Imaging in Prostate Cancer

There are several different imaging modalities used in prostate cancer. These include:

 

  1. Ultrasound - primarily for biopsy
  2. Bone scan
  3. Computerized Tomography (CT) scan
  4. Magnetic Resonance Imaging (MRI)

 

Imaging plays different roles depending on what the intended purpose of the scan is and not all modalities are necessary or even helpful in the different 'states' of prostate cancer. The results of any imaging test must be interpreted in the context of the complete clinical picture and should not be used in isolation.

 

Use of Imaging in Screening for Prostate Cancer

 

Imaging has no established role in screening for prostate cancer. The factors important in determining a man's risk of harboring prostate cancer are currently limited to age, race, family history, PSA, finger exam findings and prior biopsy results (if available).

 

The use of MRI is currently experimental but may be discussed based on your particular circumstances.

 

Use of Imaging in the Diagnosis of Prostate Cancer


Prostate biopsy remains the only test capable of establishing a diagnosis of prostate cancer. There is no imaging test which can make the diagnosis.

 

If prostate cancer is suspected based on the screening evaluation, ultrasound guided biopsy is the gold standard for establishing a diagnosis. The purpose of the ultrasound is to ensure that the biopsy samples are representative of the prostate gland, rather than clustered in just one area. Ultrasound is poor for identifying tumors, however.

 

The use of MRI is currently experimental but may be discussed based on your particular circumstances. There are 2 features of prostate cancers that can help in identifcation of tumors:

  1. Low signal intensity on T2 weighted images
  2. Restricted diffusion of water

Neither of these features is diagnostic of prostate cancer and biopsy is still required to establish a diagnosis. There is a spectrum of findings on MRI and therefore a uniform approach has been proposed for the interpretation of these scans. It is critical that the 'protocoling' and 'reading' of these scans be performed by physicians who are knowledgeable in this area otherwise the MRI may be useless.

 

 

CT and bone scan have no use for the diagnosis of localized prostate cancer (but are useful in staging - see below). If there is a strong suspicion that the cancer has spread then CT and bone scan may be used to make a tentative diagnosis - e.g. in the setting of a very high PSA a bones scan suspicious for spread is usually enough to start treatment.

 

Use of Imaging for Biopsy Proven Prostate Cancer

 

If a diagnosis of prostate cancer has been made on biopsy, imaging may be used for further staging. NOT ALL CANCERS REQUIRE ADDITIONAL IMAGING. In many cases the combination of PSA, digital rectal exam findings and the biopsy results will provide all the required information. Imaging is often over-utilized and may provide no additional information or in some cases false information.

 

MRI may be used for assessment of lymph nodes as well as for operative planning - especially if it is unclear if nerve-sparing surgery to preserve erections may compromise cancer control.

 

CT scan can be useful for assessing lymph node involvement. Note that not all enlarged lymph nodes are cancer - some may be 'reactive' to the biopsy or infection. The larger and 'rounder' the lymph node, the more likely it is to be cancer.

 

Bone scan can be useful for assessing spread to bones. Note that prior trauma and osteoarthritis can cause a 'positive' bone scan. Prostate cancer has an affinity for the large and axial skeletal bones - that is, the pelvis, spine and large bones of the legs. The pattern of spread and the likelihood that a patient might have spread to the bones is important in interpretation. For example, a bone scan showing uptake in the leg of a patient with a low risk of spread (e.g. low PSA and grade of tumour) is likely reactive where in a patient with a high chance of spread (e.g. high PSA and grade) is more likely to have actual cancer.

 

Note that the because of the small amounts of cancer which have spread outside the prostate may not be detectable, the absence of spread on CT or bone scan does not guarantee that there is no spread. However, if the Ct scan or bone scan suggest that the disease has spread it is unlikely that the disease has not spread.