Radiotherapy (RT) is an essential part of the treatment of inoperable stage III lung cancer. Combination of RT with platinum-based chemotherapy provides a survival benefit when compared to RT alone, nevertheless most patients relapse and the overall survival rate is poor. The standard RT dose for stage III lung cancer is 60 Gy in daily fractions of 2 Gy given over 6 weeks. The target volume includes the primary tumor and mediastinal nodes suspected of harbouring tumour, based on PET-CT imaging and surgical staging. The accuracy of RT planning and delivery has greatly improved over the years since the introduction of computerized tomography planning including 4D imaging, advanced delivery techniques including IMRT and the use of on-board imaging. It was hoped that these techniques would allow the safe delivery of larger doses of RT resulting in increased tumor control and increased survival. RTOG 0617 randomised patients to receive chemo-RT to 60 Gy or 74 Gy. There was no improvement in local control and reduced survival in the high dose arm. This may be attributed to increased cardiotoxicity at the higher RT dose. Cardiotoxicity is not only a late effect but can occur in the first years after chemo-RT. This is becoming more important in the era of adjuvant immunotherapy with more patients surviving several years after chemo-RT. I will discuss new technology to reduce the dose of radiation to the heart, including real-time MR imaging during RT, which will be needed to reduce the cardiotoxicity of RT for lung cancer.