If you're using perl, you can do something like..
With an invisible man at the center point:
	Code:
	sub EVENT_SPAWN
   {
   $x = $npc->GetX();
   $y = $npc->GetY();
   $z = $npc->GetZ();
   quest::set_proximity($x - 5, $x + 5, $y - 5, $y + 5, $z - 6, $z + 10);
   }
sub EVENT_ENTER
   {
   #something happens
   }