Obligación de quienes realicen actividades vulnerables

A major controversy that exists is how do invasive ductal breast tumours progress? While there is evidence to suggest that in situ disease represents the preinvasive form o f breast disease that would naturally progress to invasive disease (see section 1.3.2), the mechanism, pathway and time-frame of the biological progression to invasive disease is uncertain. Do invasive ductal tumours progress by dedifferentiation? That is do tumours arise as well differentiated grade I tumours which then evolve to poorly differentiated grade III breast tumours over time or do grade I tumours arise as grade I tumours and remain as grade I tumours, spreading and metastasising as well-differentiated tumours? This is an important biological question that has implications for both research and treatment of these tumours. Critically there is evidence both to support and refute the hypothesis of dedifferentiation.

Once a tumour has been identified, it is necessarily treated by excision. Clearly, it is not ethically possible to follow the natural history of the disease in situ.

Therefore the evidence that is used both to support and refute this hypothesis is derived from a number of sources. Some studies have used mammographie data. This comes from a variety o f different trials, which were established in the 1980s. The hypothesis being that breast tumours picked up by screening theoretically represent an ‘earlier version’ in the natural history of tumours when compared with the tumour which is picked up incidentally, and so deductions can be made about the biology of breast tumours using this data in a number of ways.

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Some authors compare the features of screened versus non-screened tumours for example (Anderson, T. J. et al. 1986; Duffy, S. W. et al. 1991; Rajakariar, R. and Walker R. A. 1995). Some have studied the actual tumours (Meyer, J. S. and Wittliff J. L. 1991) and others have used mathematical modelling of the data (Tubiana, M. and Koscielny S. 1991; Tabar, L. et al. 1996; Tabar, L. et al. 1999). Another approach is to compare various features o f tumours and their métastasés, for example the S-phase fraction and ploidy (Auer, G. U. et al. 1984). Although how valid this approach is in assessing progression is uncertain, as it makes assum ptions that ploidy/S-phase fractions and histological grade are interchangeable and this may not be so (see discussion later). Even when studies which have possible errors in their interpretation are excluded, the data ore frequently directly conflicting which does make it difficult to determine what is occurring biologically.

Before discussing the evidence it is necessary to clarify a few points with regard to breast screening. The first screening investigation is known as the prevalence screen and will have a length-time bias. This simply means that it will pick up more well-differentiated tumours as these will tend to be more slow growing compared to the poorly differentiated tumours. Therefore, these will still be small when picked up by screening and not necessarily palpable, whereas the poorly differentiated tumours will grow faster and so will present earlier. In order to eliminate this bias, only tumours from the steady-state i.e. those identified during the subsequent rounds o f screening, should be studied and compared with non­ screened tumours. These are known as the incidental screens. Several studies although appearing to support dedifferentiation are confounded by not considering the lead time bias (Joensuu, H. et al. 1991; Rajakariar, R. and Walker R. A. 1995).

The evidence in support of dedifferentiation comes from a number of studies. Duffy and colleagues (Duffy, S. W. et al. 1991), compared data from the Swedish two county trial, in which 66 741 women aged between 40-69 were invited for mammography. 48 678 women were not invited and so formed the control arm. After exclusion o f length bias they found that tumours in the screened group were still smaller and more well differentiated than in the control

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group. Their interpretation of this data was that as tumours grow their malignant capacity increases thus becoming poorly differentiated. This theory is supported by the work of Tubiana and Koscielny (Tubiana, M. and Koscielny S. 1991) who have modelled data taken from a series of 3000 patients at their institute. To investigate the relationship between size and histological grade they plotted the proportion of tumours with each of the three grades as a function o f tumour size. From this they found the percentage o f grade II tumours remained constant whereas the percentage of grade I tumours decreased with increasing size with the inverse being true for grade III tumours. Thus suggesting to the authors that during growth some grade I tumours progress to grade II while some grade II tumours progress to grade III. Furthermore, the constancy in proportion of grade II tumours is compatible with an equal flow into and out o f this grade i.e. a constant rate of progression during breast tumour growth. Tabar and colleagues (Tabar, L. et al. 1996; Tabar, L. et al. 1999) have applied mathematical models to the data from the Swedish two-county trial to not only provide confirmatory evidence for dedifferentiation but to assess more precisely the number of tumours capable of progression. It should be noted that grade was classified into just two groups, grades I-II and grade III. Interestingly in their model dedifferentiation was much more likely to occur in younger women (40-54) than older women (55-69) with predicted frequencies of 91% and 38% respectively.

Other evidence cited to support dedifferentiation is generally weaker. For example, the thymidine-labelling index is used as a measure o f the proliferative activity of a tumour. As tumours show heterogeneity for this index (Meyer, J. S. and Wittliff J. L. 1991) it is argued that areas of high proliferation could outgrow the areas of low proliferation so becoming larger higher grade tumours.

Similarly, the evidence which refutes the hypothesis of dedifferentiation comes from a number of studies. Anderson and colleagues (Anderson, T. J. et al. 1986) provide evidence which is in direct contradiction to the results of Duffy. Examining data from the Edinburgh screening study, it was found that tumours identified during the incidence screening rounds were not well-differentiated. Indeed they were more aggressive and poorly differentiated. This was the predicted result which conforms to screening theory, in that good prognosis slow

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growing tumours are detected at the prevalence screen with the incidence tumours including a higher proportion of faster growing aggressive tumours. Other groups have found no difference between the screen detected and control tumours (Hatschek, T. et al. 1989). There was also no difference in S-phase fraction between incidence tumours and controls, and a similar frequency of aneuploidy. Hakama and colleagues (Hakama, M. et al. 1995) took data from a screening study in Finland, where 34 000 women aged between 40-68 were invited for screening. They did find that tumours in the screened group were smaller than the controls (which were cancers arising before the introduction of screening), but when they analysed the tumours by DNA flow cytometry, which they used as a measure of tumour aggressiveness and therefore indirectly of high grade, they found no difference between the screened tumours and the controls. Therefore suggesting that biological aggressiveness does not increase during breast cancer development. Other studies have confirmed this consistency in ploidy status between screen detected tumours and controls. A study by Auer and colleagues (Auer, G. U. et al. 1984) comparing ploidy status between primary tumours and métastasés found concordance between the primary tumour and métastasés in 17/18 tumours.

Millis and colleagues (Millis, R. R. et al. 1998) examined 115 patients with infiltrating ductal carcinoma, and compared the grade of primary tumour with the type o f in situ component o f the disease and the grade of the 169 locally recurrent and metastatic lesions. They found that in the lymph nodes, the local recurrences and distant métastasés there was concordance for grade. Furthermore, when the in situ component was compared with the primary tumour there was general concordance between grade too. Other studies have found similar findings, with concordance between grade o f primary tumour and metastatic disease ranging between 71%-82% (Haagensen, C. D. 1933; Bloom, H. J. 0 . and Richardson W. W. 1957b). An interesting study by Hitchcock and colleagues (Hitchcock, A. et al. 1989) compared the DNA ploidy status between primary and métastasés and found that 56% of tumours were concordant for both grade and ploidy. However, these were independent variables, thus lending support to the idea that grade and ploidy are not interchangeable, i.e. not all grade I tumours are diploid and not all grade III tumours are aneuploid.

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However, of the 44% of tumours which did change grade, the change was not unidirectional; not all tumours became more poorly differentiated. If dedifferentiation does occur then it might be expected that tumours would frequently be identified which show areas of different grades. However, this is so rarely seen that pathologists do not even know the frequency.

So what conclusions can be drawn from this? With such conflicting data it would certainly appear likely that there are no absolutes; not all grade I tumours progress. Although this might appear to be an obvious statement, until fairly recently the statement that all grade I tumours do progress was an accepted fact. When Hitchcock and colleagues (Hitchcock, A. et al. 1989) published their findings, they suggested that their data did not support the ‘longstanding dogma that selection o f clones with metastatic potential results in metastatic tumours becoming less well differentiated than the primary tumour’. Conversely, the weight of evidence in favour of dedifferentiation makes it difficult to suggest all grade I tumours do not progress, but equally it is not possible to prove a negative. So perhaps the important question to resolve is what generally happens, in the majority of cases? From the evidence currently available it is not possible to answer this question, but we clearly need to.