X-ray scattering effect on the heterogeneous dose distribution in a cell box

The dose distribution in a cell box is very important factor for the radiobiology experiment since it affects the cell survival probabilities. When the x-ray is irradiated to the closed (or sealed) cell box, which is frequently used for the experiment of oxygen enhancement ratio (OER) research, it is possible that x-ray scattering effect by cell boxes may cause the inhomogeneous dose distribution within a cell box. In this study, we have investigated the inhomogeneity of the dose distribution with three different sizes of cubical cell boxes, whose side lengths are 10, 15 and 20 cm. The difference of the doses at the center of 10 cm box and 20 cm box was measured to be less than 2 % for both 6 MV and 15 MV x-ray irradiation. That indicates the absolute dose at the center of cell box doesn’t much depend on the size of cell box. Unlike the absolute dose value at the center of box, the relative dose distribution in a cell box has the strong dependency upon the condition of the cell box. The results show that the percentage showing more than 10 % dose difference to the average dose value goes up to ~ 40 %. Our experimental evidence suggests that when the closed box is used for the radiobiological experiment, the dose inhomogeneity within the cell box is not negligible and the location of the cells within a box should be carefully designed.