How many different cancers are in a solid tumour?

Modelling cancer has become a popular activity among mathematical biologists and physicists. However, present models omit a very important (from a practical point of view) aspect of cancer growth: the accumulation of neutral mutations. These mutations, called "passengers", do not carry any fitness advantage under normal circumstances, but may do so when treated with drugs. Due to the lack of experimental data it has become clear only very recently that different passenger mutations can occupy different locations in the tumour, and so the tumour is genetically very heterogeneous. In this talk I will explain my attempts at modelling genetic heterogeneity in tumours, and make some (grim) predictions for the possibility of any substantial improvement in cancer chemotherapy. I will also discuss interesting parallels between tumours and bacterial populations growing on solid substrates (2d) or suspended in gels (3d).