Changing the way of how meta description appears in header.php?

Hi, is there a way to change how meta description($forum_head['descriptions']) appears in header.php?

This is what I want:
- on index.php or viewforum.php, meta description, should be just the title
- but on viewtopic.php, meta description, should be a part of the topic contents(the first 300 characters)

Any ideas?