retaining a value in a controller variable

[eluser]ssachan[/eluser]
ok, I am trying to setup a forum. So basically I am passing the topic id to this controller with handles all the comments for this topic.
Based on the topic id I load the view showing all the comments and for any new comments posted on this view, I am directed back to the same controller and I need to store the comment with the topic id.
What I want to do is store the topic id so that I don't need to pass it back and forth while I traverse between controller and view.
And clearly this is not a session data since I may open a lot of topics and hence I cannot override this data