Lung cancer is the most common cause of cancer death. Things have improved with the use of drugs like pembrolizumab,which is often used following diagnosis of advanced non-small cell lung cancer (NSCLC). Pembrolizumab uses the immune system,the body’s natural defence. The immune system sends cells called T cells to fight infections and diseases. Cancer cells hide in the ‘PD-1/PD-L1 pathway’,allowing them to grow and spread. Pembrolizumab blocks the pathway. This prevents cancer cells from hiding,meaning they can be killed by T cells. The cancer may shrink or disappear as a result. Treatment is usually every 6 weeks,sometimes with chemotherapy. Treatment can last 2 years,and this may be too much. Research has not shown that length increases benefit,and many people who stop before 2 years continue to benefit from pembrolizumab after it has finished. There is a possibility that researchers can reduce treatment frequency without effectiveness being reduced. This is the main aim of REFINE-Lung,and has the potential to bring a number of positive consequences: • Similar effectiveness but less side effects • Improved quality of life • Less visits to and savings by hospitals Eligible participants will be about to or already receiving pembrolizumab as first treatment for NSCLC. After 6 months,they will be randomly allocated to one of the following with or without chemotherapy until the cancer grows significantly: • Pembrolizumab 6 weekly – the ‘control’ group,or standard treatment; • Pembrolizumab 12 weekly. After 150 participants are recruited,the researchers will see if 12 weekly is similar to 6 weekly,and if so 3 further groups will be opened: • Pembrolizumab 9 weekly; • Pembrolizumab 15 weekly; • Pembrolizumab 18 weekly. Up to 350 participants will be recruited into each group from 27-35 participating UK hospitals,1750 in total. Each participant will be followed up for 18 months.