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Bone scans in diagnosis of prostate cancer

Posted Sep 17 2008 1:52am

A bone scan uses tiny amounts of radioactive materials called tracers (or properly “radionuclides”) that are injected into the patient. These tracers accumulate in certain organs and tissues, such as bones, and give off a type of radiation called “gamma” radiation. The gamma rays can be “seen” by using a special camera, which produces images that can be interpreted by radiologists or by specialists in nuclear medicine.

A clever way to think about a bone scan is that it is the complete opposite of a X-ray examination. In an X-ray examination the radiation passes into or through your body from an external machine to create an image on film placed on the other side of your body. In a bone scan, the source of the radiation is inside your body and travels to the surface, where a camera detects it.

For patients who are newly diagnosed with prostate cancer, the value of a bone scan is limited. It would be unusal for the doctor to request a bone scan for any patient with a Gleason score of less than 7 and a PSA level of less than 20 ng/mL. 

Patients with a Gleason score of 7 or higher may, however, be candidates for a bone scan, whatever their PSA level. A bone scan may be considered important in any patient with a Gleason score of 7 or higher who appears to be at high risk of bony metastatic disease. A bone scan is also indicated for any patient with prostate cancer who has symptoms suggesting bony metastases.

However, it is important to understand that a bone scan may not show positive results until 5 years after micrometastasis (the very earliest stage of metastatic disease) has occurred. Therefore, a bone scan with negative results does not prove that there is no metastasis.

When the radiologist or nuclear medicine specialist looks at the results of a bone scan, (s)he is looking for evidence of abnormal bone metabolism on the scans. These may appear as darker “hot spots” or lighter “cold spots” where the tracers have or haven’t accumulated. The image shows a bone scan of a patient with metastatic prostate cancer.

Bone scans are very good at identifying abnormalities in bone metabolism, but they aren’t always so good at helping to tell what the exact problem may be.

If a patient has lower back pain, an elevated PSA, and a positive prostate biopsy with a Gleason score of 8, and he also has a positive bone scan showing accumulation of tracer in his lower spine and pelvis, it’s a pretty safe bet that the bone scan is showing metastatic prostate cancer. By contrast, however, in a patient with significant arthritis, no lower back pain, the same elevated PSA and the same positive biopsy with a Gleason score of 8, it may be harder to tell whether a positive bone scan is a consequence of the arthritis or metastatic prostate cancer.

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