or three weeks. The dog should wear an Elizabethan collar to prevent self-trauma to the surgical site and be rechecked two weeks after surgery for fecal incontinence or anal stricture.

Because of the difficult location of the tumor and local invasiveness, marginal resection is often performed, and recurrence is common, reported in 45% to 50% of dogs.3,4 Adjuvant treatment is recommended in cases of marginal excision to increase local control and decrease the risk of recurrence.

Radiation therapy

Radiation therapy can be used preoperatively or postoperatively or palliatively as the sole method of treatment.

Preoperative. The goal of preoperative radiation is to kill tumor cells existing as microscopic extensions at the tumor periphery, which increases a surgeon's ability to obtain clean margins. Radiation may also shrink the tumor, thereby increasing the ease of surgical resection. The total dose delivered to the region is decreased in preoperative radiation to prevent healing complications with the intended future surgery. At our hospital, preoperative megavoltage radiation therapy consists of 22 daily fractions of 2.5 Gy, compared with 24 daily fractions of 2.5 Gy for postoperative radiation. Surgery can be performed when the acute radiation side effects have healed, typically within two to four weeks.

Postoperative. Postoperative radiation is used in cases in which clean margins are not obtained with surgery. Radiation is started at suture removal, when the surgical incision has healed, and is administered over a course of three to five weeks. Radiation to the lymph node bed should also be considered after iliac lymph node resection in which disease breaches the capsule since a margin of normal tissue cannot be obtained in this location. The lymph node bed may also be radiated prophylactically in patients that did not have enlarged lymph nodes at presentation since that is the most common location for the development of future metastatic disease.7

3A. Typical radiation-induced moist desquamation two weeks after completion of 24 fractions of 2.5 Gy to the perineum after incomplete resection of anal sac adenocarcinoma.
Definitive radiation can also be used to treat metastatic lymph nodes that could not be resected at surgery in hopes of decreasing their size and slowing growth for an extended period. In one study, three dogs with anal sac adenocarcinoma had 15 daily fractions of 3.2 Gy of megavoltage radiation therapy to enlarged iliac lymph nodes.7 Two dogs were reevaluated with abdominal ultrasonography: one had complete resolution of the enlarged lymph nodes for two years, and the other dog had one enlarged lymph node that completely responded and another node that reduced in size by more than 50%.7

3B. The same dog as in Figure 3A one month later showing resolution of the erythema and moist desquamation.
Complications. Because of the location of anal sac adenocarcinoma and the iliac lymph nodes, many sensitive normal tissues, including the perineal skin, anus, rectum, and colon, may receive unavoidable radiation, and, consequently, side effects can be severe preoperatively and postoperatively. Acute complications that occur during and several weeks after treatment include colitis, tenesmus, moist desquamation of the perineal skin, and perineal discomfort (Figure 3A).7,8 Topical treatments (vitamin E, aloe, commercially available radiation creams), Elizabethan collars, and intensive pain management are required in most cases. Antibiotics may be required to treat secondary skin infections in some patients. Colitis can be managed with fiber supplementation and metronidazole. The side effects are typically self-limiting and resolve several weeks after radiation is finished (Figure 3B).

Late complications occur months to years after radiation therapy and may present as chronic problems such as chronic diarrhea, chronic tenesmus, rectal stricture, fecal incontinence, and colonic perforation.13 The use of smaller fractions of radiation per dose is associated with a lower risk of these complications.13

Treatment failure is another complication, defined as regrowth of a tumor or lymph node in the area of previous radiation. In a study of 15 dogs with anal sac adenocarcinoma treated with surgery, radiation, and mitoxantrone chemotherapy, four dogs developed recurrence within the radiation field.7

4A. A lateral caudal abdominal radiograph of a dog with anal sac adenocarcinoma and metastatic iliac lymphadenopathy. The enlarged iliac lymph nodes can be seen and are causing ventral deviation and partial obstruction of the colon. (The pellet was an incidental finding.) Surgical resection of the iliac lymph nodes was attempted but aborted because of a high risk of hemorrhage. The primary anal sac tumor was resected. Palliative radiation therapy (three weekly doses of 8 Gy) to the iliac lymph nodes resulted in resolution of clinical signs.
Palliative. Radiation can also be used as a palliative treatment in dogs that are not candidates for definitive treatment with surgery and radiation. Palliative or hypofractionated radiation therapy typically consists of larger doses (6 to 8 Gy) of radiation given weekly for three or four weeks. The goal of this therapy is to provide a good quality of life by relieving clinical signs associated with the anal sac mass or enlarged iliac lymph nodes for a limited amount of time. Palliative radiation therapy is often used in patients with a guarded prognosis due to distant metastatic disease, extensive local disease, or concurrent illnesses. Also, some owners may decline definitive treatment because of cost, time commitment, or concerns about morbidity. In our experience, partial regression or stabilization of tumor size with alleviation of clinical signs is achieved in about three quarters of cases treated with hypofractionated megavoltage radiation therapy (Figures 4A & 4B).


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