Since different cancers vary widely in response to treatment, only a proportion of patients will respond to any particular treatment. In older therapy approaches using standard chemotherapy, treatment plans were attempted without customization and were tried in sequence until the patient responded. One or several therapies may be attempted with this approach before the best treatment was identified. Not only would these repeated rounds of ineffectual treatment expose the patient to needless, and often severe side effects, but the cost often became very burdensome for both the patient and the healthcare system as a whole.
