Broadcasting multiple scalable video streams over wireless broadband access networks in real time is a challenging problem, because of the limited channel capacity and variable bit rate of the videos. The difficulty is further increased in the presence of receiver buffer size limitations which may introduce buffer overflow possibilities. The Multicast/Broadcast Service feature of mobile WiMAX network is a promising technology for providing wireless video broadcast services. In this article, we describe a substream selection problem which arises when multiple scalable video streams are broadcast based on the Multicast/Broadcast Service feature to a number of buffer size constrained receivers. We first show that the problem is NPComplete and design a polynomial time approximation algorithm based on convex optimization and dynamic programming techniques. We mathematically prove that the solution obtained through our algorithm is always within a constant factor of the optimal solution. Through simulation we show that under real time requirements our algorithm provides solutions which are within 1dB of the optimal solutions.