IMRT optimization is an iterative process in which many dose calculations may be necessary. A fast and accurate dose calculation engine is essential for the optimization program to converge to a reliable solution in a reasonable time. Previously we developed a fast dose calculation method for IMRT optimization based on the table lookup (TLP). The TLP kernel is constructed from the dose distributions computed from an accurate superposition/convolution (SC) algorithm at the beginning. Dose calculations at subsequent iterations are performed by looking up the kernel in memory, therefore are very fast. However, it is assumed by the TLP that the beamlet only deposits doses to the voxels on its path. In this paper, we propose an improved table lookup based dose calculation method that includes the scatter contribution (TLPS). The intensity map for each beam is decomposed into an even and an odd checkbox-type pattern for the construction of the kernel for primary and scatter components. The performance of this algorithm is compared with the methods of SC only and TLP without scatter on a few clinical cases and the advantages of speed and accuracy are demonstrated. This method is especially useful for those optimizations that utilize the IMRT optimization as an inner loop, such as the beam angle optimization.