Lung cancer has the highest mortality rate of all cancers in Australia and is an area of unmet need. It starts in the lung (the primary tumour), and then spreads (metastasise) to other organs. The PRIME-Lung clinical trial will investigate if radiotherapy to the primary lung tumour, in addition to standard drug therapies, for patients with metastatic lung cancer will lead to less spread of cancer, prolong life, improve patient well-being and be cost-effective for the health care system.