Frustum - 2d matrix

Hi all;
Im using a quadtree to store my data (a terrain) and culling it with the frustum…but when a lot of nodes are on screen, the app slows down a lot( i tested and the problem is the amount of recursion and frustum tests)…
as my octree leaf nodes represent a 2d matrix, how to project the frustum in a square in Y plane to get the starting cell in x and z, and the last cell in x and z, to do the test with a
for x_init to x_end
for z_init to z_end.

instead of recursion? thanx