Theranostics is a term derived from the combination between the words therapeutics and diagnostics. This emergent field of medicine deals with the simultaneous diagnosis and treatment of some medical conditions. For some types of cancer, the use of radiopharmaceutical therapies (RPTs) allows for the simultaneous targeted delivery of cytotoxic radiation to the tumor cells, as well as the clinical imaging of the active tumor areas with PET. So far, only a few mathematical modeling studies have considered RPTs. Our aim is to lay the groundwork for the mathematical description of RPTs, providing the first works that consider spatial aspects, imaging data, and bystander effects as integral components. Hopefully, we will provide a way to produce digital twins of patients treated with RPTs, suggesting the best therapy combinations or treatment schemes to maximize the efficiency of this emergent field.