PURPOSE: To determine whether radiation therapy (RT) could mobilize viable tumor cells into the circulation of non-small cell lung cancer (NSCLC) patients. METHODS AND MATERIALS: We enumerated circulating tumor cells (CTCs) by fluorescence microscopy of blood samples immunostained with conventional CTC markers. We measured their DNA damage levels using γ-H2AX, a biomarker for radiation-induced DNA double-strand breaks, either by fluorescence-activated cell sorting or by immunofluorescence microscopy. RESULTS: Twenty-seven RT-treated NSCLC patients had blood samples analyzed by 1 or more methods. We identified increased CTC numbers after commencement of RT in 7 of 9 patients treated with palliative RT, and in 4 of 8 patients treated with curative-intent RT. Circulating tumor cells were also identified, singly and in clumps in large numbers, during RT by cytopathologic examination (in all 5 cases studied). Elevated γ-H2AX signal in post-RT blood samples signified the presence of CTCs derived from irradiated tumors. Blood taken after the commencement of RT contained tumor cells that proliferated extensively in vitro (in all 6 cases studied). Circulating tumor cells formed γ-H2AX foci in response to ex vivo irradiation, providing further evidence of their viability. CONCLUSIONS: Our findings provide a rationale for the development of strategies to reduce the concentration of viable CTCs by modulating RT fractionation or by coadministering systemic therapies.