Hi melady
I am sorry to hear about your terminal cancer. What to do next depends on your energy. You already have the enhanced rate of care, so telling them wouldn't make any difference to your current award. Telling them won't necessarily stop them from sending you a review form. (This is the DWP we are talking about, and they frequently don't take on board what claimants tell them). So it's up to you. You could write to them now and send them the evidence, or leave it until you see what position you are in when the form arrives.
Best wishes
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems