I'm Aliza, a BC patient and the site's unofficial resident Medical Librarian. Med Librarians shy away from offering medical advice because it constitutes practicing medicine without a license which is a bit illegal (we like to avoid that...;)), BUT I'm allowed to speak from my own experience as a Cancer patient.
Obviously, lemongrass tea is an herbal tea which must have some kind of properties with which I'm not familiar, so I'd have to say that if you enjoy the flavor, I don't think it will hurt you. You probably should check with your Oncologist though. I highly doubt that it will offer any sort of cure.
I have found that ginger tea, which I make by using hot water and a piece of candied ginger is excellent for a sore throat.
I've also found that chamomile tea is relaxing, but again, I'd check with your doctor before using.