Yes and no. Kind of and not. If you get treated for a tumor, the first round of chemo can be incredibly effective. Say it kills most of the cancer cells. The remaining few are a huge problem for many people. The chemo agent has evoked a specific cellular response, provoked expression of gene signals like any drug, even tylenol, does. The cells not killed in early rounds do change their reaction to the original chemo agent, they over or under express certain gene activity to survive the chemo. This is a way overly simplified example of the phenomenon of drug resistant cancer, or multidrug resistant cancers. So - cancer cells to have some adaptation response to drugs and do develop treatment resistances.