Simulation of dose dependent retardation of colony growth and its impact on cell survival curves based on the damage distributions as predicted by the GLOBLE model

MOTIVATIONS The aim of this master project is to develop a radiobiological model to describe the retardation of colony growth that is observed after the irradiation of a population of cells. This extension of the GLOBLE model would be useful to predict the colony sizes that are measured in colony assays using the same parameters that are included in the GLOBLE model to describe cell survival after irradiation. An important feature of this work was to determine the threshold criterion to discriminate survivors from non-survivors and to see how different criteria could affect the expected fraction of surviving cells. The most commonly used criterion is to consider as survived the cells that after a week lead to colonies bigger that 50 cells. This investigation is essential to determine the appropriate criterion to use if colony size measurements are taken at earlier stages (i.e. three days). METHODS To develop my model I focused on some specific aspects so I defined a method to proceed with the analysis. At first I had to find the parameters that determine the time delay. In order to do that I simulated colony size distributions for different irradiated doses. Then I had to understand why simulated distributions did not appear as broad as in experimental results and how to fix this problem. Then I simulated survival curves and derive the α and β values to have a direct comparison with the Linear Quadratic model. At last , to reproduce the fact that in real experiments some colonies don’t grow anymore after a certain time especially for higher doses, I wrote a new equation describing the number of cells after irradiation. Every simulation was compared with experimental data obtained by G.Böhrnsen using photon radiation. RESULTS The final parameter from the GLOBLE model that affects cell cycle time delay is the total effect εtot that includes both the effects due to isolated and clustered double strand breaks. Using this parameter, I defined an equation to describe the time delay. With this equation, I simulated colony sizes and produced histograms that show how colony sizes are distributed at a certain time after irradiation. The shape of these distributions is well described by my model but the position of the peaks sometimes differs from experimental data. I produced survival curves and described how α and β parameters change with increasing time after irradiation. An interesting aspect is that β seems to be almost constant in every analyzed curve. The last step of the thesis was to include the description of those colonies that don’t grow anymore after a certain amount of time and I did that writing a new equation for the number of cells. I simulated dose- survival curves using different survival criteria and I analyzed the differences between those graphs. The alpha and beta behavior is not strongly affected by the choice of the survival criterion.