Anal and perianal tumours in dogs – adenoma and cancer

Overview

Perianal means around the anus. Cutaneous hepatoid glands are modified sebaceous glands. Their tumour cells resemble liver cells, explaining the name hepatoid. Benign adenomas have been estimated to account for 58–96% of tumours in the anal region.

Perianal adenoma is the third most common tumour type in entire male dogs.

Hepatoid gland adenocarcinoma is a malignant skin gland tumour. Epithelioma has behaviour intermediate between benign and malignant. Apocrine gland anal sac adenocarcinoma, or AGASACA, instead arises from tissue in the sac beside the anus. Adenoma and cutaneous adenocarcinoma can look alike, so appearance does not confirm a benign tumour.

Causes

A hormone-dependent skin tumour

Perianal adenoma growth depends on testosterone, the male sex hormone. Androgens, or male hormones, stimulate hepatoid gland development. Reducing testicular testosterone effects through castration can therefore shrink an adenoma. Hepatoid glands also occur elsewhere, including tail skin and the abdominal midline. However, around 90% of their adenomas are around the anus.

Anal sac cancer can affect the whole body

Anal sacs are small pouches on either side of the anus. Their walls produce brown, strong-smelling fluid released during defecation. Anal sac cancer can produce PTHrP, a protein resembling parathyroid hormone, and raise blood calcium. The tumour and enlarged lymph nodes can also mechanically compress the rectum.

Risk factors

Cutaneous perianal adenomas are most common in older dogs. Hepatoid gland overgrowth occurs 3 times as often in entire males as females. Malignant hepatoid gland adenocarcinomas occur 10 times as often in males as females.

Hepatoid gland tumours have also been associated with testicular and adrenal tumours. They occur in dogs with Cushing's disease, or hyperadrenocorticism, too.

Anal sac cancer has a different sex distribution: males and females are affected in roughly equal numbers. The mean age is 10 years. All breeds can be affected, but spaniels more often than usual. Usually only one sac is affected, although tumours may develop in both simultaneously or later on the opposite side.

Typical symptoms

A perianal adenoma's surface may be smooth or lumpy. Overlying skin can break down and become irritated. An inflamed tumour may be painful.

A skin adenoma usually grows slowly and remains under 2.5 cm.

Changes around the anus may include the following:

• A small, round, hairless pink nodule appears in the skin.

• Several nodules develop around the anus.

• Your dog constantly licks or chews the anal area.

• Your dog drags its bottom along the floor, called scooting.

• Defecation becomes difficult or requires more straining than usual.

• Blood is visible in the faeces during ordinary defecation.

• New swelling appears beside the anus.

• Your dog may also strain more than usual when urinating.

• Drinking and urination increase with anal sac cancer-associated high calcium.

Cutaneous adenocarcinoma may grow faster than an adenoma and reach 10 cm in diameter. A tumour deep within an anal sac may only be found during rectal examination. Anal sac inflammation or blockage can also cause scooting and licking.

Complications and consequences

Local growth, recurrence and spread

Even a large benign skin tumour can compress the anal canal and hinder defecation. An ulcerated adenoma can develop bacterial infection. An epithelioma can invade surrounding tissue. Hepatoid gland adenocarcinoma can spread to nearby lymph nodes.

After complete removal of a single hepatoid tumour without initial spread, local recurrence occurred in around 13% of 76 dogs. All locally recurring tumours were adenocarcinomas.

Following removal of a single hepatoid tumour, 9.2% developed metastases in the same 76-dog follow-up. Metastases were found in regional lymph nodes, lungs and liver.

The outlook for anal sac cancer depends on spread

Some dogs with anal sac cancer can be cured by tumour removal alone.

High blood calcium occurs in around 25% of anal sac cancer cases. Resulting kidney failure can be permanent. Cancer can also spread to the liver, spleen and lungs.

Survival with anal sac cancer may be less than 1 year from detection. However, dogs with aggressive disease receiving combined surgery, radiotherapy and chemotherapy have had average survival exceeding 2.5 years. Individual prognosis depends on tumour size, spread, calcium level and whether surgical removal is possible.

Diagnosis at the vet

Sampling distinguishes tumour types

Examination of fine-needle-collected cells helps distinguish cancer from bacterial infection or other inflammatory changes.

The vet examines the anal area and performs rectal palpation. Fine-needle sampling aspirates tumour cells for microscopy. Sampling an anal sac tumour may require sedation. If cytology is inconclusive, a tissue biopsy is needed.

Diagnosis of hepatoid gland adenocarcinoma relies on microscopic tissue examination. The pathologist also assesses invasion and surgical margins. A well-differentiated cutaneous adenocarcinoma can resemble an adenoma even microscopically. A poorly differentiated hepatoid tumour may instead resemble apocrine gland anal sac adenocarcinoma.

A hepatoid tumour biopsy is also assessed for variation in cell and nuclear size. Cell division and preservation of glandular structure are examined too. These structural differences help distinguish an organised adenoma from poorly differentiated adenocarcinoma.

The entire removed tumour can be sent to the pathologist instead of a separate biopsy.

Spread and kidney condition are assessed before treatment

Blood tests assess general health, calcium and kidney function. Urinalysis forms part of investigations assessing cancer spread and the dog's condition. Chest radiographs look for lung metastases. Abdominal ultrasound assesses internal organs and lymph nodes.

Abdominal lymph nodes are enlarged in around 50% of anal sac cancer cases. Lymph node samples investigate possible spread. Computed tomography, or CT, may replace radiography and ultrasound.

Treatment

Castration has a major role in adenoma treatment

Treatment choice depends on tumour type, size and spread. Up to 95% of males have a complete response of hepatoid gland adenoma after castration. A large, ulcerated or inflamed adenoma may be removed at the same time surgically or with a laser. In females, surgical removal is the primary treatment.

If a presumed adenoma does not respond to castration, pituitary and adrenal function are also investigated. It may also be necessary to reassess whether it is a low-grade adenocarcinoma. Hepatoid gland adenocarcinoma does not usually respond to castration. Treatment involves sufficiently wide removal, removal of affected lymph nodes where needed, and radiotherapy.

If adenomas are numerous or close to the anal sphincter, the vet may castrate before removal. The tumours are then allowed several months to shrink before surgery. A smaller tumour may be safer and easier to remove.

Radiotherapy can also treat benign hepatoid gland tumours. A cure rate of 69% was reported over 2 years of follow-up. Tyrosine kinase inhibitors, targeted medicines affecting tumour-cell growth signals, may be considered for malignant hepatoid gland tumours.

Anal sac cancer is removed where possible

Surgery is the foundation of treatment for anal sac adenocarcinoma. Proximity to the rectum and anus limits removal of healthy tissue around the tumour. Enlarged abdominal lymph nodes may be removed through a separate abdominal incision. Their removal may provide prolonged relief from constipation they cause.

High calcium and kidney failure may need intravenous fluid treatment before anaesthesia. Removing the tumour and metastatic lymph nodes may correct tumour-associated hypercalcaemia. Radiotherapy may be used if complete removal is impossible or surgery is unsuitable. Chemotherapy is considered particularly because of metastatic risk.

Recovery after surgery

Most dogs go home 1–2 days after anal sac tumour surgery. A protective collar is usually used for 10–14 days to prevent wound licking or biting. Exercise is restricted for around 2 weeks while the wound heals. The vet may prescribe a stool softener until swelling settles.

Possible surgical complications include wound infection and breakdown. Faecal continence problems occur in up to 33% of dogs operated on for anal sac tumours, particularly after removal of large tumours. The problem is usually temporary. Pain is treated with veterinary-prescribed oral medicines.

Prevention

After castration and adenoma removal, benign tumour recurrence risk is less than 10%. Without castration, adenomas often regrow and new ones may develop.

Castration is the most effective way to prevent testosterone-dependent perianal adenomas in males. It also helps prevent new adenomas forming. Anal sacs should be examined during routine health checks. Early detection of an anal sac tumour improves survival prospects. Even a painless skin nodule needs examination because adenoma and adenocarcinoma can look alike.

Important for dog owners

Record when the nodule appeared and whether its size or surface has changed. Note defecation difficulties, bleeding and changes in drinking, urination and appetite. Bring previous histopathology results and a medication list. Before discharge, confirm what was removed, when the wound will be checked and how to report continence changes. Keep the treating vet's contact details available to other carers too.

If anal sac cancer involved high calcium, another rise may relate to tumour progression. Removing recurrent enlarged lymph nodes may sometimes relieve obstruction to defecation too.

Frequently asked questions

Is a lump near the anus always cancer?

No: a cutaneous perianal adenoma is benign. However, malignant adenocarcinomas can occur in the same area. Appearance does not reliably distinguish adenoma from cutaneous adenocarcinoma. Microscopic tissue examination identifies tumour type and helps assess behaviour.

Is castration enough treatment?

A testosterone-dependent hepatoid gland adenoma may disappear after castration. Particularly large, ulcerated or inflamed adenomas may also need removal. Malignant hepatoid gland adenocarcinoma does not usually respond to castration, so the same treatment principle does not apply to every tumour around the anus.

Can a small anal sac tumour already have spread?

Yes: anal sac adenocarcinoma may metastasise while small. Investigations therefore include abdominal and chest imaging. Lymph nodes may be sampled to assess spread. The tumour can spread beyond lymph nodes to sites such as the liver and lungs.

Why does an anal tumour make a dog drink a lot?

Anal sac cancer can raise blood calcium excessively. High-calcium symptoms include increased drinking and urination. Severe cases may cause heart and kidney damage. Intravenous fluids and medication may be needed to stabilise the dog before anaesthesia.

Will my dog have faecal continence problems after surgery?

Anal sac tumour surgery may affect faecal continence, particularly after removal of a large tumour. However, postoperative continence problems usually resolve and are generally temporary. After a one-sided tumour, continence is usually only partly reduced rather than faeces leaking constantly.