I recently found an article that relates the effectiveness of chemotherapy and radiation to the genetic makeup of the patient and also the variations in genetics that caused the cancer.

It seems that our genes help to determine the rate of cell adaptation and the heartiness of certain cells. while you would think that having less hearty cells would make radiation more effective due to the fact that cancer cells would be easier to kill, it also increases the amount of collateral damage that nearby cells endure and how quickly they are damaged. This can lead to more cancer that might not even be the same type that was intended to be irradiated. In addition some people produce specific proteins that counter some chemotherapy drugs or cause them to have unintended and detrimental effects. while research in this field is ongoing, it is apparent that a more effective cancer destroying process is necessary. http://www.sciencedirect.com/science/article/pii/S0092867402006256