Vertical fogging (Ilford Delta 400 and 3200)

I've had this issue on 2 films now, Ilford Delta 400 and 3200, both developed in Ilford DD-X. There's a vertical fogging which appears stronger at the start of the film roughly 2 frames apart, then reduces in distance towards the end of the film and becomes lighter. The pattern of the fogging would seem to suggest it was somehow fogged in the canister. It also looks like the fogging is showing the pattern of having gone through layers of the film. Both are in date. (It looks like this film is underexposed/underdeveloped, but my main concern is the fogging.)

I've double checked my Paterson developing tank, I've double checked my camera is light tight by loading, shooting a film and shining a very bright torch at the edges and film window where light leaks could happen, and haven't been able to recreate the effect. In my view this leaves an issue with the film itself or its storage. So far as I'm aware it hasn't been x rayed. Maybe it's possible heat has caused this?

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That's a very helpful picture - much better than words alone.

It looks as though it was fogged through the light-trap or, remotely possibly, by x-ray. Did you use a 'film retriever' to get the end of the film out of the cassette prior to loading it on the reel? The marks look somewhat like that sort of light-damage. X-ray would be less likely as the cassette probably wouldn't be so perfectly orientated with the beam in a checked-baggage scanner.

Alternatively, it may have been exterminated by a dalek . . . as seen in the exposure near your finger.

I use a camera which leaves the leader out (Minolta Dynax 9). If light entered through the trap I think the only time it could have occurred is when pulling out to load in the camera.