Norton Simon Model

In Skipper's model, the exponential growth of tumors is presumed to be homogeneous. The Norton-Simon model takes into account the heterogeneity of a tumor cell population following the Gompertzian growth curve.5,6 The log kill would be greater for very small cancers than for larger tumors. However, smaller cancers also regrow faster. The greater fractional kill, such as against micrometastases in the adjuvant setting, is offset by fractional repopulation of tumor cells at the same fast rate. Thus, tumors are difficult to eradicate under this model. As already stated, this model predicts the observation that adjuvant chemotherapy does not have much impact on overall survival, as opposed to the improvement in disease-free survival. Survival can thus be improved only when tumor cell populations are eradicated or rendered

figure 2.1. Skipper's model of tumor growth (x-axis, time for tumor growth; y-axis, tumor volume).

dormant during the early growth phase. Another implication is that effective therapy should be delivered at reduced intervals to maximize the chances of tumor eradication and to minimize tumor regrowth in between cycles of therapy.

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