In this paper, we consider an optimal topology control problem that dynamically configures the path and capacity under the varying traffic load for constructing a multihop backbone network in the TDD-OFDMA system. In order to deal with the co-channel interference among the different relay stations, K orthogonal subzones of OFDMA data regions are spatially reused. We present a joint path and capacity design problem that maximizes the average throughput while properly trading off the outage performance. Due to the complexity of optimal solution approach to this problem, we resort to a genetic algorithm (GA) to solve the dynamic topology design problem and compare its solution to the results from the existing heuristic algorithms.