Cancer research is all about discovering, detecting, preventing, and treating. So, when you understand the fundamental biology of cancer you will be able to come with some interesting topics that could suit your purpose related to your field and available resources.
You could take up a topic in therapeutics. Patients primarily receive drugs whose activity against the tumor is limited by unacceptable toxicity. The development of rationally designed agents that can target many of the molecular characteristics of the tumor could provide greater protection to patients.