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Fig. 9.7 Tumor progression if the initial mutant cell line has only shifted the balance between promoters and inhibitors slightly in favor of promotion. This cell line can only give rise to self limited growth. Further tumor growth requires the generation of further mutants. The new mutant in the simulation is depicted by the dashed line. Parameters were chosen as follows: r = l;d = 0.1; ap =5 ;bp = 0.1; aj = 3; b/ = 0.01; Dc = 0.00001; Dj = 0.001, L = 2. For mutant: ar = 0.5; aP = 20.

become fixed in the population (Figure 9.7). The change can either be the generation of multiple lesions, or invasion of the whole tissue, depending on the amount by which the level of promotion has been enhanced by the mutant cell population. The chances that the mutant cell population drifts to levels high enough to cause such a change in tumor growth depend on the population size of the lesion. The larger the number of tumor cells,

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