Roxy: You do not need to tell the TSA folks anything about your health. You can opt out without explanation. At least in O'Hare, there are prominent signs stating you may opt out of the back scatter. I opt out not because of a health issue, but because I do not trust the TSA to competently maintain and handle those machines. In a hospital, they are attended to by specialists and not used nearly as frequently. The TSA won't even allow their personnel to wear the badges that track their exposure to radiation.

It may help to crack a joke if you are apprehensive. I don't find the pat down particularly invasive. The TSA person always tells you what she is about to do, and does it quickly. Not the most comfortable event ever, but a lot less invasive than most doctor/dentist visits.