parameter, while comparison of age-specific death rates in the two groups would
be a more reliable method of analysis.
Seltser and Sartwell, however, did not
prove that the exposure to radiation of radiologists had no effect on their
life-span.
339.
Similar reservations about the method used by Warren [W2] were expressed
by Lewis [L18] in a review on radiation-induced leukaemia.
He pointed out after
appropriate calculations that a difference of at least 6 years in excess in the
life-span of radiologists would be expected by comparison with other non-exposed
physicians, solely on the basis of differences in the age distributions among
the two samples compared.
If this were true, radiologists might in fact have a
slightly longer life-span than other non-exposed doctors.
340.
At approximately the same time the results were published of a survey on
British radiologists by Court-Brown and Doll [C27].
The study concerned life
expectation and cancer mortality among 1377 male radiologists, mostly diagnosticians, who had been members of specialist Societies in Great Britain during
1897-1956.
It proved impossible to assess the exposure to radiation of this
group: it was simply assumed that the average dose received prior to 1921
(when
the first recommendations on radiation protection were issued) was very high,
whereas the average exposure of those registered as specialists after that date
had been within the limits recommended.
341.
Mortality data were calculated from the population at risk at each age
and in each year.
The expected numbers of deaths were first estimated by as-
suming that mortality might be the same as for all men in England and Wales in
the same age groups and over the same time period.
Expected deaths were also
calculated according to those expected in the upper social class or in the medical class as a whole, with some corrections concerning the relative mortality
of people in various social groups aged 65 or more.
By similar methods the num-
ber of deaths to be attributed to all types of cancer (appropriately corrected
for occupation and social class) were obtained.
All the data were kept separate
for radiologists registering before or after 1921.
342.
Regarding expectation of life, observed deaths were 463, less than ex-
pected on any of the assumptions mentioned, which would have been between 499
and 525.
If deaths attributable to cancer were excluded, the relative differ-
ences between observed and expected cases became more marked and approached