Perianal Tumor in Dogs

The perianal area contains sebaceous-like structures in the perianal skin of dogs, located close to the anus. Perianal tumors originate from these tissues, and most commonly develop in intact, older male dogs. While benign tumors are generally harmless and can be easily treated, the malignant variety can create life-threatening conditions for your dog, and will need to be treated aggressively.

Perianal tumors develop close to, or directly on, the anus in dogs, and originate from the perianal tissues.  These tumors can be single or multiple nodules in the skin that can be benign or malignant, and can cause signs that involve elimination. If hypercalcemia is also present, your dog can also suffer from kidney issues that can lead to failure.

The presence of a perianal tumor is often not painful for your dog, but it can cause your dog to pay more attention to the area.  Some of these tumors grow slowly for months or years, and they can eventually rupture and bleed, leaving your dog susceptible to infections. Tumors may appear as single or multiple masses in the anal area, and can create a thickened ring around the anus. Signs include:

• Round, nodules in the skin near anus
• Thickened ring of tissue surrounding anus
• Bleeding nodules
• Excessive licking of the anal area
• Scooting rear end on ground
• Colorectal obstruction 
• Rectal pain
• Protruding rectal mucous membrane 
• Straining to defecate 
• Bloody feces 
• Vomiting blood
• Increased thirst
• Increased urination 
• Decreased appetite
• Lethargy
• Reduced exercise tolerance 
• Growth on the testes
• Hypercalcemia, or elevated blood calcium levels

Types

• Perianal tumors are categorized as either benign adenomas or malignant carcinomas
• Benign adenomas are masses of the perianal area that are not classified as cancerous, but that may spread, regardless.  

Malignant carcinomas, or adenocarcinomas, are also masses of the perianal area. These are uncommon cancerous masses that are hard to distinguish from benign adenomas, but can spread to other locations in the body and cause hypercalcemia, or increased blood calcium levels that can lead to kidney failure. Malignant carcinomas require more intensive diagnostic testing and treatments, therefore it is necessary to determine if your dog’s tumors are malignant.

While the definitive cause of perianal tumors is still unknown, testosterone is believed to influence cell proliferation in the glands. Intact male dogs have an increased risk compared to females in developing these tumors. For females, adrenocortical hormones may play an important role in tumor growth. 

Additionally, genetic factors may also influence the possible progression of this type of tumor. Breeds who seem to be at a higher risk include:

• Beagles
• Cocker Spaniels
• English Bulldogs
• Samoyed breeds 

A physical exam can help your veterinarian determine if the masses or nodules on your dog’s anal area may be tumors. Often a urinalysis is performed, as well as blood work that can reveal an elevated blood calcium level, a sign of these types of growths. 

Many perianal tumors will be benign, but a small percentage of these growths can be malignant. A tissue sample by a fine-needle aspirate or by a surgical biopsy will be examined under the microscope to determine what kind of tumor your dog has. Chest and abdominal X-rays and ultrasounds can also help to determine of the tumors have spread, such as to the lymph nodes or prostate gland.

Treatment for these tumors can vary, and can be more difficult if they have spread. Treatment for tumors that are diagnosed as malignant aims to improve your dog’s quality of life, while benign masses can be completely resolved. 

For intact, male dogs, the primary treatment is the removal of both testes. This eliminates much of the testosterone, which can result in regression of small and benign tumors. Your dog is monitored after castration to assess if further removal is necessary. 

In both male and female dogs, small tumors can also be removed with cryotherapy, which freezes off the masses. For larger or malignant tumors, surgical removal may be necessary. Fecal incontinence can result from surgery due to damage to the sphincter muscle. 

Chemotherapy and radiation therapy may also be prescribed in addition to castration, and are generally given over several weeks. Estrogen therapy can be administered to shrink tumors, and is used especially in cases where castration is not an option in male dogs. A side effect of estrogen therapy is a life threatening condition of bone marrow suppression.

Supportive treatments can include soothing creams for irritated anal skin, and medications to reduce blood calcium levels, improve kidney function, and alleviate pain and nausea.

For benign perianal tumors, recovery is very good. Neutering leads to a complete regression in the majority of these cases. Your dog may need post-operative care, and should be seen by your veterinarian in 1 to 3 months to monitor tumor regression. In a small percentage of cases, tumors may recur, but can be treated. 

If your dog has a malignant tumor, post-operative care may be longer and more intense. You may need to monitor your dog for complications, such as infections and fecal incontinence. Your dog may need frequent veterinary visits for treatments. Recovery is poor, as many dogs can die from the effects of the tumors within a year from diagnosis, or are euthanized.