A tumour in a dream is a symbol, not a diagnosis: it is read as a worry, habit or situation that has grown out of proportion. Dream books see it as something taking up more room than it should.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.