Anal Sac Adenocarcinoma in Dogs
Anal sac adenocarcinoma is a malignant tumor that develops in one of the two small pouches located on either side of the anus. It occurs mainly in older dogs. It forms a firm lump next to the anus, often causing little discomfort. It can be...
What is it?
Anal sac adenocarcinoma is a malignant tumor that develops in one of the two small pouches located on either side of the anus.
It occurs mainly in older dogs. It forms a firm lump next to the anus, often causing little discomfort. It can be small and invisible from the outside: only a rectal examination by the veterinarian may reveal it.
This tumor has a distinctive feature: it can produce a substance that raises blood calcium levels. Excess calcium prevents the kidneys from concentrating urine properly: the dog urinates frequently, then drinks heavily to compensate. Sometimes this change, rather than the lump itself, prompts a visit to the veterinarian.
Key point
An older dog that starts drinking and urinating excessively should be examined, including a rectal examination: the excess calcium from this tumor can damage the kidneys.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Firm lump next to the anus, on one side only, often causing little discomfort; the area may appear swollen.
The dog drinks and urinates much more than before; it may have accidents indoors.
Straining to defecate, thinner stools than before, constipation.
Occasionally discharge or a small wound next to the anus.
In more advanced cases: fatigue, weakness, loss of appetite or vomiting, related to excess calcium or tumor spread.
Progression is slow, over weeks or months; the lump is sometimes discovered by chance during an examination.
A very painful and warm area that appeared over a few days points more toward an anal sac abscess.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The exact cause of this tumor is not known.
Like many cancers, it develops mainly with advancing age.
Advanced age: the tumor occurs mainly in older dogs.
Both males and females are affected: large studies show no sex-based predisposition.
No other clear risk factors have been established.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Your veterinarian will examine the area around the anus and perform a rectal examination: this is often the only way to detect a small tumor.
A fine needle aspirate or biopsy sample of the mass helps determine its nature.
Blood work measures calcium levels and checks kidney function.
Abdominal ultrasound looks for spread to lymph nodes; chest X-rays or CT scan look for spread to the lungs.
These tests guide the choice of the most suitable treatment.
Treatment
The main treatment is surgical removal of the tumor, and sometimes the affected lymph nodes.
Depending on the extent of disease, chemotherapy or radiation therapy may be recommended as follow-up treatment, often at a specialized center.
Very high calcium levels are corrected first, usually with intravenous fluids and medications prescribed by your veterinarian. Removing the tumor often brings calcium levels back down.
When cure is not possible, comfort care helps your dog maintain good quality of life: easing bowel movements, pain relief, and monitoring calcium.
The decision is made together with your veterinarian, based on your dog's age, overall condition, and the extent of disease.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There is no known way to prevent this tumor.
Early detection is the best approach: in older dogs, a rectal examination during the annual checkup can identify a small mass before it causes symptoms.
Report to your veterinarian any new-onset thirst and any lump near the anus, even if it seems painless.
Possible complications
High blood calcium, which can cause lasting kidney damage and lead to weakness and vomiting.
Spread to abdominal lymph nodes, which may compress the intestine and cause difficulty with bowel movements.
Spread to other organs, such as the lungs, in advanced cases.
Tumor recurrence after surgery.
When to see a veterinarian
Your dog is not defecating or is not urinating: seek emergency care.
Your dog is drinking and urinating excessively and seems weak or lethargic: see your veterinarian the same day.
You feel or see a lump next to the anus, even if it seems painless.
Your dog is drinking noticeably more water than before, with no obvious reason.
Your dog is straining longer to defecate, or the stools have become thinner.
Prognosis
The outlook depends mainly on the size of the tumor, whether it has spread to lymph nodes or other organs, and the calcium level at the time of diagnosis.
A small tumor found early and removed surgically offers the best chances of recovery, which is why rectal examination is valuable in older dogs.
Recurrence is possible, so regular check-ups are recommended after treatment.
Frequently asked questions
Lump near the anus: anal sac abscess or tumor?
An abscess appears within a few days, is warm, red-purple, and very painful, often in a small dog, usually without increased thirst. A tumor grows slowly, remains often relatively painless, and affects especially the elderly dog, which may begin to drink and urinate excessively. Only examination, with a sample, makes it possible to determine which it is.
Does your dog drink a lot: is it necessarily cancer?
No. Increased thirst has many causes: diabetes, kidney disease, Cushing's syndrome, uterine infection in females. A blood test, which notably measures calcium, and a thorough examination, including a rectal exam, help the veterinarian find the cause.
Why does the veterinarian perform a rectal examination?
Because a small anal sac tumor can be invisible from the outside. Digital rectal examination is a quick procedure that allows it to be detected early, when surgery has the best chance of being effective.
Is this tumor treatable?
It is treated first by surgery, sometimes supplemented by other treatments. The results depend on the size of the tumour and its extent at the time of diagnosis: the earlier it is discovered, the better the prognosis.
References
Content reviewed by the TunisieVet editorial team · Updated 10 October 2026
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