Monday, October 24, 2011

Screening for cancer

To most people it seems sensible that if we can screen for occult disease then we ought to. Strangely, the figures don't add up that way. If it were the case that every occult case of disease would eventually turn up and need to be treated, and if it were the case that there were no false positives picked up by the screening test and if it were the case that none of those false positives came to any harm for being detected, then, no matter the cost, it would be worthwhile to screen for occult disease.

The two areas where the value of screening tests is disputed are breast cancer and prostate cancer.

The problem with mammography for breast cancer is first that the premeonpausal breast is too dense to see cancer in and second that nobody advocates screening for the over-70s when breast cancer is at its most prevalent. Third, many of the precancerous lesions found will never require any treatment during the lady's lifespan. Fourth, there are many false positives. Fifth, managing those false positives cause harm to the patient who at very least has a period of unjustified anxiety but may need a biopsy and in extreme cases a mastectomy.

Time and again the figures have shown that mammography for this population is not wise. It is true that life expectancy for breast cancer has increased since mammography was introduced, but this is also true for the under-50s and over-70s who were not screened. It is most likely due to the introduction of Nolvadex for treatment.

Now the US Preventative Services Task Force has ruled that the PSA blood test should not be used to screen for prostate cancer on the grounds that it does not save lives and can lead to harm. The review concludes, "The widespread US practice of annual PSA-based screening for prostate cancer in men aged 50 years and older is not supported by results from randomized controled trials. PSA-based screening may modestly reduce prostate cancer mortality; but this absolute benefit is small relative to other causes of death in this age group and is associated with substantial harm. The vast majority of men treated for PSA-detected prostate cancer will not have death from prostate cancer prevented but there is convincing evidence that treatment of prostate cancer detected through PSA screening causes at least moderate harm such as erectile dysfunction, urinary incontinence, bowel dysfunction and death.

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