There would be no cache for logged in users. We actually tell the browser this (no-cache, no-store, must-revalidate, max-age=0, s-maxage=0). What you are stating about using the back button is different than loading a completely new browser session and the browser does not open a previous session.
Keep in mind that if you're using a block to display new topics, they have their own internal cache and would not update frequently. As opentype mentioned.