Encounter problem with Apache mod_proxy

I was trying to confirgure the Apache to be a proxy using ProxyPass
and ProxyPassReverse :
ProxyPass / http://backend_domian.com/
ProxyPassReverse / http://backend_domain.com/

So I am expecting all traffics coming into the Apache will be
redirected to the backend server. It works. However, when I receives
the response html page, the anchor link that originally pointing to
the apache server, ie. <a
href=http://apache_domain.com/testing/dowload?...> from the backend
server is being overwritten to <a
href=http://backend_domain.com/testing/download?...>. And therefore
this link is not working cos it points to the backend server.

Is it due to the above configuration using ProxyPass &
ProxyPassReverse?? Did anyone encounter the similar problem?? How to
overcome it??