public static bool FitInCircle(int y, int x, int oY, int oX, int radius2)
{
            bool ret = false;

                int dX = (oX > x) ? (oX - x) : (x - oX);
                int dY = (oY > y) ? (oY - y) : (y - oY);

                if (dX * dX + dY * dY <= radius2)
                                   ret = true;

            return ret;
}

Doesn't seem to select the same tiles as:

private void AddPresence(Location point)
{
            int radius = (int)Math.Round(Math.Sqrt(CollationRadius2), 0, MidpointRounding.AwayFromZero);
            int dY, dX;
                for (int y = -radius - 2; y <= radius + 2; y++)
                    for (int x = -radius - 2; x <= radius + 2; x++)
                        if (x * x + y * y <= CollationRadius2)
                        {
                            dY = (point.Y + y) % Smurfer.Height;
                            if (dY < 0) dY += Smurfer.Height;

                            dX = (point.X + x) % Smurfer.Width;
                            if (dX < 0) dX += Smurfer.Width;

                            World.Presence[dY, dX]++;
                            Presence[dY, dX]++;
                        }
        }