I would suggest something in the adaptive radiation response arena (i.e., radiation hormesis). By the time you're through, you'll be one of the few experts in the world on adaptive radiation response, and very much in demand...!
Given your interest in dosimetry and biology, the relevant field may be biodosimetry or microdosimetry. To find your specific topics of your particular interest, you may look at the pertinent scientific papers published in the journal (and if possible, attend the international meeting, e.g., for biodosimetry, EPRBioDose2018 to be held in June this year https://www.eprbiodose2018.org/).
More broadly, Strategic Research Agendas (SRAs) prepared by experts highlight the need of the scientific community, which may be useful to consider the topic for the new project.
Take, radiation biology (and epidemiology), for example, MELODI’s SRAs will be very helpful, which are available from the 1st version published in 2010 through the latest version in 2017 (it is also nice to see temporal changes in the scientific need of the community).
http://www.melodi-online.eu/m_docs_sra.html
This is just an example, and predicated on the field of your particular interest, you can find more directly relevant information online.