What does cancer is a word not a sentence mean?

It means that the condition does not have to be fatal. The Word "sentence" in this case is used as a pun. It suggests that the Word "cancer" is only a Word and a part of a sentence. At the same time, it proposes that having cancer is not a prison "sentence".

This is a wonderful example of how a positive outlook - in a situation that may seem hopeless to many - can be considered as a motto that sheds a shining beacon for others to follow.