Abstract
Carbon-ion therapy has been shown to be effective for a treatment of malignant tumors because of a good dose distribution and a high relative biological effectiveness (RBE). We are able to calculate physical processes in a phantom irradiated by heavy-ion beams with the advancement in computer technologies. However, no general theory to calculate biological effect has not been established for heavy-ion beams due to the extremely complexities of chemical and biological processes. A certain model is necessary to predict the biological effects on any irradiation condition for the heavy-ion therapy of high precision.
So far, we found that a combination of the Kiefer-Chatterjee track structure model and the Hawkins' Microdosimetric Kinetic Model (MKM) is very useful in explaining the survival curve of aerobic in-vitro cells for high-energetic ion beams. However, there are actually hypoxic cells in a large tumor, which are typically radiation-resistant. This time, we verified practical effectiveness of the MKM for predicting the biological effect of hypoxic cells for heavy-ion beams when we have known the MKM parameters of the same cells in aerobic condition.
In the result, we could estimate the MKM parameters in the hypoxic condition from the parameters in the aerobic condition for V79 and HSG cells by assuming the oxygen enhancement ratio (OER) of 3 for X-rays and a half radius of aerobic domain which represents a considerable region for radiation effect in the MKM. The reason why the domain in the hypoxic condition became the half radius of the aerobic condition is thought of as loss of production and dispersion of active oxygen (oxygen radical, hydrogen peroxide, ozone etc.) which causes lethal damages in the cell.
