Theranostics represents a concept in personalized medicine wherein the same or similar targeting molecules are used both for diagnostics and therapy.
In oncology, theranostics has acquired greater importance in recent years, as advanced, hard-to-treat tumors have been effectively treated with low side effects (Langbein T et al., 2019). It involves drugs termed radiopharmaceuticals, which usually occur as theranostic pairs, based on small molecule compounds or antibodies that target cell surface receptors overexpressed on tumor cells.
A radiopharmaceutical consists of a tumor-targeting molecule combined with a diagnostic or therapeutic radioisotope via a chelator. The great strength of the theranostics paradigm is the precise selection of patients, at the imaging stage, who will potentially benefit from the targeted radionuclide therapy (Turner JH 2018).
Take a look at the components of a typical radiopharmaceutical. Watch this video to learn more about their mode of action.

References:
Langbein, Thomas, Wolfgang A. Weber, and Matthias Eiber. 2019. “Future of Theranostics: An Outlook on Precision Oncology in Nuclear Medicine.” Journal of Nuclear Medicine 60(Supplement 2): 13S-19S. DOI: 10.2967/jnumed.118.220566
Turner, J Harvey. 2018. “An Introduction to the Clinical Practice of Theranostics in Oncology.” The British Journal of Radiology 91(1091): 20180440. DOI: 10.1259/bjr.20180440