The Current Value of Imaging in the Diagnosis and Staging of Prostate Cancer

Pat Fulgham, MD *)

(June 3, 2009)

One of the triumphs in the management of prostate cancer has been the ability to diagnose prostate cancer while still in its early stages.  This ability has been acquired on the basis of the widespread use of PSA testing and the increasing awareness by men of the importance of regular checkups.

The primary imaging modality used to diagnose prostate cancer is transrectal ultrasound of the prostate (TRUS).  Ultrasound allows an elegant
Pat Fulgham, MD
Pat Fulgham, MD
of the anatomy of the prostate and has the advantage of being a real-time imaging study.  A real-time imaging study is one in which the examiner has the ability to receive immediate visual feedback.  This enables urologists to obtain biopsies from the areas of the prostate which are most likely to harbor prostate cancer.  As such, ultrasound is by far the most commonly used imaging modality for initial diagnosis and biopsy of the prostate.  

One of the primary problems with any imaging study designed to detect prostate cancer is that prostate cancer does not have a single identifiable characteristic on imaging.  Prostate cancer has a variety of appearances regardless of what imaging study is used.  Moreover, microscopic disease is not currently able to be demonstrated by any imaging modality.

A number of strategies have been advanced to try to make imaging more sensitive for the diagnosis of early prostate cancer.  These include, in the field of ultrasound, the use of contrast agents to attempt to detect areas of increased blood supply which are commonly associated with prostate cancers.  The use of intravenous contrast agents for ultrasound is not currently FDA approved in this country, but in countries where large scale studies have been performed the diagnostic accuracy of contrast-enhanced ultrasound has only been increased marginally.  The use of Power and Color Doppler ultrasound, which is currently available in the U.S., have also not resulted in a dramatic improvement in the ability to diagnose prostate cancer.  This is not as much a deficiency in the imaging technology as it is the variability of prostate cancer and the fact that prostate cancer does not always demonstrate increased regional blood flow.  The difficulty in identifying microscopic or early disease is also encountered with CT scan and MRI scan, neither of which can reliably identify early disease.  

Although CT scanning and MRI scanning provide beautiful cross-sectional imaging of the prostate, neither is a real-time study.  That is, both obtain data sets and then display those data sets after the fact.  As such, there is no facile way to perform a biopsy at the same time either CT or MRI is being performed.  A number of attempts have been made to produce MRI and CT scanners which allow the patient to be removed from the imaging machine 

for an attempt at immediate biopsy but the biopsy still cannot be performed under real-time imaging conditions.  This is the primary reason that ultrasound remains the predominant imaging modality for early diagnosis and biopsy. One exciting innovation in imaging is the introduction of elastography as an adjunct to ultrasound scanning.  Elastography applies pressure to the surface of the prostate.  Prostate cancer tends to produce firmness or stiffness of the tissue that can be identified by its failure to compress.  Early work with elastography indicates that it may increase the diagnostic sensitivity of ultrasound. 

Another potential use of advanced imaging for prostate cancer treatment planning is the use of MRI and CT to determine whether the cancer has extended beyond the confines of the prostate.  The reason this is important is that, if it can be shown that cancer has extended through the capsule of the prostate into surrounding tissue, it makes the probability of cure by radical prostatectomy significantly lower.  It, likewise, signals a significantly higher probability of recurrent disease outside the pelvis.  If local extension beyond the capsule of the prostate can be shown before a treatment modality is selected it could influence whether (for instance) nerve sparing is used during radical prostatectomy.  Definite evidence of capsular penetration (that is, gross evidence of tumor outside the gland) would usually result in the selection of radiation or hormonal ablative therapy rather than surgery as the definitive treatment.

While both CT and MRI have shown some promise for detecting disease which has extended through the capsule, neither modality is able to identify microscopic disease which penetrated through the capsule.  

Endorectal MRI provides dramatic and beautiful images of the prostate but at this time it provides no significant benefit over existing imaging technologies in terms of either early diagnosis or definitive treatment planning.

The imaging technology that is most likely to provide a dramatic improvement in our ability to care for prostate cancer patients will be an imaging study which is able to tag antibodies on the surface of cells that are elaborated only by cancer cells and not by benign prostate cells.  The ability to fuse functional studies such as PET or SPECT scans with anatomic studies such as ultrasound or CT scanning hold significant promise.  Claims by investigators that any imaging study can reliably diagnose microscopic or early disease are premature and have not been borne out by carefully performed studies.  


Prostate cancer diagnosis and tests

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*) Dr. Fulgham is a member of the Urology Clinics of North Texas
    

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