BOOP is now called organising pneumonia, and is known to be associated with radiation therapy, as well as some underlying disorders called connective tissue disorders. Sometimes no reason can be found for it. It can resolve on its own, or usually responds to steroids. This is a rare disease, have you considered a requesting, or have you had a specialist lung physician opinion? Also there is a fact sheet on this on the lung foundation website, which is quite informative, written by a specialist at the Austin hospital.
I hope it settles down for you. Xx Tinks