DEAR DOCTOR K:

I recently had a prostate biopsy and was diagnosed with prostate cancer. Now my doctor wants to do an MRI. Why? What new information will the MRI provide?

DEAR READER:

I can understand why you're puzzled. A biopsy is the gold standard for diagnosing prostate cancer, so why do you need any other test?

In a biopsy, bits of tissue are removed from different areas of your prostate gland. They are examined with a microscope to see if there are cancerous cells. If the biopsy shows cancer, the diagnosis has been made. The biopsy also can help determine how aggressive the cancer will be and how likely it is to spread.

The biopsy tells you for sure if you have prostate cancer. But it doesn't tell you everything you need to know about how bad the cancer is and what kind of treatment you should receive. Once you've been diagnosed, decisions about your treatment will depend on the stage of your cancer. "Staging" is a way of describing how far your cancer has spread.

Ultrasound imaging tests have traditionally been used to determine how much of the prostate gland is filled with cancer, and whether the cancer has spread to the tissues next to the prostate. A newer kind of MRI called endorectal MRI may give a clearer picture than ultrasound of the local spread of prostate cancer. That's important because, in the majority of men diagnosed with prostate cancer, it exists in multiple locations in the prostate gland. This new kind of MRI is not yet widely available, but it may be available to your doctor.

Bone scans can determine if prostate cancer has spread through the blood to bones. MRI scans of the bones are another technique for detecting the spread of cancer.

The pictures produced by the MRI will tell your doctor whether and to what extent your cancer has spread to surrounding tissue, lymph nodes and other parts of your body. Your doctor will use these details along with other information to predict how aggressive your tumor is and what treatment will be best.

Many new techniques are under development to improve the diagnosis and staging of prostate cancer. New types of blood tests can spot prostate cancer cells that have escaped from the prostate gland and are circulating in the blood and threatening to lodge in bones.

Biopsy specimens are not only being examined under a microscope; they also are being studied to see what genes are turned on and off in the cancerous tissue. These "gene expression" patterns are likely to point to the best treatment.

Finally, new MRI imaging techniques are in development. For example, doctors may inject a contrast agent, such as a dye, into the bloodstream. This dye is absorbed by the prostate. Cancerous tissue absorbs the agent differently than other tissue. During a contrast-enhanced MRI, the cancerous tissue appears brighter than non-cancerous tissue, making it easier to identify and treat.

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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