Headscratchers / Somewhere in Time

The following is a problem only of the book, since the film eliminated the plot point that Richard is dying from a brain tumor. In the book, Richard seems to be under the impression that traveling to the past will somehow rid him of his tumor, and indeed while in the past he notices that his morning headache (one of the most prominent symptoms of his tumor up to that point) is gone. But he never explains why this should happen, and it doesn't seem to follow from the logic of time travel outlined in the book: why should sending his body back in the past cure that body of disease? Yet not only does Richard seem to be making that assumption, he completely takes it for granted. Despite being in constant neurosis over the numerous things that could go wrong on his journey (he fears being sent back to the present, and he even fears being stranded in the past while losing Elise, or of dying due to various other causes), he never once even seems to ponder the possibility that the cancer could take him while he's in the past. It does lend credence to the theory that his entire time-traveling experience is a delusion brought upon by his disease—but it's the one piece of the puzzle that truly doesn't fit; everything else could logically be explained either way.