| Summary: | Cell cycle-related phenomena have profound effects on the ability of cells to
survive exposure to ionizing radiation. These phenomena are important because they shed
light on the underlying mechanisms of the cell cycle, and because they provide avenues to
improve the efficacy of clinical radiotherapy. Radiobiologists have developed a partial
picture of how and why radio sensitivity varies during the cell cycle. Nevertheless, our
understanding of the role of cell cycle effects on the radio sensitivity of human cells is still
far from complete.
In this thesis, cell populations which were synchronized at specific points in the
mitotic cycle were used to explore cell cycle-related effects on the radiosensitivity of
human tumor cells in vitro. The survival of synchronized cells after irradiation was
measured at low doses which are relevant to radiation therapy, using the cell-sorter assay,
which utilizes a cell-counting flow cytometer to reduce the uncertainties associated with
traditional survival assays.
The radiosensitivity of three human tumor cell lines varied significantly over the
course of the cell cycle, in a manner which was in general agreement with earlier studies
by others who examined the responses of rodent, and some human, cell types, using
similar synchronization techniques. However, discrepancies were found with other studies
which used alternative synchronization methods.
The widely-used linear quadratic model of cell survival was tested in synchronized
cell populations. Consistent, significant deviations from the model were found. A
mathematical model of synchronized cell radio sensitivity was developed to explore these
deviations. Departures from the linear-quadratic model in two cell lines could be
adequately explained by cell cycle-related heterogeneity in experimental cell populations.
In a third cell line, however, the deviations from the linear-quadratic model were not
attributable to cell heterogeneity alone.
One of the three human tumor cell lines examined here underwent a prolonged
arrest in the G₁ phase of the cell cycle after irradiation. The arrest was characterized by
following the progression of synchronized cells after irradiation at different times in G₁
phase. The characteristics of the arrest were consistent with a "checkpoint" in late G₁
phase where radiation-damaged cells stopped cycling for an extended period. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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