Thousands of patients die annually with distant metastasis after curative-intent or “radical” radiation therapy (RT). Because non-small cell lung cancer (NSCLC), the most common cause of cancer-related mortality, exhibits an especially high rate of distant metastasis after radical RT or chemo-RT for locoregionally-advanced disease, it represents a suitable model to investigate the relationship between RT and metastasis. We hypothesized that disruption of tumour architecture during RT could result in the release of viable tumor cells into the peripheral circulation.

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 sorting (FACS) or by immunofluorescence microscopy. Twenty seven RT-treated NSCLC patients had blood samples analyzed by one 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. CTCs 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), and first experiments revealed that these cells survive in bloodstream of NOD/SCID mice for several months. CTCs formed γ-H2AX foci in response to ex vivo irradiation providing further evidence of their viability.

Although of concern, these findings could represent an opportunity to monitor and target CTCs during RT. They provide a rationale for the development of strategies to reduce the concentration of viable CTCs by modulating RT fractionation or by co-administering systemic therapies.

Citation Format: Michael MacManus, Robin Anderson, Prudence Russell, Ashley Cox, Agnieszka Swierczak, Judy P. Doherty, Daphne Jacobs, Jai Smith, Shankar Siva, Patricia E. Daly, David L. Ball, Roger F. Martin, Olga A. Martin. Mobilization of tumor cells into the circulation during radiation therapy for lung cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4826. doi:10.1158/1538-7445.AM2014-4826