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Cultured human MCF-7 cells were used as a model system to investigate single cell dosimetry after irradiation. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) uses an ion beam to desorb and ionize molecules from a sample surface resulting in a mass spectrum of secondary ions. These mass spectra describe the molecules present in the cell at the subcellular level, providing a chemical map of the cellular contents. Using statistical pattern recognition techniques, we aim to identify the radiation response at the individual cell level and identify the molecules responsible for the response. We showed in cells that were irradiated with cesium 137 and then homogenized that the cytoplasmic contents of control, 2 Gy, and 5 Gy cells were separable using Principle Component Analysis (PCA) and showed a dose-response effect with radiation dose. Neither the nuclear nor the membrane homogenates could be similarly separated statistically. The mass spectra from the freeze-fractured interior surfaces of single cells irradiated at 2 or 10 Gy are statistically separable from non-irradiated controls using PCA techniques and these cells also separated in a dose-dependent manner. A loading plot of the PCA results showed mass 184 was important in the separation along with masses 57, 70, 84, 85, and 86. Mass 184 represents a fragment of phosphocholine, a component of the cells lipid bilayer. Together these experiments show that ToF-SIMS analysis of single cells can be used to detect damage, and that at least one of the mass changes is a plausible result based on the known cell membrane damaging effect of ionizing radiation. (This work was performed under the auspices of the U.S. DOE by LLNL under contract no. W-7405-Eng-48 and supported by NCI grants CA55861 and LDRD 04-ERD-104).

[Proc Amer Assoc Cancer Res, Volume 46, 2005]