Anal Sac Tumors

What are the anal sacs and anal glands?

Dogs and cats have a pair of anal sacs, one located on each side of the anus, between the external and internal anal sphincter muscles. These sacs are lined with modified sweat glands called anal glands. The anal glands normally produce foul-smelling, grey-brown, granular secretions, which are stored in the anal sacs.

What is an anal sac tumor?

An anal sac tumor is composed of cells that originate in the glands of the anal sac. The tumor usually grows quickly and has a moderate rate of spread. One of the biggest problems with anal sac tumors is that they can spread elsewhere in the body (metastasize). Their growing size can make removal difficult. The earlier these tumors can be detected, the better for your pet. Anal sac tumors are also known as anal sac adenocarcinomas.

What causes this cancer?

The reason a particular pet may develop this, or any tumor or cancer, is not straightforward. Few tumors or cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. There are no known specific causes for anal gland tumors.

How is this tumor diagnosed?

Typically, anal sac tumors are found incidentally, during routine, yearly physical examinations. Your veterinarian may see or feel a mass during a visual examination of your pet’s hind end or feel a mass during a rectal exam. Alternatively, you may notice a bump beside the anus or observe symptoms associated with anal sac disease, such as scooting and increased licking of the back end. These symptoms can also indicate anal sac impaction and infection, so it is important to have a veterinarian perform a rectal exam.

If your veterinarian detects a mass, the easiest way to confirm the presence of an anal sac tumor is by fine needle aspirate (FNA). FNA involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a microscope slide. The cells on these microscope slides can be examined by a pathologist (cytology).

In some cases, results from FNA may not be clear, and a biopsy may be necessary. A biopsy is the surgical removal of a piece of the tumor, which is then examined by a veterinary pathologist under the microscope (histopathology). Histopathology helps make a definitive diagnosis.

How does this type of tumor typically progress?

Anal sac tumors have a moderate risk of spread, dependent on many factors. Your veterinarian will be concerned about both the local disease (the mass itself, or primary tumor) and the extent of spread or potential for spread at the time of diagnosis, as these factors are important in determining treatment options.

Your pet will have tests performed to stage the tumor before any intervention. Staging means determining if cancer has spread (metastasized), and if so, where it has spread. The stage of tumor affects the treatment options, as higher stages have extensive spread to multiple organ systems. Staging usually involves blood testing and imaging, such as abdominal ultrasound and X-rays of the chest. Further sampling may be required, and the results will ultimately alter the treatment plan.

How will this type of tumor affect my pet?

Depending on the tumor’s size, your pet could produce a thin, ribbon-like stool and show signs that they are straining to defecate. Hypercalcemia (an increased calcium level in the blood found on your pet’s blood tests) can be consistent with anal sac tumors. These tumors produce a protein that causes the release of calcium into the bloodstream, so your pet may show signs of increased drinking, increased urination, lethargy, and/or vomiting. Kidney injury is also possible due to persistently high calcium levels.

How are these tumors treated?

The best and most common treatment for the primary tumor is surgery to remove the mass; however, cancerous cells may be left behind. If staging has found disease elsewhere (e.g., if a lymph node contains abnormal tumor cells), that area will typically be surgically removed as well. Radiation and chemotherapy may then be considered if there is evidence of spreading.

If surgery is not an option, your veterinarian may discuss the options for radiation therapy on a case-by-case basis. Toceranib phosphate (Palladia®) is a medication that may be used to alleviate clinical signs for a short period of time.

Is there anything else I should know?

If your pet is having difficulty defecating or is straining to defecate because of an anal sac mass, your veterinarian may recommend a stool softener to relieve your pet of this discomfort until treatment can be pursued. It is best to avoid diets that will cause bulkiness to the stool (e.g., high-fiber diets).

Prevent your pet from scooting or licking its hind end. Pets with elevated calcium levels should always have access to plenty of fresh water until treatment can be pursued.