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

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