What is the meaning of dreaming your daughter is dying of cancer?

Your dream could have several quite different interpretations depending on your daughter's age and circumstances. Most importantly it does not necessarily mean that your daughter is actually ill, and it is not predicting any future illness. In dreams, "death" almost always is a symbol for a major change, transformation or transition. In much the same way, "cancer" can symbolize anything that is "eating away" at her, such as anxiety or any sort of obsession.
... On a deeper level, the dream is not about your daughter at all, but about yourself. The "cancer" could represent your own anxiety - perhaps anxiety about your daughter - taking over too much of your thoughts and energy. Your daughter in the dream could represent yourself at her age, or the hopes, loves, ambitions and ideals that you had when you were her age. In that case, the "cancer" suggests the loss of ideals or hope for your own future. The dream may be a nudge from your own mind that you need to take serious action to bring health back into your own life.