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Screening for prostate cancer

The digital rectal exam (DRE) of the prostate is the age old and extremely crude method of screening and will only pick up definitely abnormal glands, which will be at a later stage of the disease than we would wish.

 

PSA (prostate specific antigen) is a protein that is secreted within the lumen of the prostate ducts at very high concentrations and some gets into the blood stream where the levels may be easily and accurately assessed. Assay methods have greatly improved over the last decade and with controlled calibration standards the results are very accurate. In general, the development of prostate cancer leads to a rise in the serum PSA early in the course of the disease allowing for this blood test to be a potentially important screening test for healthy males The problem is that the normal range is difficult to exactly define and rises with age and the majority of slight rises in PSA are due to benign prostatic hypertrophy. It is largely for this reason (and the fact there is a slight risk to biopsying everyone with a slightly raised PSA) that the routine adoption of the PSA screen is not universal. Several attempts have been made to refine the PSA test such that it is more specific for detecting cancer.

 

Age specific PSA cut-off’s, have been taken into account to acknowledge that PSA gradually increases with time. PSA density is a method of adjusting the serum value with respect to the gland size/volume. 

 

PSA velocity looks at the rise of the PSA over time, as the latter will be faster in cancer than in benign disease. 

 

PSA isoforms can be used to establish the ratio of free:total PSA in the serum, which leads to a lower ratio in cancer.

 

With these refinements, the PSA test is an important health screen test for males and has allowed the diagnosis of early prostate cancer to be made more frequently. Although the ‘controlled trials’ upon which the medical profession puts so much weight are still running, the PSA screening ‘revolution’ is well and truly here. 

 

(N.B. Whilst most regard PSA as also standing for 'Providentially Sent Antigen' there are those men who get hooked on measuring a trivially raised result and for them it has also been known as 'Promoting Stress and Anxiety' !).

 

Where the PSA test is slightly above normal and there is a question as to whether a biopsy needs to be performed, then a relatively new test called the PCA-3 test may be useful. This is a urine test that seeks to find the product of an oncogene that is almost always over-expressed in prostate cancer cases. Specifically, the PCA gene is highly over-expressed (median 66-fold) in more than 95% of malignant (tumours) prostate tissue. The Progensa PCA3 test measures the (post digital rectal examination) first-catch urine PCA-3 mRNA concentration and calculates a score. High score levels strongly suggest that the prostate is infiltrated by cancer. Where the PCA-3 test is positive (high score), then a biopsy should be performed as there is a high likelihood that the patient has cancer.

 

For predictive testing, see 'Causes section'


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