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BMJ No 7119 Volume 315 Letters Saturday 22 November 1997
New method for expressing survival in cancerUse of age specific relative survival is sufficientEditorNode negative breast cancer among women in India seems to confer a significant survival advantage.(1) In Scotland, women, of whatever age, have only a 50-60% chance of surviving to their normal life expectancy, the proportion depending on the initial age.(2) But this result must be expected, because life expectancy is the average, over the whole population, of the number of years of life remaining to those who are alive. Both the healthy and the sick will contribute life years to the average. Age specific relative survival curves based on local life tables will take account of local mortality experience. Clinicians can then provide patients with two items of information: their survival chances if they had been 'average,' and how survival is reduced because of their disease. Relative survival frees the survival estimates from the influence of local variations in other causes of death. Thus survival rates from different centres can be compared. Relative survival also helps to answer the question of 'cure' - not in terms of people's chances of living out their normal life expectancy but in terms of how many years it will be before their chance of death in subsequent years is no different from that of any other 'average' individual of the same age. Both may die in the next year, of either an existing or a new condition, but if the chance of death, or life, is the same for each then one could consider the patient cured. Rather than attempting to devise yet another method of assessing survival that is useful to 'someone with little statistical knowledge,' it may be preferable to try to understand the statistical methods so that both clinicians and patients are better informed. But a message that is equally important is that survival statistics relating to specific age, sex, stage, or grade of disease are more useful than crude measures that are only relevant to the 'average' patient.
Jon Cresswell Consultant in public health medicine
Public Health Agency,
Reference
1 Vaidya J S, Mittra I. Fraction of normal remaining life span:
a new method for expressing survival in cancer. BMJ 1997;
314:1682-4. (7 June.)
2 Registrar General for Scotland. Annual report
1995. Edinburgh: HMSO, 1996.
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