A Study On The Local Effect Model For The Heavy-ion Cell Survival Calculation

It is important to evaluate the heavy-ion survival through X-ray survival curve in the treatment plan of heavy-ion radiotherapy. In 1994, a local effect model (LEM) for heavy-ion survival simulation was constructed by M. Scholz and G. Kraft.In this study, the theoretical framework and simulation methods of LEM have been investigated, and some defects of the model has been pointed out. Two arbitrary parameter A and r_min was found in the LEM. The role of the two arbitrary parameter D_t and r_min in the LEM have been tested according to the simulation process proposed by M. Scholz and G. Kraft.Our theoretical analysis shows that the local effect model neglects the relationship between macro-dose and micro-dose, thus leads to a result, which is obviously contrary to experiment result, that if the cell exposure to the same macro-dose, the heavy-ion survival and the X-ray survival will be the same. According to the relationship between macro-dose and micro-dose, when biological effect of cell irradiated by ionizing radiations obeys linear-quadratic rule, the assumption that local effect in the cell obeys linear-quadratic rule can not be accepted.We find two arbitrary parameter D_t and r_min in the LEM. D_t was introduced to construct the stand X-ray survival curve, and r_min was introduced to define the minimum radius of the heavy-ion track structure for constructing the dose distribution in track structure. Our testing calculation shows that in the framework of LEM the calculated inactivation cross section and RBE depends on Dt, whose arbitrary changes will lead to arbitrary result of inactivation cross section and RBE, and the central dose in the track structure defined by the r_min has magistral influence on the calculation result of inactivation cross section and RBE.The LEM was constructed to provide a credible RBE for heavy-ion treatment plan. However, our testing calculation shows that the RBE value provided by the LEM is arbitrary. It will be dangerous to make a treatment plan through LEM.