Q My 11-year-old Shih-Tzu, Lisa, has always been in perfect health. The other day I took her to my veterinarian for her annual check-up and everything was fine in terms of blood work, heart function, etc. He performed a rectal exam and said that there was a tiny mass, the size of a pea, on her anal sac. He recommended that we remove the mass and the anal sac immediately. He said this is likely cancer and can be very serious.

He is a very good vet, but I don't get how something so small could be so serious, or how he could he know its cancer just from feeling it. Shouldn't this be biopsied first? Wouldn't this show up in her blood work?

A: Your questions, and confusion, are common when a seemingly healthy pet, or human for that matter, receives a diagnosis such as this.

The fact is that many types of cancer can progress to advanced stages with no outward symptoms or changes in blood work. That's why it is often referred to as the silent killer. This illustrates the importance of regular physical exams, which should always include rectal examinations.

The cancer type he suspects is known as apocrine gland adenocarcinoma (AGA). This cancer arises from the anal sacs, which are paired scent glands on either side of the anus.

These are the same glands that skunks utilize for self-defense. Dogs use them simply to mark territory. AGA is a relatively rare tumor, making up just 2 percent of all skin tumors in dogs and 17 percent of all tumors in the anal region. AGA is a highly malignant cancer that invades the surrounding tissues. It quickly spreads from the anal sac to lymph nodes in the abdomen, and then on to the liver and lungs. Early detection and aggressive treatment is essential to optimal management.

Staging this tumor is the next step. This means determining what it is, and where it is. A syringe and needle can be used to suck up some cells from the tumor and look at them microscopically. In very small tumors, this has the potential to miss the diagnosis, delaying treatment or to spread disease in the area. Abdominal ultrasound should then be performed to determine if this has spread to the local lymph nodes or the liver. Chest X-rays are also recommended.

Surgery is the best single treatment for this tumor type. If there is evidence that the lymph nodes are affected, they are removed along with the tumor and anal sac. If the disease has spread beyond that, surgery may provide little benefit, unless to remove a tumor that is obstructing the colon or rectum.

Chemotherapy is commonly recommended, and significantly increases survival times. Radiation also can be useful in controlling disease in areas where it cannot be removed completely.

With appropriate, early and aggressive therapy these patients can live for years. A team approach is what is necessary to give your dog the best chance at a long and symptom-free life.

Your family veterinarian is the first line in this defense, and because he was thorough, you probably are ahead of the game.

The next step is a complete staging, to determine whether, and what type of, surgery is indicated. Consultation with an oncologist is recommended to determine what combination of treatments would suit your circumstance.

Because these tumors involve the anal area, an important nerve and vascular structures, the potential for serious surgical complications is great. These procedures should be performed by a skilled surgeon with a thorough knowledge of the relevant anatomy.

Dr. Henri Bianucci and Dr. Perry Jameson are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com or veterinaryspecialtycare.com.