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.