Based on the questions the research is trying to answer, each clinical trial protocol clearly states who can or cannot join the trial.

Common criteria for entering a trial:

• Having a certain type or stage of cancer
• Having received a certain kind of therapy in the past
• Being in a certain age group

Criteria such as these help ensure that people in the trial are as alike as possible. This way doctors can be sure that the results are due to the treatment being studied and not other factors.

These criteria also help ensure:

• Safety
  
  Some people have health problems besides cancer that could be made worse by the treatments in a study. If you are interested in joining a trial, you will receive medical tests to be sure that you are not put at increased risk.
  
  
• Accurate and meaningful study results
  
  You may not be able to join some clinical trials if you already have had another kind of treatment for your cancer. Otherwise, doctors could not be sure whether your results were due to the treatment being studied or the earlier treatment.