How does screening work?

How does screening work?

As neither the causal pathways for breast cancer nor the means of preventing it are known, the only intervention possible at the moment in healthy women to improve mortality from breast cancer is screening.Trials of chemopreventive agents have produced
conflicting results. While the American NSABP study has demonstrated a 50% reduction in breast cancer incidence in women taking tamoxifen
compared with placebo, these results have not been Further follow- reproduced in the UK or in Italy.Up is required to assess the effect on long-term incidence and mortality.

Although treatment, notably chemotherapy and hormonal therapy, has improved, prognosis still deteriorates rapidly with increasing tumour burden.Screening can advance the diagnosis so that the cancer is treated at an earlier stage with more chance of success. This is reflected in the earlier stage of tumours detected in the study group in the Swedish The progression of the disease is Two-County Trial.The lead time is the interval The longer the lead time for a given case, the better one would expect the prognosis to be.

If the cancer is not detected until it becomes clinically detectable,the lead time is zero. The lead time for an individual case is unobservable, but the distribution of lead times is dependent on the distribution of the time spent in the pre-clinical detectable phase (sojourntime).

The sojourn time is also unobservable, but may be estimated using the method of Walter and Day. This method will also provide an estimate of the sensitivity of the screen, and this may be used
to estimate the optimum screening regime and the potential gains in terms of mortality. For example,if the sojourn time is long, the maximum possible lead time is correspondingly long. If the sojourn
time is short, however, the potential benefit from screening is smaller and screening must take place

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