We recently reported that large numbers of viable cancer cells enter the bloodstream during radiotherapy (RT) for non-small cell lung cancer (NSCLC). This is important because death from distant metastasis is extremely common after curative or “radical” RT, even when locoregional disease is eradicated. Although distant treatment failure has generally been attributed to the growth of occult micrometastases that were present before treatment, we hypothesised that that disruption of tumour architecture during fractionated RT could cause viable tumour cells to enter the peripheral circulation. We applied various approaches to detect and analyse CTCs in blood taken from NSCLC patients undergoing RT, including fluorescence microscopy for visualization and enumeration of tumour cells immunostained with conventional markers for CTC detection. We developed a novel approach that identified CTCs derived from the irradiated volume by their elevated γH2AX signal (γH2AX is a biomarker of radiation-induced DNA damage). After RT, elevated γH2AX foci were detectable in CTCs thereby revealing the “radiation history” of the cells. We sought and found evidence for the presence of viable or reproductively capable CTCs by culturing cells from patient blood in in vitro and in vivo experiments. We identified large numbers of γH2AX positive CTCs derived from irradiated tumours. An increase in single CTCs and tumour cell clusters was evident in a majority of patients given palliative RT, and in about half of patients with curative RT. CTCs collected after commencement of RT were viable, as they were able to grow in culture. Cultured CTCs were injected intravenously into NSG mice. Preliminary data showed that metastatic lesions formed in mouse lungs, and abundant human tumour cells were observed in blood vessels. CTCs released during RT could contribute to the risk of distant metastasis in NSCLC. New therapeutic approaches targeting CTCs released during RT could potentially improve survival.