I chose to accept it literally; now that you mentioned it:
-cancer is mentioned at the beginning of the movie when Natalie Portman's character is speaking to her Med students,
-Shepard (the cutest one) being there because her daughter had died of Lukemia.
-the psychologist, there because she is dying of cancer.
I hadn't bothered to even consider symbolism, but this more than likely not a coincidence and you're probably right that there is a second story.
However I went to see a Science Fiction movie, so I really only cared for the surface level. As far as I'm concerned Portman's husband went into the Shimmer to die or find some type of answer to life, having been briefed to some degree on the peculiar nature of the Ops. Portman went in as she said, because, "I owe him." Which I simply take to mean she betrayed him, and had come to the conclusion that he went in the Shimmer to die as having discovered her betrayal, he could not cope. She figures her best odds to help him is to investigate the Shimmer herself. As unrealistic as it is for them to send a team of just female researchers and a paramedic, I simply chose to believe that they had thought that maybe the prior missions being all soldiers and mostly men, they were taking a shot in the dark and trying a last ditch effort. It was sufficient explanation for me because I was interested in the Science Fiction and not the characters themselves. Ultimately it really doesn't make sense for her to be ok with the fake version of her husband, because he's not even a good copy.
The idea that it's actually an allegory for the trauma of battling and surviving cancer only to be deeply permanently altered makes sense, reminds me of this story:
https://nypost.com/2015/07/12/botched-cancer-surgery-turned-my-husband-into-a-stranger/