Kindly let us know what instruments do you have like Gamma camera, PET-CT, micor-PET, micro-CT or gamma probe. Based on it we can advice what would be best project to do. Are you interested in clinical or pre-clinical nuclear medicine imaging.
additionally to Madhur it is important to state that you must have available studies, therapies, physicist and medical support regarding therapies. In case you do work with therapies already please perform them very well structured, and use technologies that are standardized like MIRD, or better voxulized dose, with at best quantified input data (Read SUV based datasets) for all calculations. If your therapies are standard probe the possibilities to work with key institutes that are looking into "novel" treatments like using bismuth. I131 publications might be interesting , but done already.
I have found that I should make topics relevant to my own place of practice. I ask myself what is/are common at this place and how can I improve it? What you choose should be what other people do in a similar practice (eg bone scan, FDG-PET scan) so your own population would be similar to them and your results and findings could be applicable to them. It is, of course, not the same everywhere but what you do must be similar to what someone else does "out there"?