How can i send proxy cache file's if-modified-since info to backend server?

hi all
i use nginx proxy cache to cache file from backend server, and when
cache are timeout, nginx will proxy the first request from client to
server, and delete the old cache file in nginx

but , if the backend file are not modified , nginx will not know, and
delete the cache also.

with squid , it will send an i-m-s header to backend server to 

that cache need to be update or not

can you tell me , how could nginx do it ?