Also possible that it is a light leak through the light trap on the cassette if you wind the film right in and expose to a bright light source. This will always happen with IR film if you take the cassette out in the daylight, and the banding looks just the same...I know, I've done it!
Realize that this is not IR, but there is a slight chance this could happen?
Seeing that banding is what made me think x-ray damage, it would penetrate in a similar fashion. I remember that with the Kodak HIR, only made that mistake once!