Tumour resistance: single-cell or population effect?
There certainly can be co-operation or competition among cancer cells within a tumor that can provide some types of population-based influences on cancer-cell survival. Present evidence, however, suggests that resistance to therapy is almost always due to resistant sub-populations of cancer cells within the tumor. In many cases, such as after treatment with EGFR inhibitors, the resistance mutations have been identified. Sometimes these mutations are in such a small fraction of the cancer cells that they can't be detected in the original tumor before treatment, but in most cases they probably were already present before therapy, caused by the mutations that are inevitable during tumor growth. This review on intra-tumor heterogeneity is a good place to start learning about this topic.