Step-by-Step Surgery for Anal Sac Cancer
Anal sac cancer causes local problems, mainly because of pain and straining to defecate. Anal sac apocrine gland adenocarcinoma also leads to systemic issues, including hypercalcemia and metastasis. Affected patients are older female dogs in 90 percent of the cases. A few cases have been described in cats.
It is important to keep an open mind. Differential diagnosis for perianal disease includes anal sac impaction, abscess (which may rupture), sacculitis, perianal adenomas, perineal hernias, perineal fistulas and other malignancies, such as perianal malignant melanoma. Any of these conditions affect the quality of life of the patient.
|A preoperative view of a left anal sac adenocarcinoma in a 9-year-old male Akita mix. Photo courtesy of Dr. Phil Zeltzman.|
An anal sac tumor is suspected on rectal exam. If it is large enough, subcutaneous swelling may be visible. Occasionally, the tumor is bilateral. A diagnosis can sometimes be reached with cytology if enough cells exfoliate. Histopathology gives a definitive diagnosis. Incisional biopsies are rarely used, whereas excisional biopsy is typically the preferred course of action.
Standard preoperative workup includes a CBC and blood chemistry, including a calcium level. Approximately 25 percent of patients are affected by paraneoplastic hypercalcemia (See our bonus content on differential diagnosis of hypercalcemia). Abdominal radiographs or ultrasound are helpful to diagnose metastasis to the sublumbar lymph nodes.
It is wise to remember that a small anal sac tumor does not rule out metastasis; size doesn’t matter. If you think that sublumbar lymph nodes look enlarged on abdominal radiographs, they probably are. Abdominal ultrasound should be recommended to confirm the suspicion; up to 50 percent of patients do have metastasis to the sublumbar nodes.
An echo-guided fine-needle aspirate or biopsy will confirm the suspicion. Ultrasound also will rule out metastasis to unusual abdominal sites, such as the liver. Thoracic radiographs should also be recommended, as 10 to 20 percent of patients have lung metastasis.
Hypercalcemia causes polyuria-polydypsia as well as lethargy, anorexia and sometimes vomiting. In the long run, hypercalcemia can lead to chronic kidney failure. Hypercalcemia may be present because the tumor produces PTH-rp (parathyroid hormone related peptide). If hypercalcemia is shown on preoperative blood work, it needs to be corrected before anesthesia and surgery. Diuresis with saline and/or furosemide (1 mg/lb IV every 12 hours) will help normalize calcium concentration.
|A right anal sac carcinoma in an 8-year-old male bassett hound. Photo courtesy of Dr. Phil Zeltzman.|
Before performing anal sacculectomy, it is critical to remember the local anatomy. The anal sac is located between the external and internal anal sphincters. Malignant tumors commonly invade muscle. Fragile local structures that must be respected include the pudendal artery, vein and nerve as well as the caudal rectal nerve.
Because anal sacculectomy is considered contaminated surgery, antibiotics are given preoperatively. Even if the patient is constipated, an enema is not desirable. The liquefied feces will inevitably contaminate the surgery site. Manual emptying of feces is, however, acceptable, if not recommended, and it probably will be the highlight of your technician’s day.
The patient is placed in the standard perineal position. A rolled towel is placed under the caudal abdomen, and the hind limbs are not tied tightly in order to prevent lesions to the femoral nerves. A purse string is not desirable either because the surgeon might need access to the anal sac duct opening. In some cases, parts of the rectal wall will need to be sacrificed.
The surgery can be performed with a laser or with a scalpel blade and scissors. Blunt iris scissors can be helpful in this delicate surgery.
Whereas an inflamed anal sac can be removed with an open or modified open technique, anal sac cancer can only be excised via the closed technique, which provides a less contaminated surgical site. Excision cannot be reliably complete because sacrificing a large portion of the anal sphincter will lead to fecal incontinence.
|A right anal sacculitis in a 12-year-old female bulldog. Photo courtesy of Dr. Phil Zeltzman.|
A curvilinear skin incision is made at the 3 or 9 o’clock position. Via sharp and blunt dissection, the mass is carefully excised. Hemostasis is performed with ligatures and electrocautery. The duct, if still recognizable, can be ligated with 3/0 or 4/0 absorbable, monofilament suture material.
After copious lavage, the remainder of the external sphincter muscle is reapposed with monofilament, synthetic, 3/0 suture material.
Subcutaneous and intradermal sutures are placed. Skin sutures or staples are rarely necessary.
If the sublumbar lymph node is enlarged, excision or rather debulking is sometimes recommended. Surprisingly, involvement of the lymph node does not seem to decrease survival time.
Confinement and an Elizabethan collar are recommended for two weeks, until suture recheck or removal. The patient receives antibiotics for a few days and painkillers (NSAIDs and opioids) for one week. During the postoperative period, it is wise to be proactive and recommend a source of fiber to prevent constipation.
Complications such as infection and draining tracts are rare. The dreaded complication is fecal incontinence. All these complications can be minimized with careful planning and delicate surgical technique.
Fecal incontinence can be temporary or permanent. It is due to damage to the external sphincter, the caudal rectal nerve or the pudendal nerve. It is considered permanent after three to four months.
Guessing the diagnosis with the naked eye can be very misleading. The mass must therefore be submitted for histopathology.
Once the diagnosis of adenocarcinoma has been determined, radiation therapy and/or chemotherapy are possible follow-up options.
Prognosis depends on the size of the mass and the presence of metastasis and hypercalcemia. Without chemotherapy, up to 50 percent local re- currence can be expected.
Without metastasis, average survival can reach 18 months. With metastasis, it is an average of six months.
Follow-up visits could include calcium levels and rectal exams every two to three months. Whether radiographic or ultrasonographic exams are repeated will depend on what the owner wishes. In other words, after anal sacculectomy has been performed, what will the client do after learning that the cancer has spread?
As with many diseases, early diagnosis and early treatment are critical. Therefore, rectal exam to assess anal sacs should be part of every single physical exam, especially in older patients.
Anal sac carcinoma is surely not a great disease, but with appropriate treatment, quality of life is definitely possible. <HOME>
Dr. Phil Zeltzman is a small-animal board-certified surgeon at Valley Central Veterinary Referral Center in Whitehall, Pa. His website is DrPhilZeltzman.com.
This article first appeared in the November 2009 issue of Veterinary Practice News
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Step-by-Step Surgery for Anal Sac Cancer
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