The man with a knife without a face
Lurks waiting by the door, the door ajar.
The room is dark and small:
He can smell where I am.
He knows my name;
He knows my name.
The easy interpretation is that Shadow is about the mastectomy Sandra knew she would have to have. However, that was so far in the future (seven months) as to be almost untenable as a sole interpretation.
A deeper interpretation is that the man with no face is the cancer itself, which (because it knows her name) has total control over her. The door ajar, with the cancer lurking beside it, suggests the tantalising possibility of escape from her predicament but that, ultimately, whether she does escape depends upon the cancer itself.
I hope it isn't a betrayal of marital confidence to say that at about this time we would sometimes be sitting in our front room, not doing anything in particular, when she would suddenly start shaking all over from pure terror, and could only just manage to say, through teeth that were literally chattering with fear, "I'm terrified about what will become of me". This was what she later came to call 'The sheer sickening terror of initial diagnosis'.