Expert Testimony

Expert Testimony

1218 who present late with symptoms, although the stage distribution of cancers in this category in the Utrecht project is no different from that in ...

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who present late with symptoms, although the stage distribution of cancers in this category in the Utrecht project is no different from that in the whole of Utrecht before screening started. In the HIP study, women who refused screening had a substantially greater mortality from other causes, but for those in whom breast cancer developed survival was marginally better than that in the control group.’3 There is thus no evidence that can be pinpointed to indicate that selection of low-risk women into the screening programme accounts for the favourable result, and it is reasonable to conclude that at least part of their decreased risk of dying of breast cancer is attributable to early detection by screening. The extent of potential confounding biases might be measured by conducting similar case-control studies within a randomised controlled study, comparing the results obtained by using as controls, firstly women who refused screening and secondly women who were not offered it. These results could also be verified against those from the whole study/control group

comparison. Several other points relevant to the organisation of screening can be inferred from these results, although in each case the numbers of deaths within subgroups are too small for certainty of their statistical validity. First is the question of benefits within ’age-group, which is of interest since the HIP workers concluded that the benefit of screening women under 50 was unproven. The Utrecht project involved only women over the age of 50, and, rather surprisingly, benefit seemed to be greater with increasing age. In Nijmegen by contrast the lower age-limit was 35 and relative risk was equal in those above and below age 55. Secondly, the Nijmegen study screened by mammography only, without clinical examination, and thus has provided the first evidence that screening by mammography alone may be effective in reducing the risk of dying of breast cancer. Thirdly, the Utrecht study addresses the question of frequency of screening, indicating a lower risk in those screened twice than in those screened only once.

In Utrecht, the cohort of women offered screening has not shown the usual increase in age-specific breast cancer mortality rates, but in neither city has a fall in breast cancer mortality rate in the whole population yet been demonstrated. This is not inconsistent with the findings of the case-control studies since, at the time of these analyses, most of the deaths from breast cancer were in women whose cancer had been diagnosed before screening began. Like all other screening studies, the starting-point for these was a population none of whom had been diagnosed as having the disease. In the case of breast cancer it takes up to 10 years for such a disease-free population, even in the absence of screening, to reach the mortality rates of the general population of the same age. This long interval between entry to a trial and expected mortality 13

Shapiro

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1977; 39:

cancer

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for the extended duration of the current controlled trials, none of which can be expected to show statistically significant mortality differences within 5 years from enrolment of the complete sample size into the trial. The Utrecht and Nijmegen groups are sensibly cautious about the interpretation to be put on their results. Not only is there the possibility, discussed above, of various confounding biases accounting for the difference between ever-screened and neverscreened women, but also it is too soon to say whether the reduced risk represents cure or merely postponement of death. Additional numbers, which presumably will accumulate over time, are required to verify the age, modality, and frequency effects. This new but preliminary evidence that screening may protect against the risk of dying from breast cancer is insufficient to warrant abandonment of the present trials. Indeed it adds further weight to the necessity for their continuation, since it is from these trials that a more certain estimate of benefits, balanced against disadvantages and costs, will come. However, health authorities might be well advised to note the consistency of this new evidence, both within the two present studies and with the HIP findings. If each of the additional trials expected to report results within the next 5 years shows similar benefit, will there be sufficient trained staff for even the nucleus of a breast screening service? accounts

Expert Testimony criminal action under English law, reports of witnesses to be called for the prosecution are expert made available to the defence, which may then search for experts able and willing to challenge their findings and conclusions. The defence is under no such obligation to make its expert evidence available to the prosecution before the trial. The basis of the law is that the prosecution is responsible for ensuring that its case, technical as well as factual, is watertight and proves the guilt of the accused beyond reasonable doubt. This bias is founded on the notion that ensuring conviction of the guilty is less important than avoiding conviction of the innocent. A Bill that last week completed its passage through the Commons, and is now entering the Lords, would change this and remove the advantage the defence has so far enjoyed. Clause 72 of the Police and Criminal Evidence Bill specifies that Crown Court Rules may make provision for "(a) requiring any party to proceedings before the Court to IN

a

disclose to the other party or parties any expert evidence which he proposes to adduce in the proceedings and (b) prohibiting a party who fails to comply in respect of any evidence with any requirement imposed by virtue of paragraph (a) above from adducing that evidence without leave of the court".

What is so special aboutexpert evidence that legislators should suddenly decide that it requires to be treated differently from all other evidence that may be

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presented to the criminal courts? Irrespective of which party calls him (or her), the expert witness swears to tell the whole truth. His primary duty is to the court and his task is to explain the scientific probabilities on which his evidence is based and, by his presentation, to help the court decide what weight should be given to them. His evidence should be unbiased and as full and accurate as possible. He is unusual only in that his opinion will be sought, and here his obligation is to indicate the reliance that should be given to it. The medical expert first called upon by the prosecution is in a special position. He may well have been called to the scene of the crime, and he is certainly likely to have had early and wide access to scientific specimens. Nowadays, the medical forensic scientist is a highly trained specialist, and in serious crimes he will have access to virtually unlimited scientific facilities which by the nature of things can only rarely be available to the defence. Under these circumstances the prosecution expert has the onerous duty of ensuring that all relevant tests are in fact performed, and he must actively search for clues that might be helpful in any way to the defence. His failure to do so may well result in a, miscarriage of justice. Lately there have been embarrassing instances of cases where the prosecution expert witness failed to present a full and accurate account of the scientific data,’ and it is easy to see why the Royal College of Pathologists and the British Association of Forensic Medicine should support the proposed new law.2 The Royal College of Pathologists has recommended that "scientific matters should not be discussed, for the first time, in front of a jury" and the Secretary of the British Medical Association has written of our adversarial system that "it would be difficult to devise a system less likely to arrive at the scientific truth". No one should underestimate the difficulty and complexity of the task faced by the prosecution expert witness when he first compiles his report. He must be prepared to defend his evidence in open court, not knowing in advance how it will be attacked. This can be embarrassing for him if omissions or errors are disclosed, but all witnesses face this exposure and the only point of importance is whether or not the proposed reforms will improve or hamper the court’s chances of finding a just verdict. With the stimulus of open criticism removed, it is possible that a prosecution expert may in future become less conscientious in the preparation of his report because he knows that either the defence expert will tell him of any errors and omissions in good time, or an error will be missed entirely. Perhaps the position of the defence expert will be substantially altered by the proposed changes in that he might not only have to question the prosecution evidence for errors or debatable points, but also feel obliged to seek more deeply for’omissions. Starting at a later date, having to make his own contacts 1. Brahams D. Should medical and scientific experts be required to confer before a criminal trial. Lancet 1984, i: 918 2 Correspondence Giving expert evidence Bull Roy Coll Pathologists 1984, no 45 17-18

police, and almost inevitably with inferior laboratory facilities, he would face an even more daunting task. Finally, is there not a danger that any pretrial conference of experts might agree what with the

constitutes "truth"

on

the scientific evidence, and thus

usurp the court in

making a judgment which, because be would impervious to cross-examination? agreed, Clause 72 is a small part of a lengthy and controversial bill. It has received very little public attention, but its potential significance is great and any member of the medical profession may be affected by it. If passed, it will restrict the freedom of action of the defence lawyers, leaving the rules governing the prosecution unchanged. To this extent, therefore, it reduces the rights of the citizen. Those who would benefit are the police, the prosecution expert witnesses, and perhaps those doctors who occasionally give evidence for the prosecution. Such a new law would constitute a fundamental change in a legal code that has hitherto insisted that the prosecution has the absolute duty to prepare its case without relying on foreknowledge of the testimony that can be found to refute it.

CLUES FROM FAMILIAL CANCER RETINOBLASTOMA is commonly cited in the teaching of medical genetics as an illustration of "dominant inheritance with variable penetrance", meaning that 50% of a sufferer’s offspring are at risk but that some of those who inherit (and pass on) the "retinoblastoma gene" never manifest the disease. In fact the condition occurs in a sporadic as well as a familial form. When familial, the tumours often affect both eyes and usually appear during the first year of life. Sporadic cases are unilateral and tend to present slightly later in childhood; and, by definition, the trait is not passed to

succeeding generations.l,2 The epidemiological features can be fitted to a model, proposed by Knudson in 1971, in which malignancy requires the concurrence of two specific mutations in a single cell in the developing retina. In familial cases, one of these mutations is already present in every cell of the body (being inherited from a parent and being passed to 50% of sperm or ova)-hence, a single specific new mutation will cause a A reasonable estimate of the frequency of any new mutation is once in every million cell divisions so, if there are about two million cells in each developing retina, malignancy becomes probable though not inevitable. Conversely, in the sporadic form of the disease, no abnormality is inherited and both specific mutations have to arise de novo in the same cell-clearly a very rare mischance. The implication of Knudson’s hypothesis is that, although familial retinoblastoma displays a single gene mendelian dominant pattern of inheritance, the genetic abnormality that is inherited is itself recessive in expression. What is truly familial is the risk, not the tumour. Nothing can be inferred from this about the functions that are altered by the two tumour.

1 Vogel F. Genetics of retinoblastoma. Hum Genet 1979; 52: 1-54 2. Harnden D, Morten J, Featherstone T. Dominant susceptibility to cancer in man. Adv Cancer Res 1984; 41: 185-255 3. Knudson AG Mutation and cancer. statistical study of retinoblastoma. Proc Natl Acad Sci USA 1971, 68: 820-23.

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