Lung Biopsy - Why would this be?

My dad had a CT scan which they were sure was cancer and went for a PET scan last week. His follow up is next Weds and now have called today and said he needs a lung biopsy on Thursday.

What would they be doing in the biopsy? Checking what it is? I am really worrying now - does this mean its worse news than we initially thought (they were talking about surgery to remove it and then treatment) 

Thanks all