Day 022 - 12 Sep 94 - Page 38
1 which are specially bred to show susceptibility to a
2 certain chemical which is then perhaps given to the rat,
3 because otherwise you would be carry out your experiment
4 for many, many years. What you are trying to do is to run
5 an experiment where you get an answer quite quickly.
6 Therefore, you are using animals which are particularly
7 susceptible to certain factors to which you are going to
8 expose them.
9
10 You are also exposing the animals to these chemicals, for
11 example, in much greater concentrations than, you know,
12 you or I would ever be exposed to in normal life.
13
14 So, what one does not know is, for example, with chemical
15 exposure, is there a threshold level? Do you need to have
16 more than a certain level of exposure before you will get
17 an effect, or is there a continuing spectrum of dose and
18 response? You know, this is a problem which we have not
19 been able to answer in lots of things. For example, in
20 radiation exposure; we try to keep radiation exposure to
21 members of staff or the population as low as possible.
22 But yet we are only able to demonstrate effects from
23 radiation once the level gets to a certain point, the
24 level of exposure gets to a certain point. We do not know
25 if the spectrum continues down below that particular level
26 of exposure.
27
28 Q. I think you said before just on that subject, for example,
29 from Hiroshima it can take 30 years for cancer to develop?
30 A. It may do. I mean, some things develop rapidly.
31 Hiroshima was an extreme example, but for people who got
32 relatively low levels of exposure then it may be 20 or 30
33 years before you see a discernible effect.
34
35 Q. So you would not place a reliance on animal experiments on
36 whether something was healthy or unhealthy?
37 A. No. You know, it is like a jigsaw puzzle; there are
38 lots of pieces that have to fit into place before you have
39 an answer at the end of it. I think population studies
40 are one piece of the jigsaw; I think animal experiments
41 are another piece of the jigsaw, but they are only parts
42 and they can only give you pointers. It is rare for them
43 to actually give you the complete answer.
44
45 Q. Before I come to cohort which is obviously something you
46 feel quite positive about, just on the genetic side: You
47 say that the genetic causal process has not been
48 established?
49 A. Only in exceptional circumstances and this applies to
50 cancer as a whole. I have mentioned polyposis coli which
51 is an autosomal dominantly inherited abnormality. These
52 people will go on to develop bowel cancer usually after
53 the age of 30. But we do see, for example, cancer running
54 in families. We know, as I said this morning, the risk of
55 developing cancer is increased substantially if you have a
56 blood relative, a direct blood relative, who has had the
57 disease.
58
59 Now, we have not been able to identify specific genetic
60 abnormality which is responsible for that.
