SU‐FF‐T‐398: Study of Inhomogeneous Dose Distribution in a Cell Box Due to the X‐Ray Scattering Effect

Purpose: The ratio of hypoxic to aerated doses needed to achieve the same biological effects, called the oxygen enhancement ratio (OER), is very important factor in radiotherapy since none produces such a dramatic effect and no other agent has such obvious practical implication. Although it is usually assumed that the dose distribution in a cell box is uniform, the absorbed dose to the tumor cells could be very different according to the locations of the cells within a cell box, which affects the cell survival curves. We have studied the inhomogeneous dose distribution in a cell box due to the scattering of x‐rays with the cover and side‐walls of cell box. Method and Materials: Three different sizes of cubical cell boxes, whose side lengths are 10, 15, 20 cm, were designed and irradiated by 6 and 15 MV x‐rays using Varian 2100CD linear accelerator. Results: The differences of the absorbed doses at the center between 10×10×10 cm³ box and 20×20×20 cm³ box were less than 2 % for 6 MV and 15 MV x‐ray irradiation, which indicates that the scattering effect depending on the size of cell box is not significant for the absolute absorbed dose at the center of box. However, the relative dose distribution within the cell box shows very different behavior revealing distinct heterogeneous dose distribution in a cell box. For example, the percentage showing more than 10 % dose difference within the cell box was ∼ 40 % for 20×20×20 cm³ box with 15 MV x‐ray irradiation. Conclusion: While the dose inhomogeneity for 15 MV x‐ray irradiation increases as the size of cell box increases, the dose inhomogeneity for 6 MV x‐ray doesn't show any size dependency. Our experimental evidence suggests that the dose inhomogeneity within the cell box is not negligible.