I do not have cancer, but my husband does. He has not complained about nightmares. However, I have had vivid nightmares since his diagnosis. I believe in my case, and perhaps in yours as well, the nightmares are a way of expressing fear, anxiety, anguish. There are also some medications that can cause vivid and disturbing dreams. I am so sorry that you are going through such a difficult time; I certainly can relate. I have stopped asking "why" and ask "how"; how can I cope, how can I help my beloved husband cope during these terrible times, how can I learn and grow. I find that praying, writing about these dreams, listening to soothing music, reading uplifing material and telling myself "This too shall pass", and other positive things help. I will keep you in my prayers.