The perianal (peri = around) glands are located in the dermis not only around the anus, but can also be found on the foreskin, scrotum, tail, hind legs and trunk. These glands are considered to be sebaceous glands.

Behaviour

Benign form

Benign tumours (adenomas) can arise from perianal glands and their development and growth seem to be linked with sex hormones. The growth of these tumours is stimulated by male hormones and suppressed by female. Of note, the prevalence of these tumours regularly coincides with certain testicular tumours (Leydig cell tumour). Perianal adenomas only appear in bitches when their uterus and ovaries have been removed, although they are also known to appear in exceptional cases of excessive adrenal production of testosterone. Perianal adenomas grow slowly. They can become very large locally, but do not metastasize.

Malignant form

These tumours appear in both intact as well as neutered male and female dogs. Malignant tumours have a tendency to grow faster than benign ones, be less mobile and firmer. Perianal adenocarcinomas metastasize infrequently (in about 15% of cases). The most frequent places metastases occur are the local lymph nodes, seldom the lungs, liver, kidneys and bone.

Prevalence

The benign form (adenoma) occurs relatively frequently (more than 50% of tumours around the anus), at an average age of 10 years, whilst the malignant form (adenocarcinoma) appears less frequently (between 3-21% of tumours around the anus) at an average age of 11 years.

Breed predisposition

The benign form occurs more in the beagle, Bulldog, Cocker Spaniel and Samoyeed, whereas the malignant form mainly affects males of large breeds.

Symptoms

• Benign form: slowly growing (during months or years) tissue mass(es) which aren’t painful and usually aren’t associated with anomalies. They can also occur on the foreskin, ball sac or tail. These tissue masses can ulcerate and infect, but are rarely attached to the underlying structures. Usually, they are well delineated (average of 0.5 to 3 cm in diameter) and raised.
• Malignant form: this form resembles the benign one, but grows faster, is firmer, ulcerates, attaches to underlying tissues, relapses when the surgical margins were very narrow and is generally larger than the benign form. Large tissue masses are likely to cause obstruction and difficulties defecating, pain or irritation around the anus. Although rarely the case, metastases in the abdominal lymph nodes can significantly enlarge the affected lymph nodes. In turn, this causes pressure on the intestines and complicates bowel movements.

Physical examination

Careful rectal examination should be performed in all dogs with perianal tumours. This procedure allows the veterinarian to estimate the tumour perimeters, how likely the tumour can be excised and the possible complications associated with a surgical excision. During a rectal examination, depending on the size of the dog, it may also be possible to palpate enlarged local lymph nodes in the pelvis (these are hard to see via ultrasound because they’re located deep within the pelvis).

Imaging

Through X-rays of the lungs from different angles, it is possible to detect pulmonary metastases (if they are large enough), although lung metastases are rare for this tumour type. X-rays can also indicate whether there’s an enlargement of the lymph nodes within the pelvis. However, if the lymph nodes can be visualized through ultrasound, ultrasound will provide more detailed information than radiographs.

Cytology

The veterinarian can aspirate cells via a needle to assess the nature of the tissue mass. A tissue sample is necessary to make a definite diagnosis. Typical for the malignant form is the invasion of healthy tissue).

Benign form (perianal adenomas)

Because perianal adenomas are sensitive to hormones, the majority of these tumours will disappear (at least partially) after castration (in male dogs). They occur in uncastrated male dogs and castrated female dogs. When few delineated or large benign tissue masses are present near or in the anal sphincter, the veterinarian can first castrate the dog, then monitor the dog for a few months to assess whether the tissue mass reduces in size. If size reduction occurs, surgical resection of the tissue mass is safer and easier. Neutering will only be efficient for benign tissue masses which are sensitive to hormones. Up to 90% of male dogs will be cured via neutering and removal of the tissue mass. Adenomas can be removed with narrow margins. Surgical removal is advised for male dogs in whom the tumour is ulcerated or has relapsed. Surgery (neutering and removal of the tissue mass) is the treatment of choice.

Malignant form (perianal adenocarcinomas)

The malignant form generally does not respond to neutering. Therefore, the tissue mass must be removed with sufficient margins. Luckily, up to half the anal sphincter can be removed without causing fecal incontinence (=inability to control bowel movements).

If the tissue mass reappears, surgical excision will be more difficult and the dog may require multiple palliative removals over the course of the years. Insufficient margins obtained during the first operation are the main cause of reoccurrence.

Radiotherapy after surgery could also help subdue the local reoccurrence.

For the benign form a complete cure can be obtained when the tumour is surgically removed and the dog neutered. A complete cure is also possible for the malignant form if sufficient margins are obtained during the first surgery.