Just thought I would give a little update: my oncologist advice was to leave the mirena iud in as he felt the tamoxafen would counter any hormonal issues. It's also 'old'--been in for over 4 years. I had my check up with radiation oncologist and asked her and she said no clear evidence about mirena but bit grey area. In retrospect I'm glad I left it as I wouldn't have wanted either heavy unexpected periods or pregnancy in the last year! Now I'm going to have it as a issue for discussion with next appt gyncologist.
Best wishes