The I-START trial is designed to determine the highest doses of radiotherapy that can safely be used in locally advanced non-small cell lung cancer (NSCLC). Patients with NSCLC who are expected to live longer than three months and are fit to receive radical radiotherapy (radiotherapy given with curative intent) will be eligible to participate. All trial participants will receive 20 doses (called fractions) of radiotherapy. Evidence is available that suggests increasing the dose of radiotherapy given per fraction may improve both local control of the cancer and survival in some patients. However, high dose radiotherapy can damage normal tissues as well as the tumour. The dose of radiotherapy that can be used to treat lung cancer is limited by the normal tissues close to the cancer. For most of these normal tissues (lung, spinal cord and heart) the maximum safe radiotherapy dose that can be given is known. The maximum safe dose of radiotherapy for the oesophagus (gullet) is not currently known.