This clinical trial is focused on studying a type of lung cancer known as non-small cell lung cancer (NSCLC) in its early stages, specifically stages IB to IIIA. The study is particularly interested in patients whose cancer has specific changes in the epidermal growth factor receptor (EGFR) gene, which are known to respond to certain treatments. The treatment being tested is a medication called osimertinib, also known by its code name AZD9291. This medication is compared to a placebo to evaluate its effectiveness and safety in preventing the return of cancer after the tumor has been completely removed through surgery, with or without additional chemotherapy.

The purpose of the study is to determine how well osimertinib works in keeping the cancer from coming back, which is referred to as disease-free survival. Participants in the study will be randomly assigned to receive either osimertinib or a placebo. The study will be conducted in a way that neither the participants nor the researchers know who is receiving the actual medication or the placebo, a method known as double-blind. This helps ensure that the results are not influenced by expectations about the treatment.

Throughout the study, participants will be monitored for several years to assess the long-term effects of the treatment. This includes regular check-ups and tests to track their health and any changes in their condition. The study will also look at overall survival rates and the quality of life of the participants, using questionnaires and other assessments. The safety of the treatment will be closely watched by checking for any side effects or health changes during the study period.