Diagnosing anal cancer

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was updated on June 8, 2022.

Our doctors perform a comprehensive physical exam and a series of diagnostic tests to diagnose anal cancer.

To help answer questions on tests for diagnosing anal cancer, this guide will cover:

Digital rectal exam

This may be the first diagnostic test used to evaluate patients with anal cancer. In a digital rectal exam (DRE), a doctor inserts a lubricated, gloved finger into the rectum to feel the rectum and anus and identify lumps or changes in the body that may indicate cancer. During the exam, the doctor also feels the patient’s groin area to check for enlarged lymph nodes.

If the doctor feels something suspicious, he or she may order further tests.

CT scan

A CT (computed tomography) scan is an imaging test that helps doctors determine the stage of anal cancer. During a CT scan, an X-ray machine takes detailed images of the body. CT scans also may be used to determine whether anal cancer has spread to other parts of the body, such as the lungs or liver.

CT scans are taken throughout treatment to monitor how the cancer is responding to therapy.

A CT scan may also be used during a biopsy (more on this procedure below), as the images help guide the biopsy needle to the precise area of the body where cancer is suspected.


A pelvic MRI (magnetic resonance imaging) exam uses strong magnetic fields and radio waves to produce detailed images of the body. An MRI scan may help doctors stage the cancer. MRI allows for greater soft-tissue contrast than a CT scan, and it’s useful to help identify cancer metastases, especially in the lymph nodes, liver and spinal cord.

In some cases, the contrast material gadolinium may be injected into the body before the MRI scan to make it easier for doctors to identify cancer.

PET scan

A PET (positron emission tomography) scan is a nuclear imaging technique that creates detailed, computerized pictures of organs and tissues inside the body and shows areas of abnormal metabolic activity. A PET scan may be helpful in staging anal cancer, as well as indicating how the patient’s responding to treatment.

During the procedure, a safe, radioactive chemical is first injected into the body to help doctors identify where the cancer is located. A PET scan may also be combined with a CT scan to provide more detailed imaging, which may assist with diagnosis and determining where the cancer has spread.


An ultrasound exam is a diagnostic tool that uses sound waves to capture pictures of the internal organs of the body. It may be used to take images of the anal area for staging cancer.

During an ultrasound exam, a tool called a transducer is inserted into the rectum, but the procedure isn’t usually painful.


During a biopsy, a doctor will take a small amount of tissue and examine it under a microscope to look for cancer cells. This is the only way to definitively diagnose anal cancer.

Biopsies may be performed under local or general anesthesia, depending on the tumor’s location and size. If the doctor thinks the cancer may have spread to the lymph nodes, a nearby lymph node might be removed and biopsied as well.


An anoscopy uses an endoscopic device called an anoscope to examine the rectum. The anoscope is a long, thin tube with a light on the end that’s first covered with a gel before being gently inserted into the anus and rectum.

This typically painless procedure allows the doctor to look for cancer in the rectum, and it may be combined with a biopsy.

When will I get the results?

After being patients are tested for anal cancer, it’s normal to feel nervous or worried. Test results may take up to a few weeks, but the doctor will let the patient know how long they should expect to wait. The patient should feel empowered to call the doctor for an update if he or she is concerned.

Next topic: How is anal cancer treated?

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