Here is some further reading on the subject.
http://cf.alpa.org/internet/alp/2000/janinflight.htm
Ontariotech, there was a study done by someone from RMC (Royal Military College) in Kingston about 12 years ago. I am not sure how widespread his study was but he was interested in flying on Northern routes. A number of Vancouver crew with Air Canada who did almost exclusively YVR-LHR flying participated. They carried monitors and kept records. I met him at one point and in a nutshell, he said that the exposure was below the allowable maximums established at the time, for those in fields that involved radiation.
As far as monitoring crews for unusual rates of mortality I have not heard of any, although if there was a case made on facts, it would be a pretty hot potato.
If it is worth anything, I once asked our pension committee chairman if the actuarial figures showed that the pilot retiree group were not lasting as long as the general populace. He said that if anything, it was the opposite. I'm sure that is an overly simplistic conclusion about the subject but the retirees are not really dropping like flies.