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Cancers which can been screened

In order for valid screening to be possible, the pre-clinical phase of the cancer involved should be long enough to enable screening to be carried out within this period. The advance in diagnosis offered by such screening should then give way to active therapy, reducing, if possible, mutilations and other therapeutic complications involved in the treatment of more advanced cancer.

If the pre-clinical phase is too short (which is currently, but may not always be, the case for lung cancer and leukaemia), then screening is impossible.

If no efficient treatment exists (within current cancer discovery conditions) or if cancer has already produced metastases (for instance - for many brain tumours such as glioblastomas or pancreas carcinoma with liver metastases), screening has no utility.

We need, of course, to have a simple, reproducible and relatively inexpensive test: systematic pulmonary radiography does not permit secure early diagnosis of lung cancer (too many false negatives); nowadays an annual multi-slice helicoidal scanner for all smokers is economically too expensive even if some studies have shown its potential utility (T.Blanchon, S. Sone).

Screening tests

A screening test should be easy to perform and well accepted by healthy persons (especially if it needs to be repeated often), have only light secondary drawbacks and if possible cost as little as possible. But above all, it should be efficient.

For every screening test, we can define the following values : true positive (TP), true negative (TN), false positive(FP), false negative(FN), positive predictive value (PPV) and negative predictive value (NPV).

A good screening test should be very sensitive (it do not miss cancers) and specific (fear of cancer inducing unnecessary further examinations)

Generally speaking, the more sensitive a test is, the less specific it is (other pathologies can also induce abnormal tests: for instance PSA can also be increased in prostatitis and faecal blood can be observed during colon inflammatory diseases.

On the contrary, the more specific a test is, the less sensitive it is (many cancer patients do not have a positive result: for instance, normal pulmonary radiography for small lung cancers andabsence of elevation of the carcino-embryonic antigen (CEA) in small colon carcinoma.

A low positive predictive value will induce, in order to confirm the diagnosis, high numbers of painful, costly and unnecessary examinations to healthy persons: for instance prostate biopsies for an elevated prostatic specific antigen ( PSA) level.

A low negative predictive value will mistakenly reassure people who have cancer: for instance, the absence of faecal blood associated with the presence of a colon cancer which intermittently bleeds.