Thalidomide and cancer?

To the Editor: McBride reports that there were four deaths from cancer before the age of 40 years in a group of 480 thalidomide-affected people in the United Kingdom, an approximate cumulative mortality rate of 0.83%.1 He compares this with the annual death rate in the under-40-years age group, and concludes that the rate is increased almost 100 times in those affected by thalidomide.

The correct comparison is with the cumulative mortality rate in the general population from birth to age 40. From 1999 UK statistics,2 this is about 0.31% — that is, we would expect 1.48 deaths among 480 people. While the observed number of four is greater than this, it is only slightly greater, and the difference is not statistically significant (the mortality ratio is 2.7, with exact 95% confidence limits of 0.7 to 6.9, based on a Poisson distribution). McBride's conclusion is based on an inappropriate comparison.

A full analysis would use population death rates over the 40-year period and take account of censoring, but that is unlikely to affect the result substantially.