In this study we present two prediction methods, mean dose and summed dose, for predicting the number of A549 cells that will survive after modulated x-ray irradiation. The prediction methods incorporate the dose profile from the modulated x-ray fluence map applied across the cell sample and the linear quadratic (LQ) model. We investigated the clonogenic survival of A549 cells when irradiated using two different modulated x-ray fluence maps. Differences between the measured and predicted surviving fraction were observed for modulated x-ray irradiation. When the x-ray fluence map produced a steep dose gradient across the sample, fewer cells survived in the unirradiated region than expected. When the x-ray fluence map produced a less steep dose gradient across the sample, more cells survived in the unirradiated region than expected. Regardless of the steepness of the dose gradient, more cells survived in the irradiated region than expected for the reference dose range of 1-10 Gy. The change in the cell survival for the unirradiated regions of the two different dose gradients may be an important factor to consider when predicting the number of cells that will survive at the edge of modulated x-ray fields. This investigation provides an improved method of predicting cell survival for modulated x-ray radiation treatment. It highlights the limitations of the LQ model, particularly in its ability to describe the biological response of cells irradiated under these conditions.