Dr. Mark Thompson of PetPlace.com explains that a bump under the skin at the base of a dog's tail is referred to as a perianal adenoma. Perianal adenomas are tumors that remain stationary and do not affect other tissues. Perianal adenomas are benign. In rare cases, a malignant tumor known as a perianal adenocarcinoma can form at the base of the tail.
Perianal adenomas are typically surgically removed. Veterinarians also commonly recommend neutering as a preventive measure. Deerfield Veterinary Hospital explains that in addition to preventing perianal adenomas from returning, neutering can help to prevent testicular cancer and prostate infection. Perianal adenomas are most common in male dogs that have not been neutered.
According to PetMD, perianal adenocarcinomas can spread rapidly to the surrounding tissues. The prognosis for perianal adenocarcinomas is typically negative. Perianal adenocarcinomas tend to occur in dogs with hormone imbalances or excess calcium in the blood. Symptoms of perianal adenocarcinomas include a small mass or lump at the base of the tail, constipation, difficulty in defecating, weight loss and lethargy.
Perianal adenocarcinomas are diagnosed through physical examination and needle biopsies of the tumor cells. X-rays and ultrasound tests are also helpful in diagnosing a malignancy. Treatment of perianal adenocarcinomas consists of surgical removal and radiation.