Having had the displeasure of wearing the "Frankenboot", my experience has been similar to the other posters. Even if you don't set off the WTMD, they'll want to take of the cast and check it. Most times I had the swab test, but I did have the boot xrayed more than once. If it's a real problem, i.e. excruciating pain, to take of the cast, be patient and explain that to the TSA officer. Also, allow enough time to clear security without having to rush through. Why subject yourself to more stress than necessary.

Hope you have a speedy recovery.