Perineal Tumors

Chapter 82


Perineal Tumors



Many types of neoplasia can affect the perineum of dogs and cats. These include tumors whose development is not specific to this area, including mast cell tumors, soft tissue sarcomas, lymphoma, and benign masses such as lipomas and sebaceous gland adenomas. Although these must be considered as differential diagnoses when a dog or cat has a perineal mass, this chapter focuses on the three tumor types that arise from specialized glandular tissue in the perineum.


The perianal glands are modified sebaceous glands that are abundant in the skin around the anus. They often are referred to as circumanal glands owing to the ring pattern that they form around the anus. They also can be found scattered on the prepuce, tail, and hind legs. Perianal glands are unique to the dog (absent in the cat), and their development is androgen dependent. Tumors that arise from these structures can be benign or malignant and are referred to as perianal gland adenomas and adenocarcinomas, respectively.


The apocrine glands of the anal sac are embedded within the stroma that lies between the internal and external anal sphincters. The anal sacs represent cutaneous diverticula at the 4- and 8-o’clock positions around the anus that are lined by squamous epithelium originating at the anocutaneous junction. The anal sacs serve as reservoirs for the secretions produced by their associated apocrine glands. Tumors affecting the apocrine glands of the anal sac are almost always malignant. Unlike perianal tumors, which occur only in dogs, anal sac apocrine gland carcinomas can affect both dogs and cats.


Of the three perineal neoplasms described here, the perianal adenoma is the most common in the dog. Of the two malignant tumors, anal sac apocrine gland carcinoma occurs more frequently than perianal adenocarcinoma.



Perianal Gland Tumors



Clinical Signs and Diagnosis


Perianal adenoma, the benign form of perianal gland neoplasia, is common in older intact male dogs because of its sex hormone dependence. Dogs usually have a slow-growing mass on the hairless skin around the anus. The tumor can also arise at the tail base, on the prepuce, or on the hind limbs. Perianal adenoma usually occurs as a single and well-circumscribed mass but can be multiple or appear as generalized hypertrophy of the perianal region. The tumors generally are nonpainful and asymptomatic. Rarely, large adenomas can become ulcerated and infected. Although it is unusual, perianal adenomas can affect females or castrated males. Androgen secretion from the adrenal glands in dogs with hyperadrenocorticism and lack of estrogen in ovariohysterectomized females are possible causes of perianal adenoma development in these dogs.


Perianal adenocarcinoma should be considered as a differential diagnosis in these cases. Contrary to its benign counterpart, perianal adenocarcinoma is less common and is not androgen dependent. It can develop in any sex and is characterized by faster tumor growth and clinical signs. Perianal adenocarcinoma is more invasive and adherent to underlying tissues. It may occur as one or multiple masses. Dogs typically are brought to the veterinarian because of the presence of a mass, which may be ulcerated, or discomfort in the perianal region. Dyschezia and tenesmus can occur with large tumors.


Cytologic analysis of a fine-needle aspirate of a perianal mass should be performed before a treatment plan is formulated. Differentiation between perianal adenoma and adenocarcinoma can be difficult by cytologic examination, but other conditions can be ruled out. A tissue biopsy is often needed to confirm malignancy, which greatly impacts the treatment approach. Because of their morphologic resemblance to hepatocytes, perianal adenomas are sometimes referred to as hepatoid tumors.


Cats do not have perianal glands and therefore are not affected by these tumors.



Biologic Behavior and Staging


Perianal adenoma has a benign clinical course. Metastasis does not occur. Removal of hormonal stimulation, combined with local therapy if needed to hasten relief of clinical signs, almost always is curative. Thoracic radiography and abdominal imaging for tumor staging are not cost effective unless needed to assess non–tumor-related problems before treatment. For cases in which perianal adenocarcinoma is high on the differential diagnosis list, such as in neutered males and females, tumor staging should be considered before a treatment plan is formulated.


Perianal adenocarcinoma is a more locally invasive tumor, and it grows at a faster rate. The risk of metastasis is low early in the course of disease (15%). When metastasis does occur, it progresses via the lymphatic system to regional lymph nodes (sacral, hypogastric, and iliac) and then to lungs, liver, spleen, bone, and other organs. Tumor staging should begin with a thorough rectal examination. It is important to identify the gross tumor margins accurately for surgical planning. Palpation of the ventral aspect of the lumbar vertebral bodies during a rectal examination may reveal enlargement of sacral lymph nodes. Distant metastasis should be ruled out by thoracic radiography and abdominal imaging. Abdominal ultrasonography is more sensitive than radiography for less advanced lymph node metastasis and allows for a thorough evaluation of other organs. Lymph nodes within the pelvic canal are not well visualized with ultrasonography, so rectal palpation remains important, and radiography may be helpful to show pelvic lymphadenopathy in large dogs. Although the likelihood of metastasis at diagnosis is low for this tumor type, complete staging is recommended so that an accurate prognosis and effective treatment plan can be formulated for each patient. Clinicians should keep in mind that the presence of lymph node enlargement or splenic or hepatic nodules is not specific for metastasis. Tissue sampling with cytologic analysis is necessary to rule out a reactive or hyperplastic process.



Treatment and Prognosis


Castration is the treatment of choice for perianal adenoma in intact male dogs. Most tumors regress, and recurrence is rare in the absence of androgenic stimulus. For ulcerated tumors, surgical excision can be considered to hasten relief. If surgical resection would be associated with a high risk of fecal incontinence, castration is recommended first, followed by surgery when the adenoma regresses to a more manageable size. When females or neutered males are affected, surgery is the treatment of choice. The surgical approach need not provide wide margins of normal tissue because these benign tumors are not highly invasive.


Perianal adenocarcinoma requires a more aggressive therapeutic approach. Tumor growth is more invasive and independent of hormonal influences, so castration is not effective. Surgical excision should include a margin of normal tissue to remove microscopic cancer cells that extend beyond the gross tumor margin. Completeness of excision should be reported in the histopathology report. In the author’s practice, adjuvant radiotherapy (RT) is recommended when the microscopic margin of normal tissue is less than 5 to 10 mm and additional surgery is not possible. Incomplete or marginal tumor resections are common given the difficulty of surgery in this location. Local RT, targeting only the primary tumor bed, is justified for this tumor, which is associated with a low incidence of metastasis. The adjuvant radiation prescription used by the author is 2.7 Gy delivered daily (Monday through Friday) for 18 treatments. Information is lacking about the efficacy of RT, and variations on this prescription are common. Acute adverse effects related to RT with curative intent are self-limiting and include moist desquamation of the perianal region within the RT field. The benefit of chemotherapy is not known; often it is not recommended if RT is available since the risk of metastasis is low. For rare cases in which metastasis has occurred, chemotherapy is a logical treatment recommendation. Although efficacy information is lacking, doxorubicin and platinum-based protocols are reasonable to consider (see later for dosing information). Tumor recurrence often can be managed with multiple palliative surgeries; however, each recurrence generally is more difficult to resect. Early diagnosis followed by an aggressive first surgery is the best therapeutic approach. The reported median disease-free interval is approximately 2 years for dogs with tumors smaller than 5 cm treated with surgical resection. Prognosis worsens with more advanced disease, including larger tumors or metastasis.


For dogs with nonresectable tumors associated with dyschezia, discomfort, or tumor ulceration, palliative RT using a coarse-fraction radiation prescription is reasonable to consider. Common protocols include 6 to 8 Gy delivered once weekly for four treatments, or 4 Gy delivered daily for five treatments. Although the efficacy of RT in this setting has not been documented, responses have been observed anecdotally by the author, and acute adverse effects are minimal.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Perineal Tumors

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