We prove the convergence of a family of solutions to a two-dimensional transport equation with a non-local boundary condition modelling the evolution of a population of metastases. We show that when the data of the repartition along the boundary tend to a Dirac mass, then the solution of the associated problem converges and we derive a simple expression for the limit in terms of the solution of a 1D equation. This result permits us to improve the computational time needed to simulate the model.