Day 022 - 12 Sep 94 - Page 44
1 A. I think with out current level of knowledge we are
2 unable to influence genetic factors. Therefore, we have
3 to look at those factors where we do have some control and
4 try to modify those because, as I have said, once you get
5 the abnormality, the next stage is promotion, and it may
6 be that by modifying the factors which act at that stage
7 in the progress, then one can within the life expectancy
8 of that person prevent the development of a particular
9 illness -- but not necessarily so.
10
11 Q. If we move on to cohort studies: You said they were the
12 most reliable ---
13 A. Yes.
14
15 Q. -- studies, I believe. Again playing devil's advocate, or
16 in a responsible way, you would be expected to know what
17 the limitations of cohort studies are. Could you say what
18 some of those limitations would be?
19 A. I think one of the problems with the cohort studies is
20 that you are often examining the patient or people --
21 sorry, I do not want to say "patients" but you are
22 examining people -- over many years, because you
23 essentially take a healthy population or you take a
24 population and then find out what happens to them over the
25 coming years, because if you are trying to look at factors
26 which may influence cancer, we know that these may take
27 many years to exert their influence.
28
29 Now, the difficulty of examining something, say, like diet
30 during that period of time is that people's diets may
31 change. Therefore, you have the difficulty of knowing
32 what the exact importance of diet is over a period of 20
33 years. So, that has been put forward as a criticism of
34 these prospective studies which by their very nature must
35 take many years to conduct and complete.
36
37 The other criticism, major criticism, is that if you do
38 these studies in a country such as the United States of
39 America, there probably is less variation in diet within
40 the groups in that study than there may be, for example,
41 if you looked at populations in Europe where you -----
42
43 MR. JUSTICE BELL: That is Mr. Morris's nurses point?
44 A. Yes.
45
46 MR. MORRIS: Yes.
47 A. But the great advantage, of course, is that you are
48 looking at populations to begin with without prejudice and
49 looking at them as they go forward over the years.
50 Therefore, there is less likelihood of introducing bias in
51 the answers that you may get.
52
53 Q. Just in terms of time that you need to look back for, say,
54 the identifying causes of cancer, is it possible to know
55 when the cancer was initiated when you have a cancer
56 patient?
57 A. I do not think we know the answer to that; it may even
58 be in childhood. For example, with breast cancer it may
59 be that the critical period is during major hormonal
60 changes such as at the time the woman starts to
