Perianal tumors in dogs are primarily driven by hormones, specifically long-term exposure to testosterone in intact (unneutered) male dogs. The good news: 58 to 96% of these tumors are benign adenomas. The remainder, including a smaller percentage of adenocarcinomas and a distinct tumor called anal sac adenocarcinoma, have different causes that aren’t fully hormone-dependent.
Types of Perianal Tumors
Not all lumps near a dog’s rear end are the same tumor. Understanding which type your dog has matters because their causes, behavior, and treatment differ significantly.
Perianal gland adenomas are by far the most common, making up the vast majority of perianal tumors. These are benign growths that arise from the hepatoid glands, small oil-producing glands that ring the anus. They grow slowly and don’t spread to other parts of the body.
Perianal gland adenocarcinomas are the malignant version, accounting for roughly 3 to 21% of perianal gland tumors. Under a microscope, the cells look disorganized and divide rapidly, and the tumor can invade surrounding tissue.
Anal sac adenocarcinomas are a separate tumor entirely. These originate in the apocrine glands inside the anal sacs (the small scent glands on either side of the anus) rather than in the perianal glands themselves. They behave aggressively, often spreading to nearby lymph nodes or distant organs.
Testosterone and Hormonal Causes
The dominant cause of benign perianal adenomas is hormonal. The perianal glands contain androgen receptors, which means testosterone directly stimulates these glands to grow. In intact male dogs, years of continuous testosterone exposure can push the glands from normal size to overgrowth (hyperplasia) and eventually to tumor formation. This is why perianal adenomas are overwhelmingly a disease of unneutered males.
Both androgen and estrogen receptors have been found in normal perianal glands and in perianal tumors. The interplay between these hormones helps explain why neutering, which removes the primary source of testosterone, is effective at both treating and preventing benign adenomas. Many adenomas actually shrink after castration alone, without any additional surgery.
Female dogs develop perianal tumors far less often. When they do, the androgens produced by the adrenal glands are thought to be the driving factor, particularly in spayed females who no longer produce ovarian estrogen to counterbalance those androgens.
Why Malignant Tumors Are Different
Adenocarcinomas of the perianal gland and anal sac adenocarcinomas don’t follow the same hormonal playbook. While testosterone plays a role in the benign versions, malignant perianal tumors can occur in neutered males and females, which tells us other mechanisms are at work. A clear single cause hasn’t been identified for these cancers.
Anal sac adenocarcinomas in particular appear to have a genetic component. English Cocker Spaniels carry a recognized breed predisposition, though the disease occurs across all breeds. Other commonly affected breeds include Alaskan Malamutes, Cavalier King Charles Spaniels, Dachshunds, English Springer Spaniels, German Shepherds, and Golden Retrievers. The fact that specific breeds cluster around this cancer strongly suggests inherited genetic risk factors beyond hormones alone.
Age as a Risk Factor
Perianal tumors are a disease of older dogs. Across all tumor types in dogs, the peak diagnosis window falls between 9 and 11 years of age. Benign tumors tend to appear slightly earlier, with a median diagnosis age around 9 years, while malignant tumors skew a bit older, with a median around 10 years. This fits the pattern seen in most cancers: the longer cells divide over a lifetime, the more opportunities exist for something to go wrong.
Based on large-scale data from cancer registries, veterinary oncologists generally suggest starting cancer screening in mixed-breed dogs around age 7 to 8 and even earlier (before age 6) in breeds known to develop tumors at younger ages. For perianal tumors specifically, this means paying closer attention to any new lumps around the rear end as your dog enters its senior years.
Breed Predisposition
Genetics influence both benign and malignant perianal tumors, but the breed lists differ depending on the tumor type. For benign adenomas, the risk is highest in intact males of any breed, so the hormonal factor tends to overshadow breed effects. For anal sac adenocarcinomas, breed matters more. The breeds identified as higher risk, including English Cocker Spaniels, German Shepherds, Golden Retrievers, Dachshunds, and several spaniel breeds, suggest that certain genetic lines carry susceptibility that hormones alone don’t explain.
Regional differences in breed popularity and neutering rates also affect which tumors veterinarians see most often. Countries where early neutering is standard practice tend to report fewer benign perianal adenomas, while anal sac adenocarcinomas, which aren’t prevented by neutering, still appear at expected rates.
Complications From Anal Sac Tumors
One notable feature of anal sac adenocarcinomas is their ability to cause dangerously high blood calcium levels, a condition called hypercalcemia. The tumor produces a protein that mimics parathyroid hormone, tricking the body into pulling calcium from bones and retaining it in the kidneys. This is one of the most common cancer-related causes of hypercalcemia in dogs, alongside lymphoma.
High calcium can cause increased thirst and urination, loss of appetite, vomiting, muscle weakness, and in severe cases, kidney damage. In some dogs, these symptoms are what prompt the vet visit that leads to discovering the tumor in the first place, especially when the mass is small and not yet visible from the outside.
How Neutering Affects Risk
For benign perianal adenomas, neutering is both preventive and therapeutic. Removing the testosterone source stops the hormonal signal that drives these tumors. Dogs neutered before adenomas develop have a substantially lower risk of ever getting one, and dogs neutered after a benign adenoma is found often see the tumor regress without further intervention.
For malignant perianal tumors and anal sac adenocarcinomas, neutering does not appear to offer the same protective effect. These cancers arise through mechanisms that are at least partially independent of sex hormones, which is why they occur in neutered dogs of both sexes. If your dog is neutered and develops a lump near the anus, it’s worth having it evaluated promptly, since the hormonal “safety net” of castration doesn’t apply to malignant types.