It is a very even wavy band on the film. I would have thought to create such a precise pattern the chances of the X ray machine being exactly square to the film (laterally or tranversely) as it scanned would be very small at best. But other than a leprechaun with a torch inside your camera I can't think what might have caused such a wavy line.
Hmmm. I would have thought that Xray exposure of a negative would result in light marks on the print, rather than dark marks. Am I missing something?