A Logarithmic Formula to Describe the Relationship between the Increased Radiosensitivity at Low Doses and the Survival at 2 Gray

Objectives: Intrinsic radiosensitivity at doses used in radiotherapy is linked to hypersensitivity (HRS) and increased radio resistance (IRR) at low doses. The aim of this study was to explore this relationship. Methods: Survival curves for 18 human tumour cell lines were analysed, using two models to fit the data points in order to extract the necessary parameters relevant for this study. Results: The IRR ratio αs/αr versus the survival at 2 gray (Gy) can be described by a logarithmic relation which leads to a series of straight lines. Conclusion: The relationship obtained implies that there is a direct link between HRS/IRR and survival at clinically relevant doses of 2 Gy

Phenomenological Explanation of Cell Inactivation Cross Section in Terms of Direct and Indirect Action

Objectives: The aim of this study was to use survival curves data for the inactivation of V79 cells and CHO-K1 cells by protons, neutrons, C12 ions and He3 ions to study the role of direct and indirect action in cell inactivation. Methods: A large number of survival curves for the inactivation of V79 cells by protons, neutrons, and C12 ions and for CHO-K1 cells inactivated by He3 ions over a wide energy range were taken from published references. Experimental data points were extracted from the published survival curves using MATLAB (Version 7.0) and fitted to the linear quadratic equation. The fit parameters were used to calculate the inactivation cross section (σ) at the initial slope, the 2Gy dose and at 10% survival for each particle type separately. Results: The results, in general, showed that the inactivation cross section decreases nearly exponentially when increasing the mean free path for primary ionisation (λ), except in the case of protons, and to some extent neutrons, where the cross section takes a constant value at specific λ values. The cross section increased with increasing linear energy transfer (LET) and also became independent of LET at specific LET values. Conclusion: The results indicate that the cell damage due to the double strand breaks of DNA caused by indirect action is much larger than that caused by the direct action

Calculation of the Inactivation Cross Section of V79 Cells by Protons in Radiotherapy

Objective: For efficient applications of protons in radiotherapy, detailed knowledge of the corresponding radiobiological mechanism is necessary. The inactivation cross section is a useful tool to explore the interaction mechanism. Hence, the inactivation cross section for proton irradiation on V79 cells has been calculated using published survival data. Method: The linear quadratic equation was used to fit the survival curves and the parameter α was used to calculate the cross section. The cross section σ is plotted as a function of the mean free path λ and the linear energy transfer LET. Results: The cross section σ-versus-λ curve shows a saturation region between λ≥2 nm and λ≤5 nm. In this region, the inactivation cross section value is about the same as the geometrical area of the DNA segment (≈4 μm2). Such a saturation is also seen in the σ-LET curve. Conclusion: This implies that one DNA segment is at a risk upon the traversal of a single proton.