Screening for prostate cancer

It is strongly argued that a wide-ranging screening program should be established to try and limit prostate cancer deaths.  The issue of whether or not screening for prostate cancer will affect death rates is debated with some acrimony.  Currently 1 – 2% of UK males and 20% of US males are screened.

Prostate cancer screening consists of the measurement of the prostate specific antigen (PSA) and a rectal examination. PSA is a protease, an enzyme that breaks down proteins, which is made almost exclusively by the prostate and it functions in maintaining the patency of the prostatic ducts.  It is relatively non-specific as a tool for the detection of prostate cancer.  It is an inaccurate and insensitive test, as levels are elevated in benign prostatic hypertrophy and prostatitis.  The normal PSA value for the diagnosis  of prostate cancer is > 4 ng/ml.  But at raised levels between 4-10ng/ml just 25% of patients will have prostate cancer, and at levels between 10 and 40 ng/ml only 40% will have prostatic cancer. Conversely, the man whose PSA has been ;low (e.g. 1-2) for some years who has a rise of PSA to 4 over the course of a year, is also viewed with suspicion.

It is not clear how frequently PSA should be measured but a recent report indicates that bi-annual screening reduces mortality.

Accuracy

The lack of specificity of PSA for the early detection of prostate cancer was first described in the Physicians’ Health Study, in an analysis of 22,071 male physicians who had been recruited to a prospective study. Over a ten year period of follow-up, 520 men developed prostate cancer.  PSA levels when measured in serum stored at the start of the study were elevated in 46% of patients, so that in 54% there was no clue from PSA that a cancer was about to develop. However, the study did not delve into which cancer were potentially progressive in nature.

Screening in the UK

Whilst this topic is somewhat controversial in the UK (some health screening experts questioning the application to the entire male population) blood PSA levels (Total PSA level and free:total ratios) performed on simply blood tests have been show to bring more early cases of prostate cancer to attention and this is to be applauded. For men with marginally raised PSA values, a urine PCA-3 test (detecting RNA secreted from cancerous cells into the urine) may “refine” the population of patients who require a biopsy.

It is clear that a significant proportion of early stage prostate cancer is indolent and will not impact upon the lives of the vast majority of patients. However there is a significant proportion of patients whose tumours, regardless of microscopic appearances, have a more aggressive character. It is not possible to distinguish between these two groups using current tests.

Meanwhile, the argument from the heart faces the rationalists. The argument from the heart says, “it must be better to catch a cancer early”; whilst the rationalists say, “it’s not been proven in prostate cancer.”