How Is Anal Cancer Diagnosed?

Some cases of anal cancer in people at high risk are diagnosed by screening tests, such as the digital rectal exam and/or anal Pap test (described in “ Can anal cancer be found early?”). Sometimes a doctor will detect anal cancer during a routine physical exam or during a minor procedure, such as removing a hemorrhoid. Treating cancers found in this way is often very effective because the tumors are found early in the course of the disease. But most often anal cancers are found because of signs or symptoms a person is having.

If anal cancer is suspected, exams and tests will be needed to confirm the diagnosis. If cancer is found, further tests will be done to help determine the extent (stage) of the cancer.

Medical history and physical exam

If you have symptoms that might be caused by anal cancer, the doctor will ask about your medical history to check for possible risk factors and to learn more about your symptoms.

Your doctor will also examine you to look for possible signs of anal cancer or other health problems. He or she will probably do a digital rectal exam (inserting a gloved, lubricated finger into the anus and rectum to feel for lumps or other abnormalities).

If the results of the exam are abnormal, your doctor might do other exams or tests to help find the problem. If you are being seen by your primary care doctor, you might be referred to a specialist such as a colorectal surgeon, also sometimes known as a proctologist (a doctor specializing in diseases of the colon, rectum, and anus) for further tests and treatment.


Endoscopy is the use of a tube with a lens or tiny video camera on the end to examine an inner part of the body. Several types of endoscopy can be used to look for the cause of anal symptoms. They can also be used to get biopsy samples from inside the anal canal (described below). For these tests you either lie on your side on an examining table, with your knees bent up to your chest, or you bend forward over the table.


For anoscopy the doctor uses a short, hollow tube (called an anoscope), which is 3 to 4 inches long and about 1 inch in diameter, and may have a light on the end of it. The doctor coats the anoscope with a lubricant and then gently pushes it into the anus and rectum. By shining a light into this tube, the doctor has a clear view of the lining of the lower rectum and anus. This exam is usually not painful.

Rigid proctosigmoidoscopy

The rigid proctosigmoidoscope is similar to an anoscope, except that it is longer (about 10 inches long), so it lets the doctor see the rectum and the lower part of the sigmoid colon. You will probably need to take laxatives or have an enema beforehand to make sure your bowels are empty.


If a suspicious growth is found during an endoscopic exam, your doctor will need to take a sample of tissue to see if it is cancer. This is called a biopsy. If the growth is in the anal canal, this can often be done through the scope itself. You may get a local anesthetic to numb the area before the biopsy is taken. Then, a small piece of the tissue is cut out and sent to a lab. If the tumor is very small, your doctor might try to remove the entire tumor during the biopsy.

A doctor called a pathologist will look at the sample under a microscope. If cancer is present, the pathologist will send back a report describing the cell type and extent of the cancer. Biopsies to check for cancer spread to nearby lymph nodes

Anal cancer sometimes spreads to nearby lymph nodes (bean-sized collections of immune system cells). Swollen lymph nodes in the groin can be a sign of spreading anal cancer. Lymph nodes may also become swollen from an infection.

Fine-needle aspiration (FNA) biopsy: To see if cancer cells are enlarging a lymph node, your doctor may withdraw a small sample of fluid and tissue from the lymph node using a thin, hollow needle. A pathologist checks this fluid for cancer cells. If cancer is found in a lymph node, an operation may then be done to remove the lymph nodes in that area.

Sentinel lymph node biopsy (SLNB): If anal cancer has already been diagnosed, this test might be used to help determine if the cancer has spread to the lymph nodes. While this test has been shown to be useful for some other cancers, it’s not yet clear how helpful it is for anal cancer.

A low-level radioactive tracer material is injected around the tumor. Often a blue dye is injected into the area at the same time. After an hour or so, the lymph nodes in the groin are scanned to see where the radioactive material has traveled. The doctor removes any radioactive or blue-stained lymph nodes. A pathologist then looks at the nodes for cancer cells.

This test can help tell how far the cancer has spread, because these sentinel lymph nodes would be the ones that any cancer cells leaving the tumor would have spread to first. If these nodes don’t contain cancer cells, it is very unlikely the cancer would have spread beyond this point.

Imaging tests

Imaging tests use x-rays, magnetic fields, sound waves, or radioactive substances to create pictures of the inside of your body. Imaging tests might be done for a number of reasons both before and after a diagnosis of anal cancer, including:

  • To help find a suspicious area that might be cancer
  • To learn how far cancer has spread
  • To help determine if treatment has been effective
  • To look for possible signs of cancer coming back after treatment

Some of these imaging tests are used more often than others.


Ultrasound uses sound waves to make pictures of internal organs or masses. This test can be used to see how deep the cancer has grown into the tissues surrounding the anus.

A small microphone-like instrument, called a transducer, gives off sound waves, which bounce off organs and echo back. The echoes are picked up by the transducer and converted by a computer into an image on a screen.

For most ultrasound exams the transducer is placed on the skin. For anal cancer, though, the transducer is inserted directly into the rectum. This is known as transrectal or endorectal ultrasound. The test can be slightly uncomfortable, but it usually is not painful. Ultrasounds are very safe and use no radiation.

Computed tomography (CT) scan

CT scans use x-rays to produce detailed cross-sectional images of your body. This is a common test for people with anal cancer. It can be used to help tell if the cancer has spread into the lymph nodes or to distant sites such as the liver, lungs, or other organs.

Instead of taking one picture, like a standard x-ray, a CT scanner takes many pictures as it rotates around you. A computer then combines these into an image of a slice of your body.

A CT scanner has been described as a large donut, with a narrow table that slides in and out of the middle opening. You will need to lie still on the table while the scan is being done. CT scans take longer than regular x-rays, and you might feel a bit confined by the ring while the pictures are being taken.

Before the test, you may be asked to drink 1 to 2 pints of a liquid called oral contrast. This helps outline the intestine so that certain areas are not mistaken for tumors. You may also receive an IV line through which a different kind of contrast dye (IV contrast) is injected. This helps better outline structures such as blood vessels in your body.

The injection can cause some flushing (redness and warm feeling). A few people are allergic to the dye and get hives, or rarely, have more serious reactions like trouble breathing and low blood pressure. Be sure to tell the doctor if you have any allergies or have ever had a reaction to any contrast material used for x-rays.

CT-guided needle biopsy: CT scans can also be used to guide a biopsy needle precisely into a suspected area of cancer spread. For this procedure, you remain on the CT scanning table while the doctor moves a biopsy needle through the skin and toward the tumor. CT scans are repeated until the needle is within the mass. A needle biopsy sample is then removed to be looked at under a microscope.

Magnetic resonance imaging (MRI)

MRI scans use radio waves and strong magnets instead of x-rays to make images of the body. The energy from the radio waves is absorbed by the body and then released in a specific pattern formed by the type of tissue and by certain diseases. A computer translates the pattern into detailed images of parts of the body. As with a CT scan, a contrast material might be used, but it is not needed as often.

This test is sometimes used to see if nearby lymph nodes are enlarged, which might be a sign the cancer has spread there.

MRI scans take longer than CT scans – often up to an hour – and are a little more uncomfortable. You have to lie on a table that slides inside a narrow tube. This is confining and can upset people that suffer from claustrophobia (a fear of enclosed spaces). If you have trouble with close spaces, let your doctor know before the scan. Sometimes medication can be given just before the scan to reduce anxiety. Another option might be to use a special “open” MRI machine that is less confining. The MRI machine makes buzzing or clanging noises that some people may find disturbing. Some places will provide headphones to block this sound.

Chest x-ray

This test might be done to find out if the cancer has spread to the lungs. It isn’t needed if a CT scan of the chest is done.

Positron emission tomography (PET) scan

For a PET scan, a form of radioactive sugar (known as fluorodeoxyglucose or FDG) is injected into the blood. A very low amount of radioactivity is used, and it will pass out of the body over the next day or so. Cancer cells in the body are very active, so they absorb large amounts of the radioactive sugar. After about an hour, you will be moved onto a table in the PET scanner. You lie on the table for about 30 minutes while a special camera creates a picture of areas of radioactivity in the body. The picture is not finely detailed like a CT or MRI scan, but it provides helpful information about your whole body.

Often a PET scan is done in a machine that can do a CT scan at the same time (a PET/CT scan). It lets the doctor compare areas of higher radioactivity on the PET scan with the more detailed appearance of that area on the CT scan.

For more information about scans and x-rays, see  Imaging (Radiology) Tests.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: April 9, 2014 Last Revised: January 20, 2016

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