Julia
I had exactly the same experience. I was pregnant when I discovered the
lump in my thyroid and was told not to panic. I even had a needle biopsy
which said everything was fine. But I KNEW it wasn't. I pushed and got my
surgery done 8 months after the fact having been told again again that only
1 in a million are malignant! I was slated for a lumpectomy, and ended up
loosing 90% of the gland. It was only when I went back six weeks latter
that I was told that I'd given them "quite a fright". When I think back now
I should have taken that man to the college of surgeons, but as we all know
thyroid + female spells hypochondriac to most physicians so I doubt it
would have very far.
        I am a neonatal nurse practioner and I was told that I had probably got
the  cancer from the many x rays I done over the years. I always caution
people to wear the neck collars to protect their thyroid glands from the
radiation. In neonate we small minute amounts of radiation, but for
radiation therapy the doses used are huge, so you are very probably right
about you hypothesis.
Mary